JEE Rotational Motion Questions

The straight rod is bent until its two ends meet, so the whole length of the rod becomes the circumference of a circle.

Hence, $$\text{circumference}=2\pi R=L$$ $$-(1)$$ which gives $$R=\frac{L}{2\pi}$$ $$-(2)$$

Mass of the ring: linear mass density $$\lambda$$ means $$m=\lambda\times(\text{length})=\lambda L$$ $$-(3)$$

For a thin ring of mass $$m$$ and radius $$R$$:

• Moment of inertia about the axis perpendicular to its plane and passing through the centre is $$I_{z}=mR^{2}$$. • For any diameter (say $$x$$-axis) lying in the plane of the ring, use the perpendicular-axis theorem:   $$I_{x}+I_{y}=I_{z}$$. Because of symmetry, $$I_{x}=I_{y}=I_{d}$$ (moment of inertia about any diameter), so

$$2I_{d}=mR^{2} \;\; \Longrightarrow \;\; I_{d}=\frac{mR^{2}}{2}$$ $$-(4)$$

Substitute $$m$$ from $$(3)$$ and $$R$$ from $$(2)$$ into $$(4)$$:

$$I_{d}=\frac{\lambda L}{2}\left(\frac{L}{2\pi}\right)^{2} =\frac{\lambda L}{2}\cdot\frac{L^{2}}{4\pi^{2}} =\frac{\lambda L^{3}}{8\pi^{2}}$$

Thus, the moment of inertia of the ring about any diameter is $$\boxed{\displaystyle \frac{\lambda L^{3}}{8\pi^{2}}}$$, which corresponds to Option D.

Since the wheel has negligible mass spokes, its mass is concentrated at the rim (treating it as a hoop/thin ring):

$$I = MR^2$$

$$I = 10 \text{ kg} \times (0.1 \text{ m})^2 = 0.1 \text{ kg m}^2$$

The work done by the pull (F) over the unwound distance (s) equals the change in rotational kinetic energy:

$$\text{Work} = \Delta KE_{rot}$$

$$F \cdot s = \frac{1}{2} I \omega^2$$

$$20 \times 1 = \frac{1}{2} \times 0.1 \times \omega^2$$

$$\omega = 20 \text{ rad/s}$$

The diameter of the ring is given as $$r$$, hence its radius is
$$R = \frac{r}{2}\,.$$

Step 1: Moment of inertia about a diameter (axis in the plane, through centre)
Every mass element of a thin ring is at the same distance $$R$$ from the centre. For an axis along a diameter (say the $$x$$-axis) lying in the plane of the ring, the perpendicular distance of a point $$\bigl(R\cos\theta,\;R\sin\theta\bigr)$$ from that axis is $$R\lvert\sin\theta\rvert$$. Using $$I = \int r_\perp^{\,2}\,dm$$,

$$I_{\text{diameter}} = \int_0^{2\pi} \! \left(R\sin\theta\right)^2 \left(\frac{M}{2\pi}\,d\theta\right) = \frac{MR^2}{2}\,. $$

Step 2: Moment of inertia about a tangential axis in the plane
The required axis is parallel to the diameter axis but touches the ring at one point, so it is shifted by a distance $$R$$ from the centre. Apply the Parallel-Axis Theorem:

$$I_{\text{tangent}} \;=\; I_{\text{diameter}} + M R^2 = \frac{MR^2}{2} + M R^2 = \frac{3}{2}\,M R^2 \,.$$

Step 3: Express in terms of the given diameter
Substitute $$R = \dfrac{r}{2}$$:

$$I_{\text{tangent}} = \frac{3}{2}\,M\left(\frac{r}{2}\right)^2 = \frac{3}{2}\,M\,\frac{r^{2}}{4} = \frac{3}{8}\,M r^{2}\,.$$

Therefore, the moment of inertia of the circular ring about a tangential axis lying in its own plane is $$\frac{3}{8}Mr^{2}$$.

The correct choice is Option B.

Torque (moment of the force) about the chosen origin is defined as
$$\vec{\tau} = \vec{r} \times \vec{F} \quad -(1)$$
where $$\vec{r}$$ is the position vector of the point of application of the force $$\vec{F}$$.

Therefore expression E ( $$\vec{\tau} = \vec{r} \times \vec{F}$$ ) is directly correct.

Using Newton’s second law $$\vec{F} = \dfrac{d\vec{p}}{dt}$$ and substituting in $$(1)$$ gives
$$\vec{\tau} = \vec{r} \times \dfrac{d\vec{p}}{dt} \quad -(2)$$
so expression C is also correct.

The orbital angular momentum of the particle is defined as
$$\vec{L} = \vec{r} \times \vec{p} \quad -(3)$$
Differentiating $$(3)$$ with respect to time (for a fixed origin) yields
$$\dfrac{d\vec{L}}{dt} = \dfrac{d}{dt}(\vec{r} \times \vec{p})$$
By the rotational analogue of Newton’s second law, $$\dfrac{d\vec{L}}{dt} = \vec{\tau}$$, hence
$$\vec{\tau} = \dfrac{d}{dt}(\vec{r} \times \vec{p}) \quad -(4)$$
Thus expression B is correct.

For a rigid body of constant moment of inertia $$I$$ about a fixed axis, the relation
$$\vec{\tau}_{\text{net}} = I \vec{\alpha} \quad -(5)$$
holds, where $$\vec{\alpha}$$ is the angular acceleration. Hence expression D is correct under these usual JEE assumptions.

Check expression A: $$\vec{r} \times \vec{L}$$ has dimensions $$\bigl( \text{m}\bigr)\times \bigl(\text{kg m}^2\!/\text{s}\bigr)=\text{kg m}^3\!/\text{s}$$, whereas torque has dimensions $$\text{kg m}^2\!/\text{s}^2$$. The two are not equal, and there is no general mechanical principle equating them. Hence expression A is incorrect.

Therefore the correct expressions are B, C, D and E.

Option C is the correct choice.

The force F acts at a distance of 2R from point P, and for a solid sphere, the moment of inertia about the contact point (using Parallel Axis Theorem) is:

$$I_P = I_{cm} + MR^2 = \frac{2}{5}MR^2 + MR^2 = \frac{7}{5}MR^2$$

For rolling without slipping, we can take torque about the point of contact (P) on the ground:

$$\tau_P = I_P \alpha$$

$$F(2R) = \left( \frac{7}{5}MR^2 \right) \frac{a}{R}$$

$$a = \frac{10F}{7M}$$

$$a = \frac{10 \times 49}{7 \times 20}$$

$$a = \frac{490}{140} = 3.5 \text{ m/s}^2$$

For a wheel that rolls without slipping, the centre of mass (CM) moves with linear speed $$v_{CM}$$ and the wheel simultaneously rotates with angular speed $$\omega$$ such that $$v_{CM}=R\omega$$, where $$R$$ is the radius.

The linear velocity of any point on the rim is the vector sum of the translational velocity $$v_{CM}$$ (same for every point, directed horizontally) and the tangential velocity due to rotation $$\omega R$$ (perpendicular to the radius through that point).

The highest point on the rim has its rotational velocity in the same horizontal direction as $$v_{CM}$$, so the speeds add: $$v_{\text{top}} = v_{CM}+R\omega = v_{CM}+v_{CM}=2v_{CM}$$.

Given $$v_{\text{top}} = 8 \text{ m/s}$$, we get
$$2v_{CM}=8 \implies v_{CM}=4 \text{ m/s}$$.

Consider a point on the rim that lies at the same vertical level as the centre (rightmost point if the wheel rolls to the right).
 • Translational velocity: $$4 \text{ m/s}$$ to the right.
 • Rotational (tangential) velocity: $$\omega R = v_{CM}=4 \text{ m/s}$$ vertically downward.

These two perpendicular components give the resultant speed
$$v_{\text{side}}=\sqrt{(4)^2+(4)^2}=4\sqrt{2} \text{ m/s}$$.

Therefore, the speed of the particle on the rim at the same level as the centre is $$4\sqrt{2} \text{ m/s}$$. Hence, Option A is correct.

For the lamina to remain stationary, the net torque about the pivot O must be zero:

$$\sum \tau_O = 0$$

$$\tau=\text{Force}\times\text{Perpendicular distance}$$

$$At\ Point\ A:The\ vertical\ 10\text{ N}\ (down)\ creates\ a\ clockwise\ torque:\ \tau_1=-10\cdot L$$

$$At\ Point\ B:The\ vertical\ 10\text{ N }(up)\ creates\ a\ counter\ clockwise\ torque:\ \tau_2=+10\cdot L$$

$$The\ horizontal\ 10\text{ N}\ (right)\ creates\ a\ clockwise\ torque:\ \tau_3=-10\cdot L$$

$$At\ Point\ C:The\ horizontal\ force\ F\ (left)\ creates\ a\ counter\ clockwise\ torque:\ \tau_4=+F\cdot L$$

$$The\ vertical\ force\ passes\ through\ O,\ so\ its\ torque\ is\ 0.$$

$$\sum \tau_O = (-10 \cdot L) + (10 \cdot L) + (-10 \cdot L) + (F \cdot L) = 0$$

$$-10L + FL = 0$$

$$F = 10 \text{ N}$$

$$For\ an\ object\ rolling\ without\ slipping\ down\ an\ inclined\ plane\ with\ inclination\ \theta,\ $$

$$the\ linear\ acceleration\ (a)\ of\ the\ center\ of\ mass\ is:\ a=\frac{g\sin\theta}{1+\frac{I}{mr^2}}$$

$$For\ a\ uniform\ solid\ cylinder,\ the\ moment\ of\ inertia\ is\ I=\frac{1}{2}mr^2$$

$$Substituting\ this\ and\ \theta=45^{\circ}\ into\ the\ formula:$$

$$a = \frac{g \sin 45^\circ}{1 + \frac{1}{2}mr^2 / mr^2}$$

$$a = \frac{g \cdot \frac{1}{\sqrt{2}}}{1 + \frac{1}{2}}$$

$$a = \frac{2g}{3\sqrt{2}}$$

$$a = \frac{\sqrt{2}}{3} g$$

The given rod has mass $$M$$ and length $$L$$. For a slender rod, the moment of inertia about an axis passing through its centre and perpendicular to its length is

$$I = \frac{1}{12}\,M\,L^{2}$$

The question denotes this value by $$\alpha$$, so write 

$$\alpha = \frac{1}{12}\,M\,L^{2} \; \; -(1)$$

Next, cut the rod into two equal parts. Each part therefore has

length $$=\frac{L}{2}$$
mass $$=\frac{M}{2}$$

These two half-rods are joined at right angles through their centres to form a symmetric cross (a ‘+’ shape). The required axis passes through the common centre and is perpendicular to the plane of the cross.

For one half-rod of length $$\tfrac{L}{2}$$, the moment of inertia about this axis is again given by the same formula $$I=\tfrac{1}{12}ML^{2}$$ with $$M \rightarrow \frac{M}{2}$$ and $$L \rightarrow \frac{L}{2}$$:

$$I_{\text{one half}} = \frac{1}{12}\left(\frac{M}{2}\right)\left(\frac{L}{2}\right)^{2} = \frac{1}{12}\,\frac{M}{2}\,\frac{L^{2}}{4} = \frac{M\,L^{2}}{96}$$

The cross consists of two such identical half-rods, so total moment of inertia is

$$I_{\text{cross}} = 2 \times \frac{M\,L^{2}}{96} = \frac{M\,L^{2}}{48}$$

From $$(1)$$ we have $$M\,L^{2} = 12\,\alpha$$. Substituting this into the above result:

$$I_{\text{cross}} = \frac{12\,\alpha}{48} = \frac{\alpha}{4}$$

Hence, the moment of inertia of the cross about the stated axis is $$\alpha/4$$.

Answer: Option B $$(\alpha/4)$$

An object of mass $$m$$ is projected from the origin in the vertical xy-plane at an angle of $$45°$$ with initial speed $$v_0$$. To find its angular momentum about the origin at its maximum height, we first determine the time and position at that instant.

The time to reach the maximum height occurs when the vertical component of velocity becomes zero, namely at $$t = \frac{v_0 \sin 45°}{g} = \frac{v_0}{\sqrt{2}g}\,. $$ During this interval the horizontal component of motion remains uniform, so the x-coordinate is $$x = v_0 \cos 45° \times t = \frac{v_0}{\sqrt{2}} \times \frac{v_0}{\sqrt{2}g} = \frac{v_0^2}{2g}\,. $$ The maximum height itself is given by $$y = H = \frac{v_0^2 \sin^2 45°}{2g} = \frac{v_0^2}{4g}\,. $$ Hence the position vector at maximum height is $$\vec{r} = \frac{v_0^2}{2g}\,\hat{i} + \frac{v_0^2}{4g}\,\hat{j}\,. $$

At this instant the vertical velocity vanishes and only the horizontal component remains, so $$\vec{v} = v_0 \cos 45°\,\hat{i} = \frac{v_0}{\sqrt{2}}\,\hat{i}\,. $$

The angular momentum about the origin is given by $$\vec{L} = m\,\bigl(\vec{r}\times\vec{v}\bigr)\,. $$ Evaluating the cross product, $$\vec{r}\times\vec{v} = \Bigl(\frac{v_0^2}{2g}\hat{i} + \frac{v_0^2}{4g}\hat{j}\Bigr) \times \Bigl(\frac{v_0}{\sqrt{2}}\hat{i}\Bigr)\,. $$ Since $$\hat{i}\times\hat{i}=0$$ and $$\hat{j}\times\hat{i}=-\hat{k}$$, it follows that $$\vec{r}\times\vec{v} = \frac{v_0^2}{2g}\frac{v_0}{\sqrt{2}}\bigl(\hat{i}\times\hat{i}\bigr) + \frac{v_0^2}{4g}\frac{v_0}{\sqrt{2}}\bigl(\hat{j}\times\hat{i}\bigr) = -\frac{v_0^3}{4\sqrt{2}g}\,\hat{k}\,. $$ Therefore $$\vec{L} = m\bigl(\vec{r}\times\vec{v}\bigr) = -\frac{m\,v_0^3}{4\sqrt{2}g}\,\hat{k}\,. $$

The magnitude of the angular momentum is $$\frac{m\,v_0^3}{4\sqrt{2}g}$$ directed along the negative z-axis. The correct answer is Option 3: $$\frac{m\,v_0^3}{4\sqrt{2}g}$$ along the negative z-axis.

The moment of inertia of the disc about its diameter:

$$I = \frac{1}{4}MR^2 \implies MR^2 = 4I$$

Using the Parallel Axis Theorem ($$I = I_{cm} + Md^2$$), where $$d$$ is the perpendicular distance from the axis PQ to the center of each object:

Disc A: The axis passes through its center as a diameter ($$d = 0$$). $$I_A = \frac{1}{4}MR^2 = I$$

Solid Sphere B: The axis is at a distance $$d = R$$ from its center. $$I_B = \frac{2}{5}MR^2 + M(R)^2 = \frac{7}{5}MR^2$$

Spherical Shell C: The axis is at a distance $$d = R$$ from its center. $$I_C = \frac{2}{3}MR^2 + M(R)^2 = \frac{5}{3}MR^2$$

$$I_{sys} = I_A + I_B + I_C$$

$$I_{sys} = \frac{1}{4}MR^2 + \frac{7}{5}MR^2 + \frac{5}{3}MR^2$$

$$I_{sys} = \left( \frac{15 + 84 + 100}{60} \right) MR^2 = \frac{199}{60} MR^2$$

$$I_{sys} = \frac{199}{60} (4I)$$

$$I_{sys} = \frac{199}{15} I$$

$$x = 199$$

For any rigid body rolling without slipping down an inclined plane, conservation of mechanical energy gives

$$mgh \;=\;\frac12\,m v^{2}\;+\;\frac12\,I\omega^{2}$$
Since the body rolls without slipping, $$\omega=\dfrac{v}{R}$$. Thus

$$mgh \;=\;\frac12\,m v^{2}\;+\;\frac12\,I\dfrac{v^{2}}{R^{2}} =\frac12\,m v^{2}\Bigl(1+\frac{I}{mR^{2}}\Bigr)$$

Therefore the speed attained at the bottom is

$$v=\sqrt{\frac{2gh}{\,1+\dfrac{I}{mR^{2}}\,}}$$

Case 1 : Circular ring

For a thin ring about its symmetry axis, $$I_{ring}=mR^{2}$$, so $$\dfrac{I}{mR^{2}}=1$$.

$$v_{ring}=\sqrt{\frac{2gh}{1+1}} =\sqrt{\frac{2gh}{2}} =\sqrt{gh}$$

Case 2 : Solid sphere

For a solid sphere about a diameter, $$I_{sphere}=\tfrac{2}{5}mR^{2}$$, hence $$\dfrac{I}{mR^{2}}=\tfrac{2}{5}$$.

$$v_{sphere}=\sqrt{\frac{2gh}{1+\tfrac{2}{5}}} =\sqrt{\frac{2gh}{\tfrac{7}{5}}} =\sqrt{\frac{10gh}{7}}$$

Required ratio:

$$\frac{v_{ring}}{v_{sphere}} =\sqrt{\frac{gh}{\tfrac{10gh}{7}}} =\sqrt{\frac{7}{10}} =\sqrt{\frac{3.5}{5}}$$

Hence $$x=3.5$$.

Thus, the ratio of their velocities at the bottom of the plane is $$\sqrt{\dfrac{3.5}{5}}$$, giving $$x = 3.5$$.

The torque produced by the force acting tangentially at the rim is:

$$\tau = F \cdot R$$

$$\tau = 10 \text{ N} \times 0.2 \text{ m} = 2 \text{ N m}$$

$$I = \frac{\tau}{\alpha}$$

$$I = \frac{2}{2}$$

$$I = 1 \text{ kg m}^2$$

Let the initial mass of the solid sphere be $$M$$ and its initial radius be $$R$$. Moment of inertia of a solid sphere about any diameter is

$$I=\frac{2}{5}MR^{2}$$ $$-(1)$$

When the sphere loses mass it keeps its shape, so it remains a uniform solid sphere of a smaller radius. If the radius becomes $$\frac{R}{2}$$, the new volume is

$$V'=\frac{4}{3}\pi\left(\frac{R}{2}\right)^{3}=\frac{1}{8}\left(\frac{4}{3}\pi R^{3}\right)$$

With uniform density, mass is proportional to volume, hence

$$M'=\frac{1}{8}M$$ $$-(2)$$

The new moment of inertia about the same diameter is

$$I'=\frac{2}{5}M'\left(\frac{R}{2}\right)^{2} =\frac{2}{5}\left(\frac{M}{8}\right)\left(\frac{R^{2}}{4}\right) =\frac{2}{5}MR^{2}\left(\frac{1}{32}\right) =\frac{I}{32}$$ $$-(3)$$

The mass is lost symmetrically, so no external torque acts on the sphere. Therefore the angular momentum about the diameter remains constant:

$$I\omega_{1}=I'\omega_{2}$$ $$-(4)$$

Substituting $$I'=\frac{I}{32}$$ from $$(3)$$ into $$(4)$$, we get

$$I\omega_{1}=\frac{I}{32}\,\omega_{2} \;\;\Longrightarrow\;\; \omega_{2}=32\,\omega_{1}$$

Comparing with $$\omega_{2}=x\omega_{1}$$, we find

$$x=32$$

Hence, the angular velocity of the sphere when its radius becomes $$R/2$$ is $$32\omega_{1}$$ and the required value of $$x$$ is 32.

The angular velocity of a rotating body is usually expressed in $$\text{rad s}^{-1}$$.
Given speeds in revolutions per minute (rpm) must first be converted.

Initial speed: $$1800\;\text{rpm} = 1800 \times \frac{2\pi\;\text{rad}}{60\;\text{s}} = 60\pi\;\text{rad s}^{-1}$$.

Final speed: $$2100\;\text{rpm} = 2100 \times \frac{2\pi\;\text{rad}}{60\;\text{s}} = 70\pi\;\text{rad s}^{-1}$$.

The change in angular velocity is therefore
$$\Delta\omega = \omega_f - \omega_i = 70\pi - 60\pi = 10\pi\;\text{rad s}^{-1}$$.

Torque and angular momentum are related through the angular impulse equation:
$$\tau\,t = \Delta L = I\,\Delta\omega$$ $$-(1)$$,
where $$I$$ is the moment of inertia about the rotation axis.

Given values: $$\tau = 25\pi\;\text{N m}$$ and $$t = 40\;\text{s}$$.
Hence the angular impulse is
$$\tau t = 25\pi \times 40 = 1000\pi\;\text{kg m}^2\text{s}^{-1}$$.

Substituting into $$(1)$$ gives
$$I \times 10\pi = 1000\pi$$.

Cancel $$\pi$$ and solve for $$I$$:
$$I = \frac{1000}{10} = 100\;\text{kg m}^2$$.

The axis of rotation is a diameter lying in the plane of the thin solid disk.
For a uniform disk about a diameter, the moment of inertia is
$$I = \frac{1}{4} M R^2$$ $$-(2)$$,
where $$M$$ is the mass and $$R$$ the radius.

With $$M = 1\;\text{kg}$$, substitute $$I = 100\;\text{kg m}^2$$ into $$(2)$$:
$$100 = \frac{1}{4} \times 1 \times R^2 \quad\Longrightarrow\quad R^2 = 400$$.

Thus $$R = 20\;\text{m}$$.
The diameter is twice the radius:

Diameter $$= 2R = 2 \times 20 = 40\;\text{m}$$.

Final Answer: 40 m

$$I_{\text{remaining}} = I_{\text{total}} - I_{\text{removed}}$$

$$I = I_{\text{cm}} + md^2$$ (Parallel Axis Theorem)

$$\text{Mass of small disc } (m) = M \times \frac{\pi \left(\frac{R}{3}\right)^2}{\pi R^2} = \frac{M}{9}$$

$$\text{Distance between } O \text{ and } O' \ (d) = R - \frac{R}{3} = \frac{2R}{3}$$

$$I_{\text{total}} = \frac{1}{2}MR^2$$

$$I_{\text{removed}} = \frac{1}{2}m\left(\frac{R}{3}\right)^2 + md^2$$

$$I_{\text{removed}} = \frac{1}{2}\left(\frac{M}{9}\right)\left(\frac{R^2}{9}\right) + \left(\frac{M}{9}\right)\left(\frac{2R}{3}\right)^2$$

$$I_{\text{removed}} = \frac{MR^2 + 8MR^2}{162} = \frac{9MR^2}{162} = \frac{1}{18}MR^2$$

$$I_{\text{remaining}} = \frac{1}{2}MR^2 - \frac{1}{18}MR^2 = \left(\frac{9 - 1}{18}\right)MR^2 = \frac{8}{18}MR^2 = \frac{4}{9}MR^2$$

$$x = 9$$

A particle of mass $$m$$ is projected with velocity $$u$$ at angle $$30°$$ with the horizontal. We need the angular momentum about the point of projection at maximum height.

Find velocity and position at maximum height: at maximum height, only the horizontal component of velocity remains:

$$v_x = u\cos 30° = \frac{u\sqrt{3}}{2}$$

The maximum height is:

$$h = \frac{u^2 \sin^2 30°}{2g} = \frac{u^2 \cdot \frac{1}{4}}{2g} = \frac{u^2}{8g}$$

Compute angular momentum: the angular momentum formula about the point of projection is $$L = m \cdot v_x \cdot h$$ (since the velocity is purely horizontal and the perpendicular distance from the projection point to the velocity line is $$h$$).

$$ L = m \cdot \frac{u\sqrt{3}}{2} \cdot \frac{u^2}{8g} = \frac{\sqrt{3}}{16} \cdot \frac{mu^3}{g} $$

The correct answer is Option (1): $$\frac{\sqrt{3}}{16} \frac{mu^3}{g}$$.

Two spheres: each mass 2kg, radius 0.5m, separation 1.5m (center to center), so each is 0.75m from middle.

$$I=2\left[\frac{2}{5}mr^2+md^2\right]=2\left[\frac{2}{5}(2)(0.25)+(2)(0.5625)\right]=2[0.2+1.125]=2(1.325)=2.65=\frac{53}{20}$$ kg m².

So $$x=53$$.

The answer is $$\boxed{53}$$.

given
mass = m
projection speed = u
angle = 45°

we need angular momentum about point of projection at highest point

formula:

L = m | r × v |

for 2D motion:

$$L=m(xv_y−yv_x)$$

step 1: velocity at highest point

at highest point
vertical velocity = 0
horizontal velocity =

$$u\cos45°=u/\sqrt{2}$$

so:

v_x = u/√2 , v_y = 0

step 2: time to reach highest point

$$t=(u\sin45°)/g$$

= (u/√2)/g

= u/(√2 g)

step 3: coordinates of highest point

horizontal distance:

$$x=(u\cos45°)\times t$$

= (u/√2) × (u/(√2 g))

= u²/(2g)

vertical height:

$$y=(u\sin45°)^2/(2g)$$

= (u²/2)/(2g)

= u²/(4g)

step 4: angular momentum

$$L=m(xv_y−yv_x)$$

$$\sin ce\ v_y=0:$$

$$L=−myv_x$$

magnitude:

$$L=myv_x$$

$$=m\times(u^2/(4g))\times(u/\sqrt{2})$$

$$=mu^3/(4\sqrt{2}g)$$

Now comparing with the given expression :-

$$\frac{\sqrt{2}mu^3}{Xg}$$

we get it by multiplying and dividing our answer by $$\sqrt{\ 2}$$

so we get $$=\sqrt{\ 2}mu^3/8g$$
so Value of X is 8

The outward centrifugal force on the ball is $$F = m\omega^2r$$

$$a_r = \omega^2 r$$

$$a = v \frac{dv}{dr}$$

$$v \frac{dv}{dr} = \omega^2 r$$

Integrate both sides from the point where the ball is placed (r = 1 m) to the edge of the table (r = 3 m):

$$\int_{0}^{v} v \, dv = \omega^2 \int_{1}^{3} r \, dr$$

$$\frac{v^2}{2} = \omega^2 \left[ \frac{r^2}{2} \right]_1^3$$

$$v^2 = \omega^2 (3^2 - 1^2)$$

$$v^2 = 8\omega^2$$

$$v = \sqrt{8}\omega = 2\sqrt{2}\omega$$
$$x = 2$$

We need to find the ratio of rotational kinetic energy to total kinetic energy for a hollow sphere rolling on a plane surface about its axis of symmetry.

Since the sphere is thin-walled, its moment of inertia about an axis through its centre is $$I = \frac{2}{3}mr^2$$, where $$m$$ is the mass and $$r$$ is the radius.

For rolling without slipping with velocity $$v$$ and angular velocity $$\omega = v/r$$, the translational kinetic energy is $$K_{trans} = \frac{1}{2}mv^2$$ and the rotational kinetic energy is $$K_{rot} = \frac{1}{2}I\omega^2 = \frac{1}{2} \cdot \frac{2}{3}mr^2 \cdot \frac{v^2}{r^2} = \frac{1}{3}mv^2$$.

Next, the total kinetic energy is $$K_{total} = K_{trans} + K_{rot} = \frac{1}{2}mv^2 + \frac{1}{3}mv^2 = mv^2\bigl(\tfrac{1}{2} + \tfrac{1}{3}\bigr) = mv^2 \cdot \frac{5}{6} = \frac{5}{6}mv^2$$.

From this, the ratio of rotational to total kinetic energy is $$\frac{K_{rot}}{K_{total}} = \frac{\frac{1}{3}mv^2}{\frac{5}{6}mv^2} = \frac{\frac{1}{3}}{\frac{5}{6}} = \frac{1}{3} \times \frac{6}{5} = \frac{2}{5}$$. Since this ratio equals $$\frac{x}{5}$$, we obtain $$x = 2$$.

The answer is 2.

A solid circular disc of mass $$m = 50$$ kg rolls along a horizontal floor with center-of-mass speed $$v = 0.4$$ m/s. We need to find the work done to stop it.

We start by recalling that for a body rolling without slipping, the total kinetic energy is the sum of translational and rotational kinetic energies:

$$ KE_{total} = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2 $$

Here, $$I$$ is the moment of inertia about the center of mass and $$\omega$$ is the angular velocity.

Next, the rolling condition gives $$v = R\omega$$, so $$\omega = \frac{v}{R}$$, and for a solid disc, $$I = \frac{1}{2}mR^2$$.

Substituting into the rotational kinetic energy term yields:

$$ \frac{1}{2}I\omega^2 = \frac{1}{2} \cdot \frac{1}{2}mR^2 \cdot \frac{v^2}{R^2} = \frac{1}{4}mv^2 $$

Therefore, the total kinetic energy becomes:

$$ KE_{total} = \frac{1}{2}mv^2 + \frac{1}{4}mv^2 = \frac{3}{4}mv^2 $$

Substituting the numerical values gives:

$$ KE_{total} = \frac{3}{4} \times 50 \times (0.4)^2 = \frac{3}{4} \times 50 \times 0.16 = \frac{3}{4} \times 8 = 6 \text{ J} $$

By the work-energy theorem, the work done to stop the disc equals the total kinetic energy (in magnitude).

The answer is $$\boxed{6}$$ J.

Moment of inertia $$I = 0.40$$ kg m$$^2$$, radius $$r = 10$$ cm = 0.1 m, force $$F = 40$$ N, time $$t = 10$$ s, initial angular velocity = 0.

The torque is calculated as $$\tau = Fr = 40 \times 0.1 = 4$$ N·m.

The angular acceleration is $$\alpha = \frac{\tau}{I} = \frac{4}{0.40} = 10$$ rad/s$$^2$$.

After time $$t$$, the angular velocity is given by:

$$\omega = \omega_0 + \alpha t = 0 + 10 \times 10 = 100 \text{ rad/s}$$

The answer is $$x = \boxed{100}$$.

When the second disc is placed on the rotating one, the internal friction between their surfaces brings them to a common angular velocity. Since no external torque acts on the system about the axis of rotation, the total angular momentum is conserved.

The moment of inertia for a solid disc about its central axis is:

$$I = \frac{1}{2}MR^2 = \frac{1}{2} \times 5 \times (2)^2 = 10 \text{ kg m}^2$$

Initial angular momentum of the single disc equals the final angular momentum of the two discs rotating together:

$$I\omega_1 = (I + I)\omega_f$$

$$10 \times 10 = 20 \times \omega_f \implies \omega_f = 5 \text{ rad s}^{-1}$$

Before the second disc is added, the energy in the system is:

$$E_i = \frac{1}{2}I\omega_1^2 = \frac{1}{2} \times 10 \times (10)^2 = 500 \text{ J}$$

After both discs begin rotating together at the common velocity $$\omega_f$$:

$$E_f = \frac{1}{2}(2I)\omega_f^2 = \frac{1}{2} \times 20 \times (5)^2 = 250 \text{ J}$$

The energy lost due to friction as the discs slip against each other before reaching a common speed is the difference between the initial and final kinetic energies:

$$\Delta E = E_i - E_f = 500 - 250 = 250 \text{ J}$$

Earth's angular momentum is conserved: $$I\omega = const$$. $$I = \frac{2}{5}MR^2$$.

$$I_1\omega_1 = I_2\omega_2 \Rightarrow R^2 \omega_1 = (3R/4)^2 \omega_2 \Rightarrow \omega_2 = \frac{16}{9}\omega_1$$.

$$T_2 = \frac{2\pi}{\omega_2} = \frac{9}{16} T_1 = \frac{9}{16} \times 24 = 13.5$$ hours = 13 hours 30 minutes.

The answer is 13.

We need to find the moment of inertia of three balls placed at the midpoints of the edges of an equilateral triangle, about an axis through the centroid and perpendicular to the plane.

First, recall that the moment of inertia of a system of point masses about an axis is given by $$I = \sum m_i r_i^2$$, where $$r_i$$ is the perpendicular distance of each mass from the axis of rotation.

To determine that distance, note that in an equilateral triangle of side $$a$$, the centroid divides each median in a 2:1 ratio measured from the vertex. Since the median has length $$\frac{\sqrt{3}}{2}a$$, the distance from the centroid to a vertex is $$\frac{2}{3}\times\frac{\sqrt{3}}{2}a = \frac{a}{\sqrt{3}}$$, while the distance from the centroid to the midpoint of a side is $$\frac{1}{3}\times\frac{\sqrt{3}}{2}a = \frac{a}{2\sqrt{3}} = \frac{a\sqrt{3}}{6}$$.

Substituting $$a = 2\text{ m}$$ into the latter expression gives $$r = \frac{2}{2\sqrt{3}} = \frac{1}{\sqrt{3}}\text{ m}$$.

Because the centroid is equidistant from all three edges, each mass lies at the same distance $$r = \frac{1}{\sqrt{3}}$$ from the axis. Squaring this distance yields $$r^2 = \left(\frac{1}{\sqrt{3}}\right)^2 = \frac{1}{3}\,\text{m}^2.$$

Therefore, the total moment of inertia can be written as $$I = m_1r^2 + m_2r^2 + m_3r^2 = (m_1 + m_2 + m_3)\,r^2$$. Substituting the given masses and $$r^2$$ results in $$I = (2 + 4 + 6)\times\frac{1}{3} = 12\times\frac{1}{3} = 4\,\text{kg m}^2.$$

The correct answer is 4 kg m$$^2$$.

Using conservation of angular momentum:

$$I_1\omega_1 + I_2\omega_2 = (I_1 + I_2)\omega$$

$$4(10) + 2(4) = 6\omega \Rightarrow \omega = \frac{48}{6} = 8$$ rad/s.

Loss in KE = Initial KE - Final KE:

$$= \frac{1}{2}I_1\omega_1^2 + \frac{1}{2}I_2\omega_2^2 - \frac{1}{2}(I_1+I_2)\omega^2$$

$$= \frac{1}{2}(4)(100) + \frac{1}{2}(2)(16) - \frac{1}{2}(6)(64)$$

$$= 200 + 16 - 192 = 24$$ J.

Therefore, the answer is $$\boxed{24}$$.

A circular disc rolls and slides down an inclined plane of length $$l$$. When it slides (no rolling), it takes $$t$$ seconds. When it rolls without slipping, it takes $$\left(\frac{\alpha}{2}\right)^{1/2} t$$ seconds. Find $$\alpha$$.

When the disc slides without friction, the acceleration along the incline is:

$$a_{\text{slide}} = g\sin\theta$$

Using $$l = \frac{1}{2}a_{\text{slide}} t^2$$:

$$t^2 = \frac{2l}{g\sin\theta} \quad \cdots (i)$$

When a disc rolls without slipping, the moment of inertia about the centre is $$I = \frac{1}{2}mR^2$$, so $$\frac{I}{mR^2} = \frac{1}{2}$$.

The translational acceleration along the incline is:

$$a_{\text{roll}} = \frac{g\sin\theta}{1 + I/(mR^2)} = \frac{g\sin\theta}{1 + 1/2} = \frac{2g\sin\theta}{3}$$

Using $$l = \frac{1}{2}a_{\text{roll}} t_{\text{roll}}^2$$:

$$t_{\text{roll}}^2 = \frac{2l}{a_{\text{roll}}} = \frac{2l \cdot 3}{2g\sin\theta} = \frac{3l}{g\sin\theta} \quad \cdots (ii)$$

$$\frac{t_{\text{roll}}^2}{t^2} = \frac{3l/(g\sin\theta)}{2l/(g\sin\theta)} = \frac{3}{2}$$

$$t_{\text{roll}} = t\sqrt{\frac{3}{2}} = \left(\frac{3}{2}\right)^{1/2} t$$

We are given $$t_{\text{roll}} = \left(\frac{\alpha}{2}\right)^{1/2} t$$.

Comparing: $$\frac{\alpha}{2} = \frac{3}{2} \implies \alpha = 3$$.

The answer is 3.

Take the vertical axis through the centre of the larger disc as the reference (positive sense anticlockwise).
Initially both discs are at rest ⇒ total angular momentum about this axis is zero and, because no external torque acts, it must remain zero.

Moments of inertia about the vertical axis
Large disc (mass $$M$$, radius $$R$$): $$I_L = \frac{1}{2}MR^{2}$$
Small disc (mass $$M$$, radius $$\dfrac{R}{2}$$) about its own centre: $$I_{s,\;{\rm cm}} = \frac{1}{2}M\!\left(\frac{R}{2}\right)^{2}= \frac{1}{8}MR^{2}$$

The small disc is fixed to the circumference of the big disc, so its centre is at a distance $$R$$ from the main axis. Hence, using the parallel-axis theorem, the small disc’s moment of inertia about the main axis is

$$I_s = I_{s,\;{\rm cm}} + MR^{2}= \frac{1}{8}MR^{2}+MR^{2}= \frac{9}{8}MR^{2}$$

Angular velocities
Motor keeps the small disc spinning at a constant angular speed $$\omega$$ (anticlockwise) about its own axis. Reaction torque makes the large disc rotate clockwise with angular speed $$\dfrac{\omega}{n}$$ (-ve sign). Because the small disc is rigidly fixed to the large disc, its centre also moves in a circle of radius $$R$$ with the same angular speed as the large disc.

Angular momenta about the main axis

1. Large disc: $$L_L = I_L\!\left(-\frac{\omega}{n}\right)= -\frac{1}{2}MR^{2}\frac{\omega}{n}$$
2. Small disc (two parts)    a) Spin about its own axis: $$L_{s,\;{\rm spin}} = I_{s,\;{\rm cm}}\;\omega = \frac{1}{8}MR^{2}\,\omega$$ (anticlockwise)
   b) Orbital motion of its centre: $$L_{s,\;{\rm orb}} = MR^{2}\!\left(-\frac{\omega}{n}\right)= -MR^{2}\frac{\omega}{n}$$

Total angular momentum must stay zero:

$$L_L + L_{s,\;{\rm spin}} + L_{s,\;{\rm orb}} = 0$$ $$-\frac{1}{2}MR^{2}\frac{\omega}{n} + \frac{1}{8}MR^{2}\,\omega - MR^{2}\frac{\omega}{n}=0$$

Divide by $$MR^{2}\omega$$:

$$-\frac{1}{2n} + \frac{1}{8} - \frac{1}{n}=0$$ $$-\frac{3}{2n} + \frac{1}{8}=0$$

$$\frac{1}{n}= \frac{1}{8}\times\frac{2}{3}= \frac{1}{12} \;\Longrightarrow\; n = 12$$

Hence, the large disc rotates with angular speed $$\dfrac{\omega}{12}$$ opposite to that of the smaller disc.

Final answer: 12

Let the mass of the uniform rod be $$m$$ and let its mid-point be $$C$$. The string of length $$L$$ is attached to the rod at point $$A$$ (one end of the rod) and the other end of the string is fixed to the pivot $$O$$. Hence the points are collinear in the order $$O \;-\; A \;-\; C$$, with $$OA = L$$ and $$AC = L/2$$, so

$$OC = OA + AC = L + \frac{L}{2} = \frac{3L}{2}$$

An impulsive force delivers a (linear) impulse $$P$$ perpendicular to the rod at a point that is a distance $$x = \frac{L}{n}$$ away from the mid-point $$C$$ along the length of the rod.

Linear motion of the centre of mass
The entire impulse $$P$$ is received by the rod, so the centre of mass acquires a linear momentum $$P$$ and hence a speed

$$v_C = \frac{P}{m}$$

Angular motion about the centre of mass
The same impulse produces an angular impulse (about the centre) equal to $$P x$$. For a thin uniform rod, the moment of inertia about its mid-point is

$$I_C = \frac{1}{12}\,mL^{2}$$

Therefore the angular velocity of the rod about its own centre is

$$\omega = \frac{P x}{I_C} = \frac{P x}{\tfrac{1}{12}mL^{2}} = \frac{12\,P\,x}{mL^{2}}$$

Condition for the rod to remain aligned with the string
For the rod and string to revolve rigidly about the fixed point $$O$$, the motion after the impulse must be a pure rotation about $$O$$. In pure rotation, the linear speed of the centre of mass must satisfy

$$v_C = \omega \, OC$$

Substituting $$v_C = \dfrac{P}{m}$$, $$\omega = \dfrac{12Px}{mL^{2}}$$ and $$OC = \dfrac{3L}{2}$$ gives

$$\frac{P}{m} = \frac{12Px}{mL^{2}} \times \frac{3L}{2} = \frac{18P x}{mL}$$

Cancel $$P/m$$ from both sides:

$$1 = \frac{18x}{L} \quad\Longrightarrow\quad x = \frac{L}{18}$$

Since the point of application was specified as $$x = \dfrac{L}{n}$$ from the mid-point, we equate

$$\frac{L}{n} = \frac{L}{18} \quad\Longrightarrow\quad n = 18$$

Hence the required value is

18

A disc of mass $$M$$ and radius $$R$$ rotates with angular velocity $$\omega$$. When another disc of mass $$M/2$$ and the same radius is placed gently on it coaxially, no external torque acts on the system and therefore angular momentum is conserved: $$L_{\text{before}} = L_{\text{after}}$$.

The moment of inertia of a disc about an axis through its centre perpendicular to its plane is given by $$I = \frac{1}{2}MR^2$$. Hence the initial moment of inertia is $$I_1 = \frac{1}{2}MR^2$$ while the moment of inertia of the second disc is $$I_2 = \frac{1}{2} \cdot \frac{M}{2} \cdot R^2 = \frac{MR^2}{4}$$. The combined moment of inertia therefore becomes $$I_1 + I_2 = \frac{MR^2}{2} + \frac{MR^2}{4} = \frac{3MR^2}{4}$$.

Since angular momentum is conserved, we have $$I_1 \omega = (I_1 + I_2)\omega'$$. Substituting the expressions for the moments of inertia gives $$\frac{MR^2}{2} \cdot \omega = \frac{3MR^2}{4} \cdot \omega'$$, which simplifies to $$\omega' = \frac{MR^2/2}{3MR^2/4} \cdot \omega = \frac{2}{3}\omega$$.

Therefore, the new angular velocity is $$\frac{2}{3}\omega$$.

A heavy iron bar (12 kg) has one end on the ground and the other on a man’s shoulder, making 60° with the horizontal. We wish to find the normal force applied by the man.

Let the bar have length $$L$$. The weight $$W = 12$$ kg-wt acts at the center of the bar, which is a distance $$L/2$$ from the ground end.

The component of the weight perpendicular to the bar is $$W\cos 60° = 12 \times \tfrac{1}{2} = 6$$ kg-wt, acting at distance $$L/2$$ from the pivot at the ground end. The normal force $$N$$ applied by the man is also perpendicular to the bar and acts at distance $$L$$ from the pivot.

Balancing torques about the ground end gives $$N \times L = W\cos 60° \times \frac{L}{2}$$ and therefore $$N = \frac{6}{2} = 3$$ kg-wt.

The correct answer is Option C: 3 kg-wt.

For a hollow sphere of mass $$M$$ and radius $$R$$, the moment of inertia about its diameter is:

$$I_1 = \frac{2}{3}MR^2$$

The radius of gyration is $$k_1 = \sqrt{\frac{I_1}{M}} = \sqrt{\frac{2}{3}R^2}$$

For the solid cylinder of mass $$M$$, radius $$R$$, and length $$L = 4R$$, the axis AB passes through one end. Using the Parallel Axis Theorem:

$$I_2 = I_{cm} + M\left(\frac{L}{2}\right)^2$$

$$I_2 = M\left(\frac{R^2}{4} + \frac{L^2}{12}\right) + M\frac{L^2}{4} = M\left(\frac{R^2}{4} + \frac{L^2}{3}\right)$$

$$I_2 = M\left(\frac{R^2}{4} + \frac{(4R)^2}{3}\right) = M\left(\frac{R^2}{4} + \frac{16R^2}{3}\right)$$

$$I_2 = MR^2 \left(\frac{3 + 64}{12}\right) = \frac{67}{12}MR^2$$

The radius of gyration is $$k_2 = \sqrt{\frac{I_2}{M}} = \sqrt{\frac{67}{12}R^2}$$

$$\text{Ratio} = \frac{k_1}{k_2} = \frac{\sqrt{\frac{2}{3}R^2}}{\sqrt{\frac{67}{12}R^2}}$$ = $$\sqrt{\frac{8}{67}}$$

$$x\ =\ 67$$

When a disc of radius $$R$$ rolls without slipping, the topmost point $$A$$ traces a cycloid path.

After the disc completes half a rotation, the center of the disc moves a distance equal to half the circumference:

$$d_{center} = \pi R$$

Point $$A$$ starts at the top. After half a rotation, point $$A$$ reaches the bottom of the disc.

The horizontal displacement of point $$A$$: The center moves $$\pi R$$ horizontally, and relative to the center, point $$A$$ moves from top (0, +R) to bottom (0, -R). So the horizontal displacement of $$A$$ equals the horizontal displacement of the center = $$\pi R$$.

The vertical displacement of point $$A$$: Point $$A$$ was at the top (height $$2R$$ from ground), and after half rotation it is at the bottom (height $$0$$ from ground). So vertical displacement = $$2R$$ downward.

The net displacement of point $$A$$:

$$d = \sqrt{(\pi R)^2 + (2R)^2}$$

$$d = R\sqrt{\pi^2 + 4}$$

The displacement of point $$A$$ from its initial position is $$\mathbf{R\sqrt{\pi^2 + 4}}$$.

The rod is hinged at one end, therefore the pivot supplies no external impulse about itself during the very short collision. Hence both angular momentum and kinetic energy of the two-body system are conserved.

Data: $$M = 1.00\;\text{kg},\; L = 0.20\;\text{m},\; m = 0.10\;\text{kg},\; u = 5.00\;\text{m s}^{-1},\; r = \frac{L}{2}=0.10\;\text{m}$$

Moment of inertia of a uniform rod about one end $$I = \frac{1}{3}ML^{2} = \frac{1}{3}\times 1.00\times (0.20)^{2} = 0.01333\;\text{kg m}^{2}$$ $$-(1)$$

Conservation of angular momentum about the pivot:
Initial angular momentum $$L_i = m u r$$ (mass moves perpendicular to the rod).
After collision the mass rebounds, so its angular momentum is $$-m v r$$ and the rod carries $$I\omega$$. $$m u r = I\omega - m v r$$ $$-(2)$$

Conservation of kinetic energy (elastic impact):
$$\frac{1}{2}m u^{2} = \frac{1}{2}I\omega^{2} + \frac{1}{2}m v^{2}$$ Multiplying by 2, $$m u^{2} = I\omega^{2} + m v^{2}$$ $$-(3)$$

Substituting known numbers in $$-(2)$$:
$$0.10 \times 5.00 \times 0.10 = 0.01333\,\omega - 0.10\,v \times 0.10$$
$$0.05 = 0.01333\,\omega - 0.01\,v$$
$$\omega = 75\,(0.05 + 0.01\,v) = 3.75 + 0.75\,v$$ $$-(4)$$

Put $$-(4)$$ in the energy equation $$-(3)$$:
$$0.10\,(5.00)^{2} = 0.01333\,(3.75 + 0.75\,v)^{2} + 0.10\,v^{2}$$
$$2.50 = \frac{1}{75}(3.75 + 0.75\,v)^{2} + 0.10\,v^{2}$$

Multiplying by 75 to clear the denominator:
$$187.5 = (3.75 + 0.75\,v)^{2} + 7.5\,v^{2}$$

Expanding and simplifying gives the quadratic
$$8.0625\,v^{2} + 5.625\,v - 173.4375 = 0$$

Discriminant $$D = 5.625^{2} + 4\times 8.0625 \times 173.4375 = 75^{2}$$ $$\sqrt{D} = 75$$

Therefore $$v = \frac{-5.625 + 75}{2\times 8.0625} = 4.30\;\text{m s}^{-1}$$ (negative root would give a negative speed and is rejected).

From $$-(4)$$, $$\omega = 3.75 + 0.75 \times 4.30 = 6.98\;\text{rad s}^{-1}$$

Thus the bar acquires an angular speed of $$6.98\;\text{rad s}^{-1}$$ and the small mass rebounds with $$4.30\;\text{m s}^{-1}$$, matching

Option A which is: $$\omega = 6.98\;\text{rad s}^{-1}$$ and $$v = 4.30\;\text{m s}^{-1}$$

A solid sphere rolls without slipping on a horizontal plane. The ratio of angular momentum about its axis of rotation to total energy is $$\pi : 22$$.

Angular momentum about the axis of rotation.

For a solid sphere: $$I = \frac{2}{5}mR^2$$

$$L = I\omega = \frac{2}{5}mR^2\omega$$

Total kinetic energy.

$$E = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2 = \frac{1}{2}m(R\omega)^2 + \frac{1}{2} \cdot \frac{2}{5}mR^2\omega^2$$

$$= \frac{1}{2}mR^2\omega^2\left(1 + \frac{2}{5}\right) = \frac{7}{10}mR^2\omega^2$$

Form the ratio.

$$\frac{L}{E} = \frac{\frac{2}{5}mR^2\omega}{\frac{7}{10}mR^2\omega^2} = \frac{2/5}{7\omega/10} = \frac{4}{7\omega}$$

Apply the given ratio.

$$\frac{4}{7\omega} = \frac{\pi}{22}$$

$$\omega = \frac{4 \times 22}{7\pi} = \frac{88}{7\pi}$$

Using $$\pi = \frac{22}{7}$$:

$$\omega = \frac{88}{7 \times \frac{22}{7}} = \frac{88}{22} = 4 \text{ rad s}^{-1}$$

Therefore, the answer is $$\mathbf{4}$$.

Force $$\vec{F} = -P\hat{k}$$ acts at origin. Torque about point (2,-3,0):

$$\vec{r} = (0,0,0) - (2,-3,0) = (-2, 3, 0)$$

$$\vec{\tau} = \vec{r} \times \vec{F} = (-2,3,0) \times (0,0,-P)$$

$$= (3(-P)-0, 0-(-2)(-P), 0) = (-3P, -2P, 0) = P(-3\hat{i} - 2\hat{j})$$

Given $$\vec{\tau} = P(a\hat{i} + b\hat{j})$$: $$a = -3, b = -2$$. $$a/b = 3/2 = x/2$$, so $$x = 3$$.

The answer is 3.

We have a hollow spherical ball rolling up a curved surface with initial velocity $$u = 3$$ m/s. For a hollow sphere, the moment of inertia is $$I = \frac{2}{3}mR^2$$.

Applying conservation of energy, the total kinetic energy at the bottom equals the potential energy at the maximum height:

$$\frac{1}{2}mv^2 + \frac{1}{2}I\omega^2 = mgh$$

Since the ball rolls without slipping, $$v = R\omega$$. Substituting:

$$\frac{1}{2}mv^2 + \frac{1}{2}\left(\frac{2}{3}mR^2\right)\frac{v^2}{R^2} = mgh$$

$$\frac{1}{2}mv^2 + \frac{1}{3}mv^2 = mgh$$

$$\frac{5}{6}mv^2 = mgh$$

Now, solving for $$h$$:

$$h = \frac{5v^2}{6g} = \frac{5 \times 9}{6 \times 10} = \frac{45}{60} = 0.75 \text{ m} = 75 \text{ cm}$$

So, the answer is $$75$$ cm.

A light rope is wound around a hollow cylinder and pulled with a force. We need to find the angular acceleration.

We are given that Mass $$m = 5$$ kg, radius $$R = 70$$ cm $$= 0.7$$ m, force $$F = 52.5$$ N.

To begin,

A hollow cylinder (thin-walled cylindrical shell) has all its mass concentrated at the radius $$R$$. Its moment of inertia about its central axis is:

$$ I = mR^2 $$

This differs from a solid cylinder, which has $$I = \frac{1}{2}mR^2$$. For a hollow cylinder, since all mass is at distance $$R$$ from the axis, we simply have $$I = mR^2$$.

$$ I = 5 \times (0.7)^2 = 5 \times 0.49 = 2.45\;\text{kg m}^2 $$

Next,

The rope is wound around the cylinder at radius $$R$$, so the force $$F$$ acts tangentially at a perpendicular distance $$R$$ from the axis. The torque is:

$$ \tau = F \times R = 52.5 \times 0.7 = 36.75\;\text{N m} $$

From this,

The rotational analogue of Newton's second law states:

$$ \tau = I\alpha $$

where $$\alpha$$ is the angular acceleration. Solving for $$\alpha$$:

$$ \alpha = \frac{\tau}{I} = \frac{36.75}{2.45} = 15\;\text{rad/s}^2 $$

The angular acceleration of the cylinder is 15 rad s$$^{-2}$$.

The torque ($$\tau$$) about the origin $$O$$ is produced by the force of gravity ($$mg$$) acting vertically downward.

$$\tau = \text{Force} \times \text{Perpendicular distance} = (mg) \cdot x$$

In projectile motion, the horizontal distance at any time $$t$$ is given by: $$x = (u \cos \theta)t$$

$$\tau = mg(u \cos \theta)t$$

$$L = \int_{0}^{t} \tau \, dt = \int_{0}^{t} mgu \cos \theta \cdot t \, dt$$

$$L = mgu \cos \theta \left[ \frac{t^2}{2} \right]_0^t = \frac{1}{2} mgu \cos \theta \cdot t^2$$

$$L = \frac{1}{2} \times 0.1 \times 10 \times 20 \times \frac{1}{\sqrt{2}} \times (2)^2$$

$$L = 2 \times \frac{20}{\sqrt{2}} = \frac{40}{\sqrt{2}} = 20\sqrt{2} \text{ kg m}^2 \text{ s}^{-1}$$

$$\sqrt{K} = 20\sqrt{2}$$

$$K = (20\sqrt{2})^2$$

$$K = 400 \times 2 = 800$$

A ring and a solid sphere rotate about an axis passing through their centres with the same radii of gyration. The axis is perpendicular to the plane of the ring.

For a ring about an axis perpendicular to its plane through the center: $$I_{\text{ring}} = MR_1^2$$

Radius of gyration: $$k_1 = R_1$$

For a solid sphere about an axis through its center: $$I_{\text{sphere}} = \dfrac{2}{5}MR_2^2$$

Radius of gyration: $$k_2 = R_2\sqrt{\dfrac{2}{5}}$$

$$k_1 = k_2 \Rightarrow R_1 = R_2\sqrt{\dfrac{2}{5}}$$

$$\dfrac{R_1}{R_2} = \sqrt{\dfrac{2}{5}}$$

$$\dfrac{R_1}{R_2} = \sqrt{\dfrac{2}{x}}$$

$$x = 5$$

The value of $$x$$ is $$\boxed{5}$$.

We have a solid cylinder rolling without slipping down an inclined plane of inclination $$\theta = 30°$$ and length $$l = 60$$ cm $$= 0.6$$ m, with $$g = 10$$ m/s$$^2$$.

The height of the incline is:

$$h = l \sin\theta = 0.6 \times \sin 30° = 0.6 \times 0.5 = 0.3 \text{ m}$$

For a solid cylinder rolling without slipping, using energy conservation:

$$mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2$$

Now, for a solid cylinder $$I = \frac{1}{2}mr^2$$ and $$\omega = \frac{v}{r}$$ (rolling condition), so:

$$mgh = \frac{1}{2}mv^2 + \frac{1}{2} \cdot \frac{1}{2}mr^2 \cdot \frac{v^2}{r^2} = \frac{1}{2}mv^2 + \frac{1}{4}mv^2 = \frac{3}{4}mv^2$$

Solving for $$v$$:

$$v = \sqrt{\frac{4gh}{3}} = \sqrt{\frac{4 \times 10 \times 0.3}{3}} = \sqrt{\frac{12}{3}} = \sqrt{4} = 2 \text{ m/s}$$

Hence, the answer is $$2$$ m/s.

For a solid sphere: $$I = \frac{2}{5}mR^2$$, so $$k_{sph} = R\sqrt{\frac{2}{5}}$$

For a solid cylinder: $$I = \frac{1}{2}mR^2$$, so $$k_{cyl} = R\sqrt{\frac{1}{2}} = \frac{R}{\sqrt{2}}$$

Ratio: $$\frac{k_{sph}}{k_{cyl}} = \frac{R\sqrt{2/5}}{R/\sqrt{2}} = \sqrt{\frac{2}{5}} \times \sqrt{2} = \sqrt{\frac{4}{5}} = \frac{2}{\sqrt{5}}$$

So $$k_{sph} : k_{cyl} = 2 : \sqrt{5}$$

Comparing with $$2 : \sqrt{x}$$, we get $$x = 5$$.

This matches the answer key value of $$\mathbf{5}$$.

For a solid sphere of mass $$m$$ making pure rolling on a horizontal surface, the total kinetic energy is the sum of translational and rotational kinetic energy.

For a solid sphere, the moment of inertia is $$I = \frac{2}{5}mr^2$$, and for pure rolling, $$\omega = \frac{v}{r}$$.

Total KE = Translational KE + Rotational KE:

$$KE = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2 = \frac{1}{2}mv^2 + \frac{1}{2}\left(\frac{2}{5}mr^2\right)\left(\frac{v}{r}\right)^2$$

$$KE = \frac{1}{2}mv^2 + \frac{1}{5}mv^2 = \frac{7}{10}mv^2$$

Given: $$m = 2$$ kg, $$KE = 2240$$ J.

$$2240 = \frac{7}{10} \times 2 \times v^2$$

$$2240 = \frac{14}{10} v^2 = 1.4\,v^2$$

$$v^2 = \frac{2240}{1.4} = 1600$$

$$v = 40 \text{ m s}^{-1}$$

The velocity of the centre of mass of the sphere is $$\mathbf{40}$$ m s$$^{-1}$$.

A thin uniform rod of length $$L = 2$$ m, cross-sectional area $$A$$, and density $$d$$ rotates about its center. To determine the constant $$\alpha$$ in the relation $$\omega = \sqrt{\frac{\alpha E}{Ad}}$$, we first compute the mass of the rod and its moment of inertia about the center.

The mass of the rod is given by $$m = A \times d \times L = 2Ad$$. Substituting this into the standard formula for the moment of inertia of a rod about its center, we have $$I = \frac{mL^2}{12} = \frac{2Ad \times 4}{12} = \frac{2Ad}{3}$$.

Next, we express the rotational kinetic energy in terms of the angular speed $$\omega$$: $$E = \frac{1}{2}I\omega^2 = \frac{1}{2} \times \frac{2Ad}{3} \times \omega^2 = \frac{Ad\omega^2}{3}$$. Solving for $$\omega^2$$ gives $$\omega^2 = \frac{3E}{Ad}$$, so that $$\omega = \sqrt{\frac{3E}{Ad}}$$.

By comparing this result with the given form $$\omega = \sqrt{\frac{\alpha E}{Ad}}$$, it follows that $$\alpha = \boxed{3}$$.

$$\text{Friction force } (f_k) = \mu mg$$

$$\text{Deceleration } (a) = \frac{f_k}{m} = \mu g$$

$$v = u - at$$

$$v = 18 - (0.3 \times 10) \times 2$$

$$v = 18 - 6 = 12\text{ m/s}$$

$$KE_{total} = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2$$

For a uniform disc, the moment of inertia is $$I = \frac{1}{2}mr^2$$. In pure rolling, $$v = \omega r \implies \omega = \frac{v}{r}$$.

$$KE_{total} = \frac{1}{2}mv^2 + \frac{1}{2}\left(\frac{1}{2}mr^2\right)\left(\frac{v}{r}\right)^2$$

$$KE_{total} = \frac{1}{2}mv^2 + \frac{1}{4}mv^2 = \frac{3}{4}mv^2$$

$$KE_{total} = \frac{3}{4} \times 0.5 \times (12)^2$$

$$KE_{total} = 0.375 \times 144 = 54\text{ J}$$

A uniform solid cylinder of radius $$R$$ and length $$L$$ has moment of inertia $$I_1$$ about its axis, and a concentric cylinder of radius $$R' = \frac{R}{2}$$ and length $$L' = \frac{L}{2}$$ is carved out.

Since the moment of inertia of a solid cylinder about its axis is given by $$I = \frac{1}{2}MR^2$$, for the original cylinder the mass is $$M = \rho \pi R^2 L$$, so its moment of inertia becomes $$I_1 = \frac{1}{2}MR^2 = \frac{1}{2}\rho \pi R^4 L$$. Substituting the corresponding dimensions for the carved-out portion gives its mass as $$M' = \rho \pi \left(\frac{R}{2}\right)^2 \cdot \frac{L}{2} = \frac{\rho \pi R^2 L}{8} = \frac{M}{8}$$, and therefore its moment of inertia is $$I_2 = \frac{1}{2}M'\left(\frac{R}{2}\right)^2 = \frac{1}{2} \cdot \frac{M}{8} \cdot \frac{R^2}{4} = \frac{MR^2}{64}$$.

From the above, the ratio of the moments of inertia is $$\frac{I_1}{I_2} = \frac{\frac{MR^2}{2}}{\frac{MR^2}{64}} = \frac{64}{2} = 32$$. Therefore, the correct answer is 32.

$$I = \frac{2}{3}MR^2$$

$$K_{\text{rot}} = \frac{1}{2}I\omega^2$$

$$K_{\text{rot}} = \frac{1}{2} \left(\frac{2}{3}MR^2\right) \left(\frac{v}{R}\right)^2 = \frac{1}{3}Mv^2$$

$$K_{\text{trans}} = \frac{1}{2}Mv^2$$

$$K_{\text{total}} = K_{\text{trans}} + K_{\text{rot}} = \frac{1}{2}Mv^2 + \frac{1}{3}Mv^2 = \frac{5}{6}Mv^2$$

$$\frac{K_{\text{rot}}}{K_{\text{total}}} = \frac{\frac{1}{3}Mv^2}{\frac{5}{6}Mv^2} = \frac{1}{3} \times \frac{6}{5} = \frac{2}{5}$$

$$\frac{x}{5} = \frac{2}{5} \implies x = 2$$

If the volume shrinks to $$\frac{1}{64}$$ of the original, and volume $$\propto R^3$$:

$$\frac{R'^3}{R^3} = \frac{1}{64} \Rightarrow R' = \frac{R}{4}$$

By conservation of angular momentum (no external torque):

$$I\omega = I'\omega'$$

$$\frac{2}{5}MR^2 \cdot \omega = \frac{2}{5}MR'^2 \cdot \omega'$$

$$R^2 \omega = \frac{R^2}{16} \omega'$$

$$\omega' = 16\omega$$

Since $$T = \frac{2\pi}{\omega}$$:

$$T' = \frac{T}{16} = \frac{24}{16} = \frac{24}{16} = \frac{3}{2} \text{ h}$$

So $$\frac{24}{x} = \frac{24}{16}$$, giving $$x = 16$$.

The value of x is 16.

A semicircular ring has all its mass distributed at a constant distance $$R$$ from the center.

The moment of inertia about an axis passing through the center and perpendicular to the plane of the ring is:

$$ I = \int r^2\,dm $$

Since every element of mass is at the same distance $$R$$ from the center:

$$ I = R^2 \int dm = MR^2 $$

Comparing with $$\frac{1}{x}MR^2$$: $$\frac{1}{x} = 1 \implies x = 1$$.

The value of $$x$$ is 1.

When the person stretches his hands, the moment of inertia becomes $$3I$$ (triple).

By conservation of angular momentum (no external torque):

$$L = I\omega = 3I\omega'$$

$$\omega' = \frac{\omega}{3}$$

Initial kinetic energy: $$E = \frac{1}{2}I\omega^2$$

Final kinetic energy:

$$E_f = \frac{1}{2}(3I)\omega'^2 = \frac{1}{2}(3I)\frac{\omega^2}{9} = \frac{1}{6}I\omega^2 = \frac{E}{3}$$

Therefore $$x = 3$$.

Total kinetic energy of a rigid body that rolls without slipping is the sum of
(i) translational kinetic energy of the centre of mass and
(ii) rotational kinetic energy about the centre of mass.

Let the linear speed of the centre of mass be $$v$$. For a solid sphere of mass $$m$$ and radius $$R$$:

Moment of inertia about its centre is
$$I_{\text{CM}} = \frac{2}{5}\,mR^{2}$$.

Because the sphere rolls without slipping, the rolling condition is
$$v = \omega R$$, where $$\omega$$ is the angular speed.

Write each part of the kinetic energy:

Translational part: $$K_{\text{trans}} = \frac{1}{2}\,m v^{2}$$.

Rotational part: $$K_{\text{rot}} = \frac{1}{2}\,I_{\text{CM}}\omega^{2} = \frac{1}{2}\left(\frac{2}{5}mR^{2}\right)\left(\frac{v}{R}\right)^{2} = \frac{1}{5}\,m v^{2}$$.

Total kinetic energy therefore is
$$K_{\text{total}} = \frac{1}{2}m v^{2} + \frac{1}{5}m v^{2} = \frac{7}{10}\,m v^{2} \quad -(1)$$.

The problem gives $$K_{\text{total}} = 7 \times 10^{-3}\,\text{J}$$ and $$m = 1\,\text{kg}$$. Substitute these into $$(1)$$:

$$7 \times 10^{-3} = \frac{7}{10}\,(1)\,v^{2}$$

Divide both sides by $$\tfrac{7}{10}$$:

$$v^{2} = 10^{-2}$$

Taking the positive square root (speed is positive):

$$v = 0.1\,\text{m s}^{-1}$$.

Convert metres per second to centimetres per second:

$$0.1\,\text{m s}^{-1} = 0.1 \times 100 = 10\,\text{cm s}^{-1}$$.

Hence, the speed of the centre of mass of the sphere is 10 cm s$$^{-1}$$.

We have a solid sphere of mass $$M = 500$$ g $$= 0.5$$ kg, radius $$R = 5$$ cm $$= 0.05$$ m, spinning with angular speed $$\omega = 10$$ rad/s.

The moment of inertia about a diameter is:

$$I_{diameter} = \frac{2}{5}MR^2 = \frac{2}{5}(0.5)(0.05)^2 = 5 \times 10^{-4} \text{ kg m}^2$$

So the angular momentum about the diameter is:

$$L = I_{diameter} \times \omega = 5 \times 10^{-4} \times 10 = 5 \times 10^{-3} \text{ kg m}^2\text{/s}$$

Now, using the parallel axis theorem, the moment of inertia about a tangent parallel to the diameter is:

$$I_{tangent} = I_{diameter} + MR^2 = \frac{2}{5}MR^2 + MR^2 = \frac{7}{5}MR^2$$

$$I_{tangent} = \frac{7}{5}(0.5)(0.0025) = 1.75 \times 10^{-3} \text{ kg m}^2$$

We are given that $$I_{tangent} = x \times 10^{-2} \times L$$. Substituting:

$$1.75 \times 10^{-3} = x \times 10^{-2} \times 5 \times 10^{-3}$$

$$1.75 \times 10^{-3} = x \times 5 \times 10^{-5}$$

$$x = \frac{1.75 \times 10^{-3}}{5 \times 10^{-5}} = 35$$

So, the answer is $$35$$.

The impulse $$\vec J_0 = J_0\,\hat x$$ is applied at a point situated a vertical distance $$h$$ above the centre of mass (COM) of the annular disc. Choose the co-ordinate system   •  origin at the COM,   •  $$x$$-axis along the direction of the impulse (and of the subsequent translation),   •  $$y$$-axis horizontal and perpendicular to the plane of the disc (i.e. along the axle),   •  $$z$$-axis vertically upward.

1. Linear impulse-momentum relation
The impulse imparts a COM velocity $$v = \frac{J_0}{M}$$ along the $$+x$$-direction.

2. Angular impulse-momentum relation
The position vector of the point of application is $$\vec r = h\,\hat z$$. The impulse produces an angular impulse $$\vec \tau \,\Delta t = \vec r \times \vec J_0 = hJ_0\,\hat y.$$ Hence the disc acquires an angular momentum about the $$y$$-axis equal to $$hJ_0$$. For an annular disc the moment of inertia about a diameter (the $$y$$-axis here) is $$I_y = \frac{1}{2}M(a^2+b^2)\;.$$ Therefore the initial angular velocity about the $$y$$-axis is $$\omega = \frac{hJ_0}{I_y} = \frac{2hJ_0}{M\,(a^2+b^2)}.$$

3. Condition for instantaneous pure rolling
For rolling without slipping, the velocity of the point of contact with the floor must be zero. The lowest point is at a distance $$b$$ below the COM, i.e. $$\vec r_c = -\,b\,\hat z$$. Its velocity relative to the ground is $$\vec v_c = \vec v\;+\;\vec\omega \times \vec r_c = \frac{J_0}{M}\,\hat x \;+\;\omega\,\hat y \times (-\,b\,\hat z) = \left(\frac{J_0}{M} - \omega b\right)\hat x.$$ The rolling condition $$\vec v_c = 0$$ therefore gives $$\frac{J_0}{M} = \omega b.$$ Substituting $$\omega$$ from step 2, $$\frac{J_0}{M} = \frac{2hJ_0}{M(a^2+b^2)}\,b \quad\Longrightarrow\quad h = \frac{a^2+b^2}{2b}\;.$$

4. Optimum height
Thus the unique height at which the impulse must be delivered so that the disc starts pure rolling immediately is $$h_m = \frac{a^2+b^2}{2b}\;.$$

5. Verification of the given options
•  Take the limiting case $$a \to 0$$ (a solid disc). Then $$h_m \to \dfrac{b^2}{2b} = \dfrac{b}{2}\;,$$ which coincides with Option A. ✅

Hence, the correct statement is:

Option A which is: For $$\mu \neq 0$$ and $$a \to 0$$, $$h_m = b/2$$.

For a circular disc of mass $$M$$ and radius $$R$$, the moment of inertia about the central axis perpendicular to its plane ($$I_{CM}$$) is $$I_{CM} = \frac{1}{2}MR^2$$

According to the Parallel Axis Theorem, the moment of inertia about any parallel axis $$AB$$ at a distance $$d$$ is $$I_{AB} = I_{CM} + Md^2$$

Substituting the given distance $$d = \frac{2}{3}R$$:

$$I_{AB} = \frac{1}{2}MR^2 + M\left(\frac{2}{3}R\right)^2$$

$$I_{AB} = \left(\frac{9 + 8}{18}\right)MR^2 = \frac{17}{18}MR^2$$

$$\frac{I_{AB}}{I_{CM}} = \frac{\frac{17}{18}MR^2}{\frac{1}{2}MR^2} = \frac{17}{18} \times \frac{2}{1} = \frac{17}{9}$$

$$x = 17$$

Since the solid sphere has mass $$M_s = 5\text{ kg}$$ and the disc has mass $$M_d = 4\text{ kg}$$, both with radius $$R$$, we first determine their moments of inertia about tangents.

For the disc, the moment of inertia about a diameter is $$\frac{1}{4}M_dR^2$$. Now using the parallel axis theorem to shift the axis to a tangent in its plane (a distance $$R$$ from the center) gives

$$I_{\text{disc}} = \frac{1}{4}M_dR^2 + M_dR^2 = \frac{5}{4}M_dR^2 = \frac{5}{4}\,(4)\,R^2 = 5R^2.$$

Similarly, the moment of inertia of the sphere about a diameter is $$\frac{2}{5}M_sR^2$$. Substituting into the parallel axis theorem at distance $$R$$ yields

$$I_{\text{sphere}} = \frac{2}{5}M_sR^2 + M_sR^2 = \frac{7}{5}M_sR^2 = \frac{7}{5}\,(5)\,R^2 = 7R^2.$$

Therefore, the ratio of the disc’s moment to the sphere’s moment is

$$\frac{I_{\text{disc}}}{I_{\text{sphere}}} = \frac{5R^2}{7R^2} = \frac{5}{7} = \frac{x}{7},$$

which gives $$x = 5\,. $$

For a uniform solid sphere of mass $$M$$ and radius $$R$$: $$I_{cm} = \frac{2}{5}MR^2$$

$$I_{PQ} = I_{cm} + Md^2 = \frac{2}{5}MR^2 + Md^2$$ (Parallel axis theorem)

The radius of gyration ($$k$$) is the distance from the axis at which the entire mass of the body could be concentrated to result in the same moment of inertia. It is defined by the relation $$I = Mk^2$$.

$$Mk^2 = M \left( \frac{2}{5}R^2 + d^2 \right)$$

$$k^2 = \frac{2}{5}R^2 + d^2$$

$$k^2 = \frac{2}{5}(5)^2 + (10)^2 = 10 + 100 = 110$$

We have $$k = \sqrt{x}$$, which implies $$x = k^2$$

The value of $$x$$ is 110.

Two discs of same mass $$M$$, thicknesses $$t_1 = 1$$ cm and $$t_2 = 0.5$$ cm, density ratio $$\rho_1 : \rho_2 = 3 : 5$$.

The mass of a disc is given by $$M = \pi R^2 t \rho$$.

Since $$M_1 = M_2$$:

For a thin disc about its diameter, the moment of inertia is $$I = \frac{1}{4}MR^2$$.

Since both discs have the same mass:

Given that the ratio is $$\frac{x}{6}$$:

The answer is $$\boxed{5}$$.

We have two identical solid spheres, each of mass $$m = 2\;\text{kg}$$ and radius $$r = 10\;\text{cm} = 0.1\;\text{m}$$, fixed at the ends of a light rod with separation between centres = 40 cm. The axis passes through the middle point of the rod, perpendicular to it, so the distance of each sphere's centre from the axis is $$d = 20\;\text{cm} = 0.2\;\text{m}$$.

Using the parallel axis theorem, the moment of inertia of each sphere about the given axis is:

$$I_{each} = \frac{2}{5}mr^2 + md^2$$

Substituting values:

$$I_{each} = \frac{2}{5}(2)(0.1)^2 + 2(0.2)^2 = \frac{2}{5}(2)(0.01) + 2(0.04)$$

$$I_{each} = 0.008 + 0.08 = 0.088\;\text{kg m}^2$$

Now the total moment of inertia is:

$$I = 2 \times I_{each} = 2 \times 0.088 = 0.176\;\text{kg m}^2 = 176 \times 10^{-3}\;\text{kg m}^2$$

So, the answer is $$176$$.

The moment of inertia of a disc of mass $$M$$ and radius $$R$$ about any of its diameters is $$\frac{MR^2}{4}$$. We seek the moment of inertia about an axis perpendicular to the disc and passing through a point on its edge.

By the perpendicular axis theorem for a planar body, $$I_z = I_x + I_y$$. Since each diameter contributes $$I_x = I_y = \frac{MR^2}{4}$$, the moment of inertia about the central axis normal to the plane is $$I_{\text{center}} = \frac{MR^2}{4} + \frac{MR^2}{4} = \frac{MR^2}{2}$$.

Applying the parallel axis theorem to shift the axis to the edge, which is at a distance $$R$$ from the center, gives $$I_{\text{edge}} = I_{\text{center}} + MR^2 = \frac{MR^2}{2} + MR^2 = \frac{3MR^2}{2}$$.

Since the general form is $$I_{\text{edge}} = \frac{x}{2}MR^2$$, equating to $$\frac{3MR^2}{2}$$ yields $$\therefore x = \boxed{3}$$.

The coin is a thin uniform disc of mass $$m = 5\text{ g}=0.005\text{ kg}$$ and radius $$R=\frac{4}{3}\text{ cm}=0.013333\text{ m}$$ that lies in the horizontal $$xy$$-plane.

An impulse of magnitude $$J=\sqrt{\frac{\pi}{2}}\times10^{-2}\ \text{N·s}$$ is applied vertically upward (along +$$z$$) at a point whose distance from the centre is $$r=\frac{2}{3}\text{ cm}=0.006667\text{ m}$$. Because the line of action does not pass through the centre, the impulse produces:

• a translational momentum $$J$$ along +$$z$$,
• an angular momentum (about the centre) whose magnitude is $$L = rJ$$, perpendicular to both $$\vec r$$ and $$\vec J$$, i.e. about a diameter of the disc.

Step 1: Translational upward velocity
Impulse-momentum theorem: $$J = m v_{\text{cm}} \ \Rightarrow\ v_{\text{cm}} = \frac{J}{m}$$ Compute $$J$$ first: $$J = \sqrt{\frac{\pi}{2}}\times10^{-2} = 1.2523\times10^{-2}\ \text{N·s}$$ Hence $$v_{\text{cm}} = \frac{1.2523\times10^{-2}}{0.005}=2.5046\ \text{m s}^{-1}$$

Step 2: Time of flight
The coin is projected upward with speed $$v_{\text{cm}}$$ and returns to its initial level under gravity. Time of flight: $$T = \frac{2v_{\text{cm}}}{g} = \frac{2(2.5046)}{10}=0.5009\ \text{s}$$

Step 3: Angular speed imparted about a diameter
Magnitude of angular momentum: $$L = rJ = (0.006667)(1.2523\times10^{-2}) = 8.3487\times10^{-5}\ \text{kg m}^2\text{ s}^{-1}$$ Moment of inertia of a thin disc about a diameter: $$I = \frac{1}{4}mR^{2}$$ $$R^{2} = (0.013333)^{2}=1.7778\times10^{-4}\ \text{m}^{2}$$ $$I = \frac{1}{4}(0.005)(1.7778\times10^{-4}) = 2.2222\times10^{-7}\ \text{kg m}^{2}$$ Angular speed: $$\omega = \frac{L}{I} = \frac{8.3487\times10^{-5}}{2.2222\times10^{-7}} = 3.7569\times10^{2}\ \text{rad s}^{-1}$$

Step 4: Number of complete rotations during the flight
$$n=\frac{\omega T}{2\pi}= \frac{(3.7569\times10^{2})(0.5009)}{2\pi} =\frac{1.8803\times10^{2}}{6.2832}=29.9\approx30$$

Hence, by the time the coin returns to its initial position, it completes about 30 rotations.

Answer: 30

Let the masses be $$m_1 = 20\ \text{g}=0.02\ \text{kg}$$ and $$m_2 = 30\ \text{g}=0.03\ \text{kg}$$. The distance between them is the length of the rod, $$L = 10\ \text{cm}=0.10\ \text{m}$$.

Since the suspension wire is fixed at the centre of mass (COM), we first locate that COM along the rod. Let the distances of $$m_1$$ and $$m_2$$ from the COM be $$r_1$$ and $$r_2$$ respectively.

By definition of the centre of mass, $$m_1 r_1 = m_2 r_2$$ $$-(1)$$ and $$r_1 + r_2 = L = 0.10\ \text{m}$$ $$-(2)$$.

From $$(1)$$, $$r_1 = \dfrac{m_2}{m_1}\,r_2 = \dfrac{0.03}{0.02}\,r_2 = 1.5\,r_2$$.

Substituting in $$(2)$$ gives $$1.5\,r_2 + r_2 = 2.5\,r_2 = 0.10$$, hence $$r_2 = 0.10/2.5 = 0.04\ \text{m}$$ and $$r_1 = 1.5 \times 0.04 = 0.06\ \text{m}$$.

The moment of inertia of the two-mass system about the COM is
$$I = m_1 r_1^2 + m_2 r_2^2$$

Substituting values:
$$I = 0.02\,(0.06)^2 + 0.03\,(0.04)^2$$
$$I = 0.02 \times 0.0036 + 0.03 \times 0.0016$$
$$I = 7.2 \times 10^{-5} + 4.8 \times 10^{-5}$$
$$I = 1.2 \times 10^{-4}\ \text{kg m}^2$$.

For a torsional pendulum, the angular frequency of small oscillations is
$$\omega = \sqrt{\dfrac{\kappa}{I}}$$
where $$\kappa$$ is the torsional constant of the wire.

Given $$\kappa = 1.2 \times 10^{-8}\ \text{N m rad}^{-1}$$ and $$I = 1.2 \times 10^{-4}\ \text{kg m}^2$$,

$$\dfrac{\kappa}{I} = \dfrac{1.2 \times 10^{-8}}{1.2 \times 10^{-4}} = 10^{-4}\ \text{s}^{-2}$$.

Therefore,
$$\omega = \sqrt{10^{-4}} = 10^{-2}\ \text{rad s}^{-1} = 0.01\ \text{rad s}^{-1}$$.

This can be written as $$\omega = 10 \times 10^{-3}\ \text{rad s}^{-1}$$, hence $$n = 10$$.

Final Answer: 10

For the system to be in equilibrium, the net torque about the pivot point C must be zero ($$\sum \tau_C = 0$$)

$$W_{rod} = m \cdot g = 2\text{ kg} \times 10\text{ m/s}^2 = 20\text{ N}$$

$$\tau_{rod} = 20\text{ N} \times 0.5\text{ m} = \mathbf{10\text{ N m}}$$ (Clockwise)

$$W_{obj} = 8\text{ kg} \times 10\text{ m/s}^2 = 80\text{ N}$$

$$\tau_{obj} = 80\text{ N} \times 1\text{ m} = \mathbf{80\text{ N m}}$$ (Clockwise)

$$\tau_{clockwise} = 10 + 80 = \mathbf{90\text{ N m}}$$

The tension ($$T$$) in cable $$AB$$ provides the counter-clockwise torque to balance the weights. It acts at point $$B$$, which is $$60\text{ cm} = 0.6\text{ m}$$ from the pivot, at an angle of $$30^\circ$$.

$$\tau_{tension} = T \times r \times \sin \theta$$

$$\tau_{tension} = T \times 0.6\text{ m} \times \sin(30^\circ)$$

$$\tau_{tension} = T \times 0.6 \times 0.5 = \mathbf{0.3T}$$

$$\tau_{anticlockwise} = \tau_{clockwise}$$

$$T = \frac{90}{0.3} = \mathbf{300\text{ N}}$$

We are given angular acceleration $$\alpha = 6t^2 - 2t$$ rad s$$^{-2}$$, with initial conditions $$\omega(0) = 10$$ rad s$$^{-1}$$ and $$\theta(0) = 4$$ rad.

Integrating the angular acceleration with respect to time gives $$\omega = \int \alpha \, dt = \int (6t^2 - 2t) \, dt = 2t^3 - t^2 + C_1$$. Imposing $$\omega(0) = 10$$ rad s$$^{-1}$$ yields $$C_1 = 10$$, so $$\omega = 2t^3 - t^2 + 10$$. Further integrating this angular velocity yields $$\theta = \int \omega \, dt = \int (2t^3 - t^2 + 10) \, dt = \frac{2t^4}{4} - \frac{t^3}{3} + 10t + C_2$$, hence $$\theta = \frac{t^4}{2} - \frac{t^3}{3} + 10t + C_2$$. Applying $$\theta(0) = 4$$ rad gives $$C_2 = 4$$, and thus $$\theta = \frac{t^4}{2} - \frac{t^3}{3} + 10t + 4$$. The correct answer is Option B.

We have a tube of length $$L = 50$$ cm $$= 0.5$$ m filled with an incompressible liquid of mass $$m = 250$$ g $$= 0.25$$ kg, rotating in a horizontal plane about one end with angular velocity $$\omega$$. We need to find the force exerted by the liquid at the far end.

Consider a small element of the liquid at distance $$r$$ from the axis of rotation, of length $$dr$$. The linear mass density is $$\lambda = \frac{m}{L} = \frac{0.25}{0.5} = 0.5$$ kg/m. The mass of this element is $$dm = \lambda \, dr$$. This element needs a centripetal force $$dF = dm \cdot \omega^2 r = \lambda \omega^2 r \, dr$$.

The force at the far end (at $$r = L$$) is the net force required to keep all the liquid beyond... but actually, the force at the closed end (the far end at distance $$L$$) must provide the centripetal force for the entire liquid column. We integrate the centripetal force contribution from $$r = 0$$ to $$r = L$$: $$F = \int_0^L \lambda \omega^2 r \, dr = \lambda \omega^2 \frac{L^2}{2} = \frac{m}{L} \cdot \omega^2 \cdot \frac{L^2}{2} = \frac{m \omega^2 L}{2}$$

Now we substitute $$\omega = x\sqrt{F}$$: $$F = \frac{m (x\sqrt{F})^2 L}{2} = \frac{m x^2 F L}{2}$$

Dividing both sides by $$F$$ (assuming $$F \neq 0$$): $$1 = \frac{m x^2 L}{2}$$

Solving for $$x^2$$: $$x^2 = \frac{2}{mL} = \frac{2}{0.25 \times 0.5} = \frac{2}{0.125} = 16$$

Hence $$x = 4$$.

Hence, the correct answer is 4.

We need to find the ratio $$I_1/I_2$$ where $$I_1$$ is the moment of inertia of a rod about one end and $$I_2$$ is the moment of inertia of the same rod bent into a ring about its diameter.

For a uniform rod of mass M and length L about a perpendicular axis through one end: $$I_1 = \frac{ML^2}{3}$$

When the rod is bent into a ring, the circumference equals the length of the rod: $$2\pi r = L \implies r = \frac{L}{2\pi}$$

The moment of inertia of a ring of mass M and radius r about its diameter is $$I_2 = \frac{Mr^2}{2} = \frac{M}{2}\left(\frac{L}{2\pi}\right)^2 = \frac{ML^2}{8\pi^2}$$

Therefore, $$\frac{I_1}{I_2} = \frac{ML^2/3}{ML^2/(8\pi^2)} = \frac{8\pi^2}{3}$$ and comparing with $$\frac{x\pi^2}{3}$$ gives $$\frac{x\pi^2}{3} = \frac{8\pi^2}{3}$$ so $$x = 8$$.

The answer is 8.

We are given a pulley of radius $$r = 1.5 \text{ m}$$, a tangential force $$F = (12t - 3t^2) \text{ N}$$, and moment of inertia $$I = 4.5 \text{ kg m}^2$$. We need to find $$K$$ where the number of rotations before the direction reverses is $$\dfrac{K}{\pi}$$.

$$\tau = F \times r = (12t - 3t^2) \times 1.5 = 18t - 4.5t^2$$

$$\alpha = \frac{\tau}{I} = \frac{18t - 4.5t^2}{4.5} = 4t - t^2$$

Since the pulley starts from rest ($$\omega = 0$$ at $$t = 0$$):

$$\omega = \int_0^t \alpha \, dt = \int_0^t (4t - t^2) \, dt = 2t^2 - \frac{t^3}{3}$$

The direction reverses when $$\omega = 0$$ (after the pulley has started rotating):

$$2t^2 - \frac{t^3}{3} = 0$$

$$t^2\left(2 - \frac{t}{3}\right) = 0$$

$$t = 0$$ or $$t = 6 \text{ s}$$

So the direction reverses at $$t = 6 \text{ s}$$.

$$\theta = \int_0^6 \omega \, dt = \int_0^6 \left(2t^2 - \frac{t^3}{3}\right) dt$$

$$\theta = \left[\frac{2t^3}{3} - \frac{t^4}{12}\right]_0^6$$

$$\theta = \frac{2(216)}{3} - \frac{1296}{12} = 144 - 108 = 36 \text{ rad}$$

$$\text{Number of rotations} = \frac{\theta}{2\pi} = \frac{36}{2\pi} = \frac{18}{\pi}$$

Comparing with $$\dfrac{K}{\pi}$$, we get $$K = 18$$.

Concept:
Loss in gravitational PE = Gain in total KE (translation + rotation)

Step 1: Change in Potential Energy

• Wheel (12 kg) moves down by h: 12gh
• Block (3 kg) moves up by h: −3gh

Net loss in PE=12gh − 3gh = 9gh

Step 2: Total Kinetic Energy at bottom

• Block:

$$\frac{1}{2}(3)v^2$$

• Wheel (translation):

$$\frac{1}{2}(12)v^2$$

• Wheel (rotation):

$$\frac{1}{2}I\omega^2 = \frac{1}{2}(12r^2)\left(\frac{v}{r}\right)^2 = \frac{1}{2}(12)v^2$$

Step 3: Apply energy conservation

9gh = $$\frac{1}{2}(3 + 12 + 12)v^2$$

9gh=$$\ \frac{\ 1}{2}(27)v^2$$

$$\Rightarrow v^2 = \frac{2gh}{3}$$ 

v = $$\sqrt{\frac{2gh}{3}}$$ = $$\frac{1}{2}\sqrt{\frac{8gh}{3}}$$

Step 4: Compare with given form

v = $$\frac{1}{2}\sqrt{xgh} \Rightarrow x = \frac{8}{3} \approx 3$$

Final Answer:

3​

We need to find the downward distance ($$h$$) the solid cylinder must fall while unbinding from the two strings to achieve a linear center-of-mass speed of $$4\text{ m s}^{-1}$$.

1. Identify the Physical Principles

As the cylinder falls, it undergoes combined translational and rotational motion. Since the strings are wrapped around it and unbind without slipping, the system obeys the principle of Conservation of Mechanical Energy.

The potential energy lost by the cylinder during its descent is entirely converted into translational kinetic energy and rotational kinetic energy:

$$\Delta U = K_{\text{trans}} + K_{\text{rot}}$$

$$mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2$$

2. Analyze the System Parameters

• Linear velocity ($$v$$) = $$4\text{ m s}^{-1}$$
• Acceleration due to gravity ($$g$$) = $$10\text{ m s}^{-2}$$
• The moment of inertia ($$I$$) of a solid cylinder of mass $$m$$ and radius $$R$$ rotating about its central axis is:

  $$I = \frac{1}{2}mR^2$$

• Since the strings unbind without slipping, the relationship between linear velocity ($$v$$) and angular velocity ($$\omega$$) is:

  $$\omega = \frac{v}{R}$$

3. Substitute and Simplify the Energy Equation

Substitute the expressions for $$I$$ and $$\omega$$ into the kinetic energy terms:

$$mgh = \frac{1}{2}mv^2 + \frac{1}{2}\left(\frac{1}{2}mR^2\right)\left(\frac{v}{R}\right)^2$$

$$mgh = \frac{1}{2}mv^2 + \frac{1}{4}mv^2$$

Combine the kinetic energy fractions:

$$mgh = \frac{3}{4}mv^2$$

The mass ($$m$$) cancels out from both sides of the equation, leaving:

$$gh = \frac{3}{4}v^2$$

4. Calculate the Distance ($$h$$)

Isolate $$h$$ and substitute the given values into the equation:

$$h = \frac{3v^2}{4g}$$

$$h = \frac{3 \times (4)^2}{4 \times 10}$$

$$h = \frac{3 \times 16}{40} = \frac{48}{40} = 1.2\text{ m}$$

Convert the distance from meters to centimeters:

$$h = 1.2 \times 100 = 120\text{ cm}$$

Conclusion

The distance from the initial rest position that the cylinder should descend is 120 cm.

For the hanging block of mass $$m$$ moving downward with linear acceleration $$a$$:

$$mg - T = ma \quad \text{--- (1)}$$

For the rotating uniform disc of mass $$M$$ and radius $$R$$:

$$\tau = I\alpha \implies T \cdot R = \left(\frac{1}{2}MR^2\right)\alpha$$

Since the cord does not slip, the linear acceleration is related to the angular acceleration by $$a = R\alpha$$:

$$T \cdot R = \frac{1}{2}MR^2 \left(\frac{a}{R}\right)$$

$$\implies T = \frac{1}{2}Ma \implies a = \frac{2T}{M} \quad \text{--- (2)}$$

Substituting equation (2) into equation (1):

$$mg - T = m\left(\frac{2T}{M}\right)$$ $$\implies mg = T\left(1 + \frac{2m}{M}\right)$$ $$\implies T = \frac{mg}{1 + \frac{2m}{M}}$$

$$T = \frac{2 \times 10}{1 + \frac{2 \times 2}{4}} = \frac{20}{1 + 1} = 10\ \text{N}$$

For a cylindrical rod (uniform rod) of length $$L$$, the moment of inertia about an axis perpendicular to its length and passing through its center is:

$$I = \frac{ML^2}{12}$$

The radius of gyration $$k$$ is defined by $$I = Mk^2$$, so:

$$Mk^2 = \frac{ML^2}{12}$$

$$k = \frac{L}{\sqrt{12}} = \frac{L}{2\sqrt{3}}$$

Given $$L = 10\sqrt{3} \text{ m}$$:

$$k = \frac{10\sqrt{3}}{2\sqrt{3}} = \frac{10}{2} = 5 \text{ m}$$

The radius of gyration is $$\textbf{5}$$ m.

$$Massofdisc=1kg$$

$$Massofparticle=1kg$$

$$Radius=R$$

Initially the particle is at the top, finally at the bottom

Loss in potential energy of particle:
$$ΔPE=mg(2R)=1\times g\times2R=2gR$$

Disc’s potential energy does not change (its centre stays at same height)

So total energy converted to rotation:
$$2gR=\frac{1}{2}I_{total}ω^2$$

Now moment of inertia:

Disc about centre:
$$I_{disc}=\frac{1}{2}MR^2=\frac{1}{2}R^2$$

Particle at distance R:
$$I_{particle}=MR^2=R^2$$

So:
$$I_{total}=\frac{1}{2}R^2+R^2=\frac{3}{2}R^2$$

Now substitute:

$$2gR=\frac{1}{2}\left(\frac{3}{2}R^2ω^2\right)$$

$$2gR=(3/4)R^2ω^2$$

Solve:

$$ω^2=(8g)/(3R)$$

$$ω=\sqrt{\ \frac{8g}{3r}}$$

Given answer form:
$$ω=4\sqrt{\ \frac{x}{3r}}$$

Compare:
$$x=\frac{g}{2}$$

Given g = 10:

x = 5

Final answer:

5

A metre scale is initially balanced at its centre (the 50 cm mark). Two coins, each of mass 10 g (total mass = 20 g), are placed at the 10.0 cm mark, and the new balance point shifts to the 40.0 cm mark.

Concept: For the scale to be balanced at the new pivot (40 cm mark), the net torque about this point must be zero. The weight of the metre scale acts at its centre of gravity, which is at the 50 cm mark (since it is uniform).

Calculate the torques about the 40 cm mark: the coins (20 g) are at the 10 cm mark, which is $$40 - 10 = 30$$ cm to the left of the pivot.

The centre of gravity of the scale is at the 50 cm mark, which is $$50 - 40 = 10$$ cm to the right of the pivot.

Apply the condition for rotational equilibrium (net torque = 0):: $$\text{Anticlockwise torque} = \text{Clockwise torque}$$

$$20 \times 30 = M \times 10$$

$$600 = 10M$$

$$M = 60 \text{ g} = 60 \times 10^{-3} \text{ kg} = 6 \times 10^{-2} \text{ kg}$$

Since the mass is given as $$x \times 10^{-2}$$ kg, we get $$x = \mathbf{6}$$.

The disk begins from rest and rolls without slipping. Hence the angle turned by the disk in time $$t$$ is obtained from the constant-angular-acceleration relation

$$\theta = \frac12 \alpha t^{2}$$

With $$\alpha = \frac23 \text{ rad s}^{-2}$$ and $$t = \sqrt{\pi}\ \text{s}$$,

$$\theta = \frac12 \left(\frac23\right)(\sqrt{\pi})^{2}= \frac{\pi}{3} \text{ rad}$$

For pure rolling, a point that was at the initial contact position follows the cycloid

$$x = R(\theta - \sin\theta), \qquad y = R(1-\cos\theta)$$

with $$R = 1\ \text{m}$$. At $$\theta = \pi/3$$,

$$y = 1-\cos\left(\frac{\pi}{3}\right)=1-\frac12 = 0.5\ \text{m}$$

Thus the stone is $$0.5\ \text{m}$$ above the plane when it detaches.

The instantaneous angular speed is

$$\omega = \alpha t = \frac23\sqrt{\pi}\ \text{rad s}^{-1}$$

Differentiate the cycloid to find the velocity components (ground frame):

$$v_x = (1-\cos\theta)\,\omega,\qquad v_y = (\sin\theta)\,\omega$$

At $$\theta = \pi/3$$, $$1-\cos\theta = 0.5$$ and $$\sin\theta = \frac{\sqrt3}{2}$$, so

$$v_y = \frac{\sqrt3}{2}\,\omega = \frac{\sqrt3}{2}\left(\frac23\sqrt{\pi}\right) = \frac{\sqrt{3\pi}}{3}\ \text{m s}^{-1}$$

The stone now behaves like a projectile. The extra height it can rise is

$$\Delta h = \frac{v_y^{2}}{2g} = \frac{\left(\frac{\sqrt{3\pi}}{3}\right)^{2}}{2\times10} = \frac{\pi/3}{20}= \frac{\pi}{60}\ \text{m}$$

Total maximum height measured from the plane:

$$H = 0.5 + \frac{\pi}{60}\ \text{m}$$

The question writes this as $$H = \frac12 + \frac{x}{10}$$, hence

$$\frac{x}{10} = \frac{\pi}{60}\;\Longrightarrow\; x = \frac{\pi}{6} \approx 0.5236 \; \text{(m)}$$

Rounded to two decimal places, $$x = 0.52$$.

Answer: 0.52

The sphere moves down the plane, so take the downward direction along the incline as positive for translation and the clockwise sense as positive for rotation.

Forces along the incline
• Weight component: $$mg\sin\theta = 1 \times 10 \times \sin 30^\circ = 5\,$$N (down the plane).
• Static friction $$f$$ acts up the plane (opposite the motion) to maintain rolling without slipping.
• Two equal forces of $$1\,$$N are applied. One is up the plane and the other is down the plane, so their net translational effect cancels: $$+1\,$$N - $$1\,$$N = 0. They therefore constitute a couple that produces only a torque.

Translational equation
Using Newton’s second law, $$ma = mg\sin\theta - f$$ $$\Rightarrow\; a = 5 - f$$ $$-(1)$$

Torque equation about the centre
Moment of inertia of a solid sphere: $$I = \frac{2}{5}mR^{2} = \frac{2}{5}\times1\times1^{2} = 0.4\,$$kg m$$^{2}$$.
Rolling condition: $$a = \alpha R \Longrightarrow \alpha = a/R = a$$ because $$R = 1\,$$m.

Clockwise torques are taken positive.
• Torque due to friction: $$fR = f \times 1 = f$$ (clockwise, +).
• Torque due to the couple: each $$1\,$$N force acts at a perpendicular distance $$r = 0.5\,$m, giving a total anticlockwise torque
$$\tau_c = 2Fr = 2 $$\times$$ 1 $$\times$$ 0.5 = 1\,$$N m (anticlockwise, −).

Applying $$\sum$$\tau = I$$\alpha$$: $$f - 1 = I$$\alpha$$ = 0.4\,a$$ $$-(2)$$

Solve simultaneous equations
From $$(1):\; f = 5 - a$$.
Substitute in $$(2):$$ $$\bigl(5 - a\bigr) - 1 = 0.4\,a$$
$$4 - a = 0.4\,a$$
$$4 = 1.4\,a$$
$$a $$= $$\frac{4}{1.4} = \frac{20}{7}$$ $$\approx$$ 2.857 $$\text{ m\,s}^{-2}$$

Rounded to two decimal places, the acceleration of the sphere is $$\boxed{2.86$$\text{ m\,s}^{-2}}$$.

All four bodies have mass $$M$$ and radius $$2R$$. The standard moment of inertia formulas (with radius $$r$$) are:

Solid sphere about diameter: $$\frac{2}{5}Mr^2$$. So $$I_1 = \frac{2}{5}M(2R)^2 = \frac{8}{5}MR^2$$.

Solid cylinder about its axis: $$\frac{1}{2}Mr^2$$. So $$I_2 = \frac{1}{2}M(2R)^2 = 2MR^2$$.

Solid circular disc about its diameter: $$\frac{1}{4}Mr^2$$. So $$I_3 = \frac{1}{4}M(2R)^2 = MR^2$$.

Thin circular ring about its diameter: $$\frac{1}{2}Mr^2$$. So $$I_4 = \frac{1}{2}M(2R)^2 = 2MR^2$$.

Now we compute $$2(I_2 + I_3) + I_4 = 2(2MR^2 + MR^2) + 2MR^2 = 2(3MR^2) + 2MR^2 = 6MR^2 + 2MR^2 = 8MR^2$$.

Setting this equal to $$xI_1$$: $$8MR^2 = x \cdot \frac{8}{5}MR^2$$.

Dividing both sides by $$\frac{8}{5}MR^2$$: $$x = \frac{8}{\frac{8}{5}} = 5$$.

The value of $$x$$ is $$\mathbf{5}$$.

We need to find the magnitude of the angular momentum of a 1 kg object. The mass is $$m = 1$$ kg, the position vector is $$\vec{r} = 3\hat{i} - \hat{j}$$ m, and the velocity is $$\vec{v} = 3\hat{j} + \hat{k}$$ m/s.

Since linear momentum is $$\vec{p} = m\vec{v} = 1 \times (3\hat{j} + \hat{k}) = 3\hat{j} + \hat{k}$$, substituting into the definition of angular momentum $$\vec{L} = \vec{r} \times \vec{p}$$ gives

$$\vec{L} = \vec{r} \times \vec{p} = (3\hat{i} - \hat{j}) \times (3\hat{j} + \hat{k})$$

Using the determinant method,

$$\vec{L} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 3 & -1 & 0 \\ 0 & 3 & 1 \end{vmatrix}$$

Expanding the determinant yields

$$\vec{L} = \hat{i}[(-1)(1) - (0)(3)] - \hat{j}[(3)(1) - (0)(0)] + \hat{k}[(3)(3) - (-1)(0)]$$

$$\vec{L} = \hat{i}[-1 - 0] - \hat{j}[3 - 0] + \hat{k}[9 - 0]$$

$$\vec{L} = -\hat{i} - 3\hat{j} + 9\hat{k}$$

The magnitude is

$$|\vec{L}| = \sqrt{(-1)^2 + (-3)^2 + (9)^2}$$

$$|\vec{L}| = \sqrt{1 + 9 + 81}$$

$$|\vec{L}| = \sqrt{91} \text{ N m s}$$

Comparing with $$\sqrt{x}$$, we get $$x = 91$$, so the answer is 91.

1. Translational (Orbital) Angular Momentum

• Disk A is fixed at the origin. Since both disks have a radius of $$R$$, the center of mass (CM) of disk B moves in a circular path of radius $$r = R + R = 2R$$.
• The center of mass of disk B moves with a fixed angular speed $\omega$ around the center of A. Therefore, the linear velocity of its center of mass ($$v_{\text{cm}}$$) is: $$v_{\text{cm}} = \omega(2R) = 2\omega R$$
• The orbital angular momentum ($$L_{\text{orbital}}$$) of disk B relative to the center of A is given by: $$L_{\text{orbital}} = M \cdot v_{\text{cm}} \cdot (2R) = M(2\omega R)(2R) = 4M\omega R^2$$

2. Rotational (Spin) Angular Momentum

• Because disk B rolls without slipping on the stationary circumference of disk A, its contact point with disk A must have an instantaneous velocity of zero.
• Let $$\omega_B$$ be the spinning angular velocity of disk B about its own center of mass. The non-slip condition dictates: $$v_{\text{cm}} = \omega_B R \implies 2\omega R = \omega_B R \implies \omega_B = 2\omega$$
• The moment of inertia of a uniform disk about its central axis is $$I_{\text{cm}} = \frac{1}{2}MR^2$$.
• The intrinsic spin angular momentum ($$L_{\text{spin}}$$) about its own center is: $$L_{\text{spin}} = I_{\text{cm}}\omega_B = \left(\frac{1}{2}MR^2\right)(2\omega) = M\omega R^2$$

3. Total Angular Momentum

Since both the orbital revolution and the intrinsic spin occur in the same direction (counter-clockwise), their magnitudes add up directly:

$$L_{\text{total}} = L_{\text{orbital}} + L_{\text{spin}}$$

$$L_{\text{total}} = 4M\omega R^2 + M\omega R^2 = 5M\omega R^2$$

Conclusion

Comparing our result with the expression given in the problem, $$n M \omega R^2$$:

$$n M \omega R^2 = 5 M \omega R^2 \implies n = 5$$

For a solid sphere rolling without slipping, we need to find the ratio of rotational kinetic energy to total kinetic energy.

$$I = \frac{2}{5}mR^2$$

$$K_{rot} = \frac{1}{2}I\omega^2 = \frac{1}{2} \times \frac{2}{5}mR^2 \times \omega^2 = \frac{1}{5}mR^2\omega^2$$

For rolling without slipping, $$v = R\omega$$

$$K_{trans} = \frac{1}{2}mv^2 = \frac{1}{2}mR^2\omega^2$$

$$K_{total} = K_{trans} + K_{rot} = \frac{1}{2}mR^2\omega^2 + \frac{1}{5}mR^2\omega^2 = \frac{7}{10}mR^2\omega^2$$

$$\frac{K_{rot}}{K_{total}} = \frac{\frac{1}{5}mR^2\omega^2}{\frac{7}{10}mR^2\omega^2} = \frac{\frac{1}{5}}{\frac{7}{10}} = \frac{1}{5} \times \frac{10}{7} = \frac{2}{7}$$

Hence, the correct answer is Option C.

We need to find the torque about the origin given $$\vec{F} = 3\hat{i} + 4\hat{j} - 2\hat{k}$$ and position vector $$\vec{r} = 2\hat{i} + \hat{j} + 2\hat{k}$$.

The torque is given by the cross product of $$\vec{r}$$ and $$\vec{F}$$: $$\vec{\tau} = \vec{r} \times \vec{F}$$.

Using the determinant method, we compute $$\vec{\tau} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 2 & 1 & 2 \\ 3 & 4 & -2 \end{vmatrix}$$.

Expanding this determinant gives the components: the $$\hat{i}$$ component is $$(1)(-2) - (2)(4) = -2 - 8 = -10$$; the $$\hat{j}$$ component is $$-[(2)(-2) - (2)(3)] = -[-4 - 6] = 10$$; and the $$\hat{k}$$ component is $$(2)(4) - (1)(3) = 8 - 3 = 5$$.

Therefore, $$\vec{\tau} = -10\hat{i} + 10\hat{j} + 5\hat{k}$$.

The correct answer is Option A: $$-10\hat{i} + 10\hat{j} + 5\hat{k}$$.

We need to match the moment of inertia expressions using standard formulas and the parallel axis theorem.

(A) M.I. of solid sphere about a tangent:

M.I. about the center = $$\frac{2}{5}MR^2$$

By parallel axis theorem (shifting by distance $$R$$):

$$I = \frac{2}{5}MR^2 + MR^2 = \frac{2 + 5}{5}MR^2 = \frac{7}{5}MR^2$$

This matches (II).

(B) M.I. of hollow sphere about a tangent:

M.I. about the center = $$\frac{2}{3}MR^2$$

By parallel axis theorem:

$$I = \frac{2}{3}MR^2 + MR^2 = \frac{2 + 3}{3}MR^2 = \frac{5}{3}MR^2$$

This matches (I).

(C) M.I. of circular ring about its diameter:

By perpendicular axis theorem: $$I_z = I_x + I_y$$

For a ring, $$I_z = MR^2$$ (about axis through center, perpendicular to plane)

By symmetry, $$I_x = I_y$$, so: $$I_x = \frac{MR^2}{2} = \frac{1}{2}MR^2$$

This matches (IV).

(D) M.I. of circular disc about any diameter:

M.I. about axis perpendicular to disc through center = $$\frac{1}{2}MR^2$$

By perpendicular axis theorem and symmetry:

$$I_{\text{diameter}} = \frac{1}{2} \times \frac{1}{2}MR^2 = \frac{1}{4}MR^2$$

This matches (III).

The matching is: A-II, B-I, C-IV, D-III

The correct answer is Option A.

The flywheel starts from rest and accelerates uniformly with angular acceleration $$\alpha$$. Using the equation for angular displacement with uniform angular acceleration $$\theta = \omega_0 t + \frac{1}{2}\alpha t^2$$ and noting that $$\omega_0 = 0$$, in the first second ($$t = 1$$ s) we have $$5 = \frac{1}{2}\alpha(1)^2$$ so $$\alpha = 10 \text{ rad/s}^2$$. In the first two seconds the total angle is $$\theta_2 = \frac{1}{2}(10)(2)^2 = \frac{1}{2}(10)(4) = 20 \text{ rad}$$, and the angle rotated in the second second is $$\theta_{\text{2nd sec}} = 20 - 5 = 15 \text{ rad}$$.

Using the formula for the angle in the $$n$$th second, $$\theta_n = \omega_0 + \frac{\alpha}{2}(2n - 1)$$, for $$n = 2$$ we get $$\theta_2 = 0 + \frac{10}{2}(2 \times 2 - 1) = 5 \times 3 = 15$$ rad, confirming the result. The correct answer is Option B: 15 rad.

We need to find the ratio of the final linear velocity of a rolling solid cylinder to that of a rolling solid sphere when they start from rest and roll down the same inclined plane without slipping.

1. Derive the Velocity Formula for Pure Rolling

When an object of mass $$M$$ and radius $$R$$ rolls down an inclined plane of height $$h$$ without slipping, its initial potential energy is converted into both translational and rotational kinetic energy:

$$Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}I\omega^2$$

Since the object rolls without slipping, its angular velocity is $$\omega = \frac{v}{R}$$. Substituting this and writing the moment of inertia in terms of its radius of gyration ($$I = MK^2$$):

$$Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}(MK^2)\left(\frac{v}{R}\right)^2$$

$$Mgh = \frac{1}{2}Mv^2 \left(1 + \frac{K^2}{R^2}\right)$$

Canceling out the mass ($$M$$) and isolating the velocity ($$v$$) gives:

$$v = \sqrt{\frac{2gh}{1 + \frac{K^2}{R^2}}}$$

Since both objects fall from the same height ($$h$$), their velocities are inversely proportional to their rolling factor:

$$v \propto \frac{1}{\sqrt{1 + \frac{K^2}{R^2}}}$$

2. Calculate the Rolling Factors

• For a Solid Cylinder:

  The moment of inertia is $$I_{\text{cylinder}} = \frac{1}{2}MR^2 \implies \frac{K^2}{R^2} = \frac{1}{2}$$

  $$1 + \frac{K^2}{R^2} = 1 + \frac{1}{2} = \frac{3}{2}$$

• For a Solid Sphere:

  The moment of inertia is $$I_{\text{sphere}} = \frac{2}{5}MR^2 \implies \frac{K^2}{R^2} = \frac{2}{5}$$

  $$1 + \frac{K^2}{R^2} = 1 + \frac{2}{5} = \frac{7}{5}$$

3. Determine the Ratio ($$\frac{v_{\text{cylinder}}}{v_{\text{sphere}}}$$)

Taking the ratio of their final linear velocities:

$$\frac{v_{\text{cylinder}}}{v_{\text{sphere}}} = \sqrt{\frac{1 + \left(\frac{K^2}{R^2}\right)_{\text{sphere}}}{1 + \left(\frac{K^2}{R^2}\right)_{\text{cylinder}}}}$$

Substitute the calculated values into the ratio expression:

$$\frac{v_{\text{cylinder}}}{v_{\text{sphere}}} = \sqrt{\frac{\frac{7}{5}}{\frac{3}{2}}} = \sqrt{\frac{7}{5} \times \frac{2}{3}} = \sqrt{\frac{14}{15}}$$

Conclusion

The ratio of the velocity of the solid cylinder to that of the solid sphere is $$\sqrt{\frac{14}{15}}$$

$$Length=\sqrt{\ 34},base=3m⇒height=5m$$

$$Weight=10g=100Nactingatmidpoint$$

midpoint coordinates:
horizontal = 3/2
vertical = 5/2

Forces:

• at floor: $$F_f$$ (resultant, has horizontal + vertical)
• at wall: $$F_w$$ (horizontal only, since wall is frictionless)

Take moments about the bottom point:

Torque by weight (clockwise):
$$=100\times(horizontaldis\tan ceofCM)$$

$$=100\times(3/2)=150$$

Torque by wall reaction (anticlockwise):
$$=F_w\times height$$
$$=F_w\times5$$

Equilibrium:
$$F_w\times5=150$$
$$F_w=30N$$

Now vertical equilibrium:

Ff(vertical) = 100

Horizontal equilibrium:

$$F_f(horizontal)=F_w=30$$

So resultant floor reaction:

$$Ff=\sqrt{(100^2+30^2)\ }$$
$$=\sqrt{\left(10000+900)\right)}$$
= 10$$\sqrt{109}$$

Now ratio:

$$Fw/Ff=30/(10\sqrt{109}$$
$$=3/\sqrt{109}$$

Final answer:

$$3/\sqrt{109}$$

A thin circular ring of mass $$M$$ and radius $$R$$ rotates with angular velocity $$\omega_0 = 2$$ rad/s. Two objects each of mass $$m$$ are attached gently to opposite ends of a diameter.

Since the masses are attached gently (no external torque), angular momentum is conserved:
$$L_i = L_f$$
$$I_i \omega_0 = I_f \omega_f$$.

Initial moment of inertia (ring only) is
$$I_i = MR^2$$
and final moment of inertia (ring + two point masses at distance $$R$$) is
$$I_f = MR^2 + 2mR^2 = (M + 2m)R^2$$.

Substituting into the conservation equation,
$$MR^2 \times 2 = (M + 2m)R^2 \times \omega_f$$,
this gives
$$\omega_f = \frac{2M}{M + 2m}$$ rad/s.

The correct answer is Option C.

Let us analyze both the Assertion and Reason carefully.

Assertion A: "If we move from poles to equator, the direction of acceleration due to gravity of earth always points towards the center of earth without any variation in its magnitude."

Analysis of Assertion A:

The effective gravitational acceleration at a latitude $$\lambda$$ on the rotating Earth is:

$$g_{eff} = g - R\omega^2\cos^2\lambda$$

where $$g$$ is the gravitational acceleration without rotation, $$R$$ is the radius of the Earth, and $$\omega$$ is the angular velocity of Earth's rotation.

Variation in magnitude: At the poles ($$\lambda = 90°$$), $$g_{eff} = g$$. At the equator ($$\lambda = 0°$$), $$g_{eff} = g - R\omega^2$$, which is smaller. So the magnitude clearly varies as we move from poles to equator. This part of the assertion is false.

Variation in direction: The centrifugal acceleration at latitude $$\lambda$$ is directed perpendicular to Earth's rotation axis (i.e., radially outward from the axis, not from the center of Earth). Only at the equator and at the poles does the centrifugal acceleration align radially with respect to Earth's center. At intermediate latitudes, the centrifugal force has a component tangential to the Earth's surface, causing the net effective gravity to deviate slightly from pointing towards the center. So the direction also varies, making the assertion false.

Reason R: "At the equator, the direction of acceleration due to gravity is towards the center of earth."

Analysis of Reason R:

At the equator, the centrifugal acceleration $$R\omega^2$$ is directed radially outward from Earth's center (since at the equator, the direction perpendicular to the rotation axis coincides with the radial direction from Earth's center). The gravitational pull is directed radially inward towards the center. Since both are along the same line (radial direction), the resultant effective gravity still points towards the center of Earth. Only the magnitude is reduced. This makes Reason R true.

Conclusion:

The Assertion is false (both magnitude and direction vary) but the Reason is true (at the equator specifically, the direction is towards the center).

Hence, the correct answer is Option D.

We have the position vector $$\vec{r} = 2\hat{i} + 2\hat{j} + \hat{k}$$ and the force $$\vec{F} = 5\hat{i} + 3\hat{j} - 7\hat{k}$$. The torque about the origin is $$\vec{\tau} = \vec{r} \times \vec{F}$$.

We compute the cross product using the determinant formula: $$\vec{\tau} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 2 & 2 & 1 \\ 5 & 3 & -7 \end{vmatrix}$$

The $$\hat{i}$$ component is $$(2)(-7) - (1)(3) = -14 - 3 = -17$$. The $$\hat{j}$$ component is $$-\left[(2)(-7) - (1)(5)\right] = -(-14 - 5) = 19$$. The $$\hat{k}$$ component is $$(2)(3) - (2)(5) = 6 - 10 = -4$$.

Therefore $$\vec{\tau} = -17\hat{i} + 19\hat{j} - 4\hat{k}$$.

Hence, the correct answer is Option 3.

Given that it is an spherical shell

We know that moment of inertia of a spherical shell with respect to its centre is $$I=\frac{2}{3}MR^2$$

But with respect to origin it is $$I+MR^2$$ (Here R is the perpendicular distance between origin and centre of the sphere

So moment of inertia with respect to origin is $$I=\frac{5}{3}MR^2$$

We also know that Angular momentum $$L=I\omega\ $$

substitute I

$$L=\frac{5}{3}MR^2\omega\ $$

Given that M=1

$$L=\frac{5}{3}R^2\omega\ $$

So a will be 5

Particle P moves along $$\vec{A} = \hat{i} + \hat{j}$$ and then along a direction normal to the plane containing $$\vec{A}$$ and $$\vec{B} = \hat{j} + \hat{k}$$. So the direction of P after the collision is $$\vec{A} \times \vec{B}$$.

$$\vec{A} \times \vec{B} = (\hat{i} + \hat{j}) \times (\hat{j} + \hat{k}) = \hat{i}\times\hat{j} + \hat{i}\times\hat{k} + \hat{j}\times\hat{j} + \hat{j}\times\hat{k}$$ $$= \hat{k} - \hat{j} + 0 + \hat{i} = \hat{i} - \hat{j} + \hat{k}$$

Particle Q moves along $$\vec{A} = \hat{i} + \hat{j}$$ and then along a direction normal to the plane containing $$\vec{A}$$ and $$\vec{C} = -\hat{i} + \hat{j}$$. So the direction of Q after the collision is $$\vec{A} \times \vec{C}$$.

$$\vec{A} \times \vec{C} = (\hat{i} + \hat{j}) \times (-\hat{i} + \hat{j}) = \hat{i}\times(-\hat{i}) + \hat{i}\times\hat{j} + \hat{j}\times(-\hat{i}) + \hat{j}\times\hat{j}$$ $$= 0 + \hat{k} + \hat{k} + 0 = 2\hat{k}$$

The angle $$\theta$$ between these two directions is found using the dot product formula:

$$\cos\theta = \dfrac{(\hat{i} - \hat{j} + \hat{k}) \cdot (2\hat{k})}{|\hat{i} - \hat{j} + \hat{k}| \times |2\hat{k}|}$$

The numerator is $$2$$, and the denominator is $$\sqrt{1^2 + (-1)^2 + 1^2} \times 2 = \sqrt{3} \times 2$$.

$$\cos\theta = \frac{2}{2\sqrt{3}} = \frac{1}{\sqrt{3}}$$

Comparing with $$\cos^{-1}\!\left(\dfrac{1}{\sqrt{x}}\right)$$, we get $$x = 3$$.

We need to determine the time $$t > 0$$ at which the angular momentum of the particle equals its initial angular momentum at $$t = 0$$.

1. Determine the Position and Velocity Vectors

From the problem statement , the position vector $$\vec{r}(t)$$ of the particle of mass $$m$$ is given by:

$$\vec{r}(t) = 10\alpha t^2 \hat{i} + 5\beta(t - 5)\hat{j}$$

To find the velocity vector $$\vec{v}(t)$$, we take the first derivative of the position vector with respect to time ($$\vec{v} = \frac{d\vec{r}}{dt}$$):

$$\vec{v}(t) = \frac{d}{dt}\left(10\alpha t^2\right)\hat{i} + \frac{d}{dt}\left(5\beta t - 25\beta\right)\hat{j}$$

$$\vec{v}(t) = 20\alpha t \hat{i} + 5\beta \hat{j}$$

2. Formulate the Angular Momentum Equation

The angular momentum $$\vec{L}$$ of a particle about the origin is defined by the cross product of its position and linear momentum vectors:

$$\vec{L} = \vec{r} \times \vec{p} = m(\vec{r} \times \vec{v})$$

Let's calculate the cross product $$\vec{r} \times \vec{v}$$ explicitly using their unit vector components:

$$\vec{r} \times \vec{v} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 10\alpha t^2 & 5\beta(t - 5) & 0 \\ 20\alpha t & 5\beta & 0 \end{vmatrix}$$

$$\vec{r} \times \vec{v} = \left[(10\alpha t^2)(5\beta) - (5\beta(t - 5))(20\alpha t)\right] \hat{k}$$

$$\vec{r} \times \vec{v} = \left[50\alpha\beta t^2 - 100\alpha\beta t(t - 5)\right] \hat{k}$$

$$\vec{r} \times \vec{v} = \left[50\alpha\beta t^2 - 100\alpha\beta t^2 + 500\alpha\beta t\right] \hat{k} = \left[500\alpha\beta t - 50\alpha\beta t^2\right] \hat{k}$$

Therefore, the angular momentum vector at any time $$t$$ is:

$$\vec{L}(t) = m\alpha\beta\left(500t - 50t^2\right)\hat{k}$$

3. Equate Angular Momentum States

First, evaluate the initial angular momentum at $$t = 0$$:

$$\vec{L}(0) = m\alpha\beta\left(500(0) - 50(0)^2\right)\hat{k} = \vec{0}$$

We are looking for a non-zero time $$t$$ where the angular momentum returns to this initial value ($$\vec{L}(t) = \vec{L}(0)$$):

$$m\alpha\beta\left(500t - 50t^2\right)\hat{k} = \vec{0}$$

Since the mass $$m$$ and dimensional constants $$\alpha, \beta$$ are non-zero, we can divide them out:

$$500t - 50t^2 = 0$$

$$50t(10 - t) = 0$$

This quadratic equation gives two solutions:

• $$t = 0\text{ seconds}$$ (Initial state)
• $$t = 10\text{ seconds}$$

Conclusion

The angular momentum of the particle becomes the same as it was for $$t = 0$$ at time $$t =$$ 10 seconds.

We have a solid disc of mass $$M = 10\;\text{kg}$$ and radius $$R = 20\;\text{cm} = 0.20\;\text{m}$$ rotating with an initial angular velocity of $$600\;\text{rpm}$$ about its central axis. First, we convert this angular speed to SI units.

The relation between revolutions per minute and radians per second is

$$\omega = \text{(rev/min)} \times \frac{2\pi\;\text{rad}}{1\;\text{rev}} \times \frac{1\;\text{min}}{60\;\text{s}}.$$

Substituting the given value,

$$\omega_i = 600 \times \frac{2\pi}{60} = 600 \times \frac{2\pi}{60} = 10 \times 2\pi = 20\pi\; \text{rad s}^{-1}.$$

The disc has to be brought to rest in $$t = 10\;\text{s}$$, so the final angular velocity is $$\omega_f = 0$$. For constant angular deceleration $$\alpha$$ we use

$$\omega_f = \omega_i + \alpha t.$$

Rearranging,

$$\alpha = \frac{\omega_f - \omega_i}{t} = \frac{0 - 20\pi}{10} = -2\pi\;\text{rad s}^{-2}.$$

The minus sign merely shows that the acceleration is retarding; its magnitude is $$|\alpha| = 2\pi\;\text{rad s}^{-2}.$$

Now we need the moment of inertia of a solid disk about its central axis. The formula is stated as

$$I = \frac{1}{2} M R^{2}.$$

Substituting $$M = 10\;\text{kg}$$ and $$R = 0.20\;\text{m},$$

$$I = \frac{1}{2}(10)(0.20)^2 = \frac{1}{2}(10)(0.04) = 0.2\;\text{kg m}^{2}.$$

The magnitude of the required retarding torque $$\tau$$ is connected to angular acceleration by

$$\tau = I\,|\alpha|.$$

Substituting $$I = 0.2\;\text{kg m}^{2}$$ and $$|\alpha| = 2\pi\;\text{rad s}^{-2},$$

$$\tau = 0.2 \times 2\pi = 0.4\pi\;\text{N m}.$$

We express $$0.4\pi$$ in the form $$k \times \pi \times 10^{-1}$$. Noting that $$0.4 = 4 \times 10^{-1},$$ we write

$$\tau = 4\,\pi \times 10^{-1}\;\text{N m}.$$

Thus the numeral to be filled in the blank is $$4$$.

Hence, the correct answer is Option 4.

A uniform thin bar of mass $$M = 6$$ kg and length $$L = 2.4$$ m is bent into a regular hexagon. Since a regular hexagon has 6 equal sides, the length of each side is $$a = \frac{L}{6} = \frac{2.4}{6} = 0.4$$ m, and the mass of each side is $$m = \frac{M}{6} = \frac{6}{6} = 1$$ kg.

For a regular hexagon with side $$a$$, the perpendicular distance from the centre to the midpoint of each side is $$d = \frac{\sqrt{3}}{2} a = \frac{\sqrt{3}}{2} \times 0.4 = 0.2\sqrt{3}$$ m.

The moment of inertia of each side (a thin rod of mass $$m$$ and length $$a$$) about the axis through the centre of the hexagon and perpendicular to its plane is found using the parallel axis theorem. Each rod has its own moment of inertia about its centre given by $$\frac{ma^2}{12}$$, and using the parallel axis theorem with distance $$d$$, the moment of inertia of each rod about the central axis is $$I_{\text{rod}} = \frac{ma^2}{12} + md^2$$.

Substituting the values: $$I_{\text{rod}} = \frac{1 \times (0.4)^2}{12} + 1 \times (0.2\sqrt{3})^2 = \frac{0.16}{12} + 0.12 = 0.01333 + 0.12 = 0.13333$$ kg m$$^2$$.

Since the hexagon has 6 identical sides, the total moment of inertia is $$I = 6 \times I_{\text{rod}} = 6 \times 0.13333 = 0.8$$ kg m$$^2$$.

Expressing this as $$x \times 10^{-1}$$ kg m$$^2$$: $$0.8 = 8 \times 10^{-1}$$.

Therefore, the answer is $$8$$.

We are given that the angular speed of a truck wheel increases uniformly from $$\omega_1 = 900$$ rpm to $$\omega_2 = 2460$$ rpm in $$t = 26$$ seconds, and we need to find the total number of revolutions during this time.

For uniform angular acceleration, the total angle swept (in revolutions) equals the average angular speed multiplied by the time. The average angular speed is $$\omega_{avg} = \frac{\omega_1 + \omega_2}{2} = \frac{900 + 2460}{2} = \frac{3360}{2} = 1680$$ rpm.

Now we convert the time to minutes: $$t = 26 \text{ seconds} = \frac{26}{60}$$ minutes.

The number of revolutions is $$N = \omega_{avg} \times t = 1680 \times \frac{26}{60} = 1680 \times \frac{13}{30} = \frac{1680 \times 13}{30} = \frac{21840}{30} = 728$$.

Therefore, the number of revolutions by the truck engine during this time is $$\boxed{728}$$.

For a body rolling without slipping down an inclined plane, energy conservation gives:

$$mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2$$

Since rolling without slipping: $$\omega = \frac{v}{R}$$, so:

$$mgh = \frac{1}{2}mv^2\left(1 + \frac{I}{mR^2}\right)$$

$$v = \sqrt{\frac{2gh}{1 + \frac{I}{mR^2}}}$$

For a ring: $$I_{\text{ring}} = mR^2$$, so $$\frac{I}{mR^2} = 1$$, giving:

$$v_{\text{ring}} = \sqrt{\frac{2gh}{2}} = \sqrt{gh}$$

For a solid cylinder: $$I_{\text{cylinder}} = \frac{1}{2}mR^2$$, so $$\frac{I}{mR^2} = \frac{1}{2}$$, giving:

$$v_{\text{cylinder}} = \sqrt{\frac{2gh}{\frac{3}{2}}} = \sqrt{\frac{4gh}{3}}$$

The ratio of velocities:

$$\frac{v_{\text{ring}}}{v_{\text{cylinder}}} = \frac{\sqrt{gh}}{\sqrt{\frac{4gh}{3}}} = \sqrt{\frac{3}{4}} = \frac{\sqrt{3}}{2}$$

Given that this ratio equals $$\frac{\sqrt{x}}{2}$$, we have $$\sqrt{x} = \sqrt{3}$$, so $$x = 3$$.

The initial angular speed is $$\omega_1 = 600 \text{ rpm}$$ and the final angular speed is $$\omega_2 = 1800 \text{ rpm}$$. The time taken is $$t = 10$$ s.

The average angular speed during uniform acceleration is:

$$\omega_{\text{avg}} = \frac{\omega_1 + \omega_2}{2} = \frac{600 + 1800}{2} = 1200 \text{ rpm}$$

The total number of rotations is:

$$N = \omega_{\text{avg}} \times t = 1200 \text{ rpm} \times \frac{10}{60} \text{ min} = 1200 \times \frac{1}{6} = 200 \text{ rotations}$$

The intersection of the $$x = 2$$ plane and the $$x$$-axis is the point $$(2, 0, 0)$$. The force is $$\vec{F} = 4\hat{i} + 3\hat{j} + 4\hat{k}$$ applied at point $$A = (2, 0, 0)$$.

We need the torque about point $$B = (2, 3, 4)$$. The position vector from $$B$$ to $$A$$ is $$\vec{r} = A - B = (2-2)\hat{i} + (0-3)\hat{j} + (0-4)\hat{k} = -3\hat{j} - 4\hat{k}$$.

The torque is $$\vec{\tau} = \vec{r} \times \vec{F}$$:

$$\vec{\tau} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 0 & -3 & -4 \\ 4 & 3 & 4 \end{vmatrix}$$

$$= \hat{i}[(-3)(4) - (-4)(3)] - \hat{j}[(0)(4) - (-4)(4)] + \hat{k}[(0)(3) - (-3)(4)]$$

$$= \hat{i}[-12 + 12] - \hat{j}[0 + 16] + \hat{k}[0 + 12]$$

$$= 0\hat{i} - 16\hat{j} + 12\hat{k}$$

The magnitude is $$|\vec{\tau}| = \sqrt{0^2 + 16^2 + 12^2} = \sqrt{256 + 144} = \sqrt{400} = 20$$.

The magnitude of torque is $$\boxed{20}$$.

A particle of mass $$m$$ moving with velocity $$u$$ hits one end of a rod of mass $$M$$ and length $$L$$ perpendicularly, with the collision being completely elastic, and the particle comes to rest after the collision.

Let $$V_{cm}$$ be the velocity of the centre of mass of the rod and $$\omega$$ be its angular velocity after the collision. Conservation of linear momentum gives $$mu = MV_{cm}$$, so $$V_{cm} = \frac{mu}{M}$$.

Conservation of angular momentum about the centre of the rod gives $$mu\cdot\frac{L}{2} = I_{rod}\,\omega = \frac{ML^2}{12}\,\omega$$, so $$\omega = \frac{6mu}{ML}$$.

Conservation of kinetic energy (elastic collision): $$\frac{1}{2}mu^2 = \frac{1}{2}MV_{cm}^2 + \frac{1}{2}I_{rod}\,\omega^2$$.

Substituting: $$mu^2 = M\cdot\frac{m^2u^2}{M^2} + \frac{ML^2}{12}\cdot\frac{36m^2u^2}{M^2L^2} = \frac{m^2u^2}{M} + \frac{3m^2u^2}{M} = \frac{4m^2u^2}{M}$$.

Therefore $$m = \frac{M}{4}$$, giving $$\frac{m}{M} = \frac{1}{4} = \frac{1}{x}$$, so $$x = 4$$.

We need to find the number of revolutions $$n$$ made by a uniform circular disk to reach a given angular speed under the action of a constant force.

The moment of inertia $$I$$ of a uniform circular disk of mass $$M$$ and radius $$R$$ about an axis passing through its center is given by: $$I = \frac{1}{2} M R^2$$.

We are given the following values:

Mass of the disk, $$M = 20\text{ kg}$$

Radius of the disk, $$R = 0.2\text{ m}$$

Substituting these values gives: $$I = \frac{1}{2} \times 20 \times (0.2)^2 = 10 \times 0.04 = 0.4\text{ kg m}^2$$.

The torque $$\tau$$ acting on the disk due to the constant force $$F = 20\text{ N}$$ applied at its periphery is: $$\tau = F \times R = 20 \times 0.2 = 4\text{ N m}$$.

According to Newton's second law for rotation, the angular acceleration $$\alpha$$ is given by: $$\alpha = \frac{\tau}{I} = \frac{4}{0.4} = 10\text{ rad s}^{-2}$$.

The disk starts from rest, so its initial angular speed is $$\omega_0 = 0$$. To find the total angular displacement $$\theta$$ required to attain a final angular speed of $$\omega = 50\text{ rad s}^{-1}$$, we use the third equation of rotational motion: $$\omega^2 = \omega_0^2 + 2\alpha\theta$$.

Substituting the known parameters into the equation yields: $$50^2 = 0^2 + 2 \times 10 \times \theta$$, which simplifies to: $$2500 = 20\theta$$.

Solving for the angular displacement gives: $$\theta = \frac{2500}{20} = 125\text{ rad}$$.

Since one complete revolution corresponds to a rotation of $$6.28\text{ rad}$$, the total number of revolutions $$n$$ is: $$n = \frac{\theta}{2\pi} = \frac{125}{6.28} \approx 19.90$$.

Rounding to the nearest integer, the value of $$n$$ is 20.

We need to determine the total moment of inertia of the badminton racket about an axis perpendicular to the handle, lying in the plane of the ring, and located at a distance of

$$\frac{r}{2}$$

from the free end A of the handle.

1. Identify the Given Parameters

From the problem statement , the badminton racket is split into two geometric components:

• The Handle (Linear Rod): Mass = $$M$$, Length = $$6r$$
• The Head (Circular Ring): Mass = $$M$$, Radius = $$r$$ (Diameter = $$2r$$)

The target axis of rotation is positioned at a distance of $$\frac{r}{2}$$ from end A of the handle.

2. Calculate the Moment of Inertia for the Handle

The handle is treated as a uniform thin rod. We determine its moment of inertia using the parallel axis theorem:

• Center of Mass Location:
  The center of mass of the rod lies at its midpoint, which is at a distance of $$3r$$ from end A.

  $$I_{\text{cm, handle}} = \frac{1}{12} M L^2 = \frac{1}{12} M (6r)^2 = 3Mr^2$$

• Parallel Axis Distance Shift:
  The distance from the rod's center of mass to the target axis (at $$\frac{r}{2}$$ from A) is:

  $$d_1 = 3r - \frac{r}{2} = \frac{5r}{2}$$

Applying the Parallel Axis Theorem ($$I = I_{\text{cm}} + Md^2$$):

$$I_{\text{handle}} = 3Mr^2 + M\left(\frac{5r}{2}\right)^2 = 3Mr^2 + \frac{25}{4}Mr^2 = \frac{37}{4}Mr^2$$

3. Calculate the Moment of Inertia for the Ring Head

The head of the racket is treated as a uniform circular ring. The rotation axis lies along the coplanar diameter plane of the ring:

• Center of Mass Axis:
  The moment of inertia of a thin ring about its coplanar diameter axis passing through its center is:

  $$I_{\text{cm, ring}} = \frac{1}{2}Mr^2$$

• Parallel Axis Distance Shift:
  The center of the ring is located at the far end of the handle, a distance of $$6r + r = 7r$$ from end A. The shift distance to the target axis is:

  $$d_2 = 7r - \frac{r}{2} = \frac{13r}{2}$$

Applying the Parallel Axis Theorem ($$I = I_{\text{cm}} + Md^2$$):

$$I_{\text{ring}} = \frac{1}{2}Mr^2 + M\left(\frac{13r}{2}\right)^2 = \frac{2}{4}Mr^2 + \frac{169}{4}Mr^2 = \frac{171}{4}Mr^2$$

4. Determine Total Moment of Inertia

The total moment of inertia of the entire composite racket system is the scalar sum of both individual components:

$$I_{\text{total}} = I_{\text{handle}} + I_{\text{ring}}$$

$$I_{\text{total}} = \frac{37}{4}Mr^2 + \frac{171}{4}Mr^2 = \frac{208}{4}Mr^2 = 52Mr^2$$

Conclusion

The multiplier value for the final total moment of inertia observed for the badminton racket is 52.

We need to determine which of the four given bodies will reach the bottom of the inclined plane first when allowed to roll down simultaneously without slipping.

1. Relate Rolling Acceleration to the Moment of Inertia

When a body rolls down an inclined plane of angle $$\theta$$ without slipping, its linear acceleration ($$a$$) is given by the formula:

$$a = \frac{g \sin\theta}{1 + \frac{I}{mR^2}}$$

Where $$I$$ is the moment of inertia, $$m$$ is the mass, and $$R$$ is the radius of the body.

The body with the highest linear acceleration ($$a$$) will take the least amount of time to travel down the incline, meaning it will reach the bottom first. Looking at the formula, acceleration is inversely proportional to the factor $$\frac{I}{mR^2}$$. Therefore, the body with the smallest value of $$\frac{I}{mR^2}$$ will have the highest acceleration.

2. Analyze the Given Bodies

Let's calculate the value of $$\frac{I}{mR^2}$$ for each body mentioned on the  page:

3. Compare the Values

• The solid sphere has the lowest factor of $$\frac{I}{mR^2} = 0.40$$.
• Substituting this value into the acceleration formula gives it the maximum acceleration:

  $$a_{\text{sphere}} = \frac{g \sin\theta}{1 + 0.4} = \frac{5}{7}g \sin\theta \approx 0.714\, g \sin\theta$$

Conclusion

Since the solid sphere has the maximum linear acceleration, it will reach the bottom of the inclined plane first. This matches numbering 4.

We have a uniform steel rod of mass $$m = 2\;\text{kg}$$ and length $$L = 0.6\;\text{m}$$. It is clamped at its lower end, so that end acts as a fixed horizontal axis. The upper (free) end is given a slight push and the rod is then allowed to fall only under the action of gravity. Because the clamp is friction-less, mechanical energy is conserved.

At the instant of release the rod is perfectly vertical, pointing upwards from the pivot. After falling through half a rotation (an angle of $$180^{\circ}$$), the rod becomes vertical again but now points downwards. That latter orientation is the position in which the free (upper) end attains its lowest point in the circular path. We wish to find the linear speed of that free end at this lowest position.

First we calculate the change in gravitational potential energy of the rod’s centre of mass (C.M.). For a uniform rod the C.M. lies at its mid-point, i.e. at a distance $$\dfrac{L}{2}$$ from the pivot.

Initial height of C.M. above the pivot:$$h_i = +\frac{L}{2}$$ Final height of C.M. (after a $$180^{\circ}$$ rotation):$$h_f = -\frac{L}{2}$$

Thus the vertical drop of the centre of mass is

$$\Delta h = h_i - h_f = \frac{L}{2} - \left(-\frac{L}{2}\right)=L.$$

The loss in gravitational potential energy is therefore

$$\Delta U = m g \,\Delta h = m g L.$$

Substituting the numerical values,

$$\Delta U = 2\;\text{kg} \times 10\;\text{m s}^{-2} \times 0.6\;\text{m} = 12\;\text{J}.$$

This entire loss of potential energy appears as rotational kinetic energy about the pivot. The formula for rotational kinetic energy is

$$K = \frac12 I \omega^2,$$

where $$I$$ is the moment of inertia of the rod about the pivot and $$\omega$$ is its angular speed at the considered instant.

For a uniform rod hinged about one end, the moment of inertia is given by

$$I = \frac{1}{3} m L^2.$$

Putting the numbers,

$$I = \frac13 \times 2\;\text{kg} \times (0.6\;\text{m})^2 = \frac13 \times 2 \times 0.36\;\text{kg m}^2 = 0.24\;\text{kg m}^2.$$

Conserving mechanical energy, we equate the gravitational energy lost to the rotational kinetic energy gained:

$$m g L = \frac12 I \omega^2.$$

Substituting $$m g L = 12\;\text{J}$$ and $$I = 0.24\;\text{kg m}^2,$$

$$12 = \frac12 \times 0.24 \times \omega^2,$$ $$12 = 0.12\,\omega^2,$$ $$\omega^2 = \frac{12}{0.12} = 100,$$ $$\omega = 10\;\text{rad s}^{-1}.$$

The linear speed of the free end is related to the angular speed by

$$v = \omega L.$$

Hence,

$$v = 10\;\text{rad s}^{-1} \times 0.6\;\text{m} = 6\;\text{m s}^{-1}.$$

So, the answer is $$6$$.

For a solid disc rolling without slipping on an inclined plane, the acceleration is given by $$a = \frac{g\sin\theta}{1 + \frac{I}{mR^2}}$$.

The moment of inertia of a solid disc about its center is $$I = \frac{1}{2}ma^2$$, where $$a$$ is the radius. So $$\frac{I}{ma^2} = \frac{1}{2}$$.

Therefore, $$a = \frac{g\sin\theta}{1 + \frac{1}{2}} = \frac{g\sin\theta}{\frac{3}{2}} = \frac{2}{3}g\sin\theta$$.

Comparing with the given expression $$\frac{2}{b}g\sin\theta$$, we get $$b = 3$$.

The value of $$b$$ is $$\boxed{3}$$.

We need to find the value of $$x$$ representing the ratio of the rotational inertias ($$\frac{I_1}{I_2}$$) for two connected wheels having the same rotational kinetic energy.

1. Analyze the Belt Connection (Linear Velocity)

From the problem, the two wheels $$P$$ and $$Q$$ are linked by a non-slipping belt $$B$$. Because the belt moves at a uniform speed, the linear tangential velocities at the rims of both wheels must be equal:

$$v_P = v_Q$$

Using the rotational relationship $$v = r\omega$$, we can rewrite this equality as:

$$r_1\omega_1 = r_2\omega_2$$

Where:

• $$r_1 = 3R$$ (radius of wheel $$P$$)
• $$r_2 = R$$ (radius of wheel $$Q$$)
• $$\omega_1$$ and $$\omega_2$$ are their respective angular velocities.

Substituting the radius values gives:

$$(3R)\omega_1 = R\omega_2 \implies \frac{\omega_1}{\omega_2} = \frac{1}{3}$$

2. Apply the Rotational Kinetic Energy Condition

The problem states that both wheels possess the same rotational kinetic energy ($$K_{\text{rot}}$$). The formula for rotational kinetic energy is:

$$K_{\text{rot}} = \frac{1}{2}I\omega^2$$

Equating the kinetic energies of both wheels:

$$\frac{1}{2}I_1\omega_1^2 = \frac{1}{2}I_2\omega_2^2$$

3. Calculate the Ratio of Rotational Inertias ($$x$$)

Rearranging the kinetic energy equation to isolate the ratio of moments of inertia ($$\frac{I_1}{I_2}$$):

$$\frac{I_1}{I_2} = \left(\frac{\omega_2}{\omega_1}\right)^2$$

Since we found earlier that $$\frac{\omega_1}{\omega_2} = \frac{1}{3}$$, its reciprocal is:

$$\frac{\omega_2}{\omega_1} = 3$$

Now, substitute this value back into our ratio equation:

$$\frac{I_1}{I_2} = (3)^2 = 9$$

The problem states that the ratio $$\frac{I_1}{I_2}$$ is expressed as $$x : 1$$. Comparing the terms:

$$x = 9$$

Conclusion

The value of $$x$$ is 9.

For a wheel of radius $$R$$ rolling without slipping on a flat surface, the centre of the wheel moves with speed $$v_0$$. The rolling condition gives the angular velocity as $$\omega = v_0/R$$, directed (for rightward motion) clockwise, i.e. $$\vec{\omega} = -\dfrac{v_0}{R}\hat{k}$$.

A particle on the rim at the same height as the centre is positioned either to the left or right of the centre. Consider the particle at position $$\vec{r} = R\hat{i}$$ relative to the centre (i.e., at the rightmost point of the wheel, at the same height as the centre).

The velocity of this particle is the sum of the centre-of-mass velocity and the rotational velocity:

$$\vec{v} = v_0\hat{i} + \vec{\omega} \times \vec{r} = v_0\hat{i} + \left(-\frac{v_0}{R}\hat{k}\right) \times (R\hat{i}) = v_0\hat{i} - v_0(\hat{k}\times\hat{i}) = v_0\hat{i} - v_0\hat{j}$$

The speed is:

$$|\vec{v}| = \sqrt{v_0^2 + v_0^2} = v_0\sqrt{2} = \sqrt{2}\,v_0$$

Comparing with the given form $$\sqrt{x}\,v_0$$, the value of $$x$$ is $$2$$.

$$I = \frac{2}{5}ma^2$$

$$K_i = \frac{1}{2}mv_0^2 + \frac{1}{2}I\omega_0^2$$

$$K_i = \frac{1}{2}mv_0^2 + \frac{1}{2}\left(\frac{2}{5}ma^2\right)\left(\frac{v_0}{a}\right)^2 = \frac{1}{2}mv_0^2 + \frac{1}{5}mv_0^2 = \frac{7}{10}mv_0^2$$

$$K_i = mgh \implies \frac{7}{10}mv_0^2 = mgh \implies h = \frac{7v_0^2}{10g}$$

Distance traveled along the inclined plane ($$s$$):

$$h = s\sin\theta \implies s = \frac{h}{\sin\theta} = \frac{7v_0^2}{10g\sin\theta}$$

We need to find the moments of inertia of four bodies, all having the same mass $$M$$ and radius $$R$$.

$$I_1$$ is the M.I. of a thin circular ring about its diameter. For a thin ring, the mass is distributed uniformly at distance $$R$$ from the centre. By the perpendicular axis theorem, the M.I. about an axis perpendicular to the ring through its centre is $$MR^2$$, and this equals the sum of M.I. about two perpendicular diameters. By symmetry, both diameters give the same M.I. So $$MR^2 = I_d + I_d = 2I_d$$, giving $$I_1 = I_d = \frac{1}{2}MR^2$$.

$$I_2$$ is the M.I. of a circular disc about an axis perpendicular to the disc and passing through its centre. Using the standard integration result for a uniform disc, $$I_2 = \frac{1}{2}MR^2$$.

$$I_3$$ is the M.I. of a solid cylinder about its own axis. Each thin disc element of the cylinder has M.I. $$\frac{1}{2}(dm)R^2$$ about the axis. Summing over all elements, $$I_3 = \frac{1}{2}MR^2$$.

$$I_4$$ is the M.I. of a solid sphere about its diameter. Using the standard result, $$I_4 = \frac{2}{5}MR^2$$.

Now we compare: $$I_1 = I_2 = I_3 = \frac{1}{2}MR^2 = 0.5\,MR^2$$ and $$I_4 = \frac{2}{5}MR^2 = 0.4\,MR^2$$.

Since $$0.5\,MR^2 > 0.4\,MR^2$$, we have $$I_1 = I_2 = I_3 > I_4$$.

Hence, the correct answer is Option C.

At the north pole, the body weighs 49 N. Using $$W = mg$$, the mass of the body is $$m = \frac{49}{9.8} = 5$$ kg.

At the equator, the effective acceleration due to gravity is reduced because of Earth's rotation. The effective gravity at the equator is $$g' = g - R\omega^2$$, where $$R = 6400 \times 10^3$$ m is the radius of Earth and $$\omega = \frac{2\pi}{T}$$ is the angular velocity of Earth with $$T = 24 \times 3600 = 86400$$ s.

Calculating $$\omega = \frac{2\pi}{86400} = 7.27 \times 10^{-5}$$ rad/s. Then $$R\omega^2 = 6.4 \times 10^6 \times (7.27 \times 10^{-5})^2 = 6.4 \times 10^6 \times 5.285 \times 10^{-9} = 0.0338$$ m/s$$^2$$.

So the effective gravity at the equator is $$g' = 9.8 - 0.0338 = 9.766$$ m/s$$^2$$.

The weight at the equator is $$W' = mg' = 5 \times 9.766 = 48.83$$ N.

The correct answer is 48.83 N.

We need to determine the total time it takes for a solid rolling sphere to travel up an inclined plane and return to its starting point $$A$$.

1. Identify the Physical Properties and System Parameters

From the problem statement, we have:

• Mass of the sphere ($$m$$) = $$2\text{ kg}$$
• Radius of the sphere ($$R$$) = $$0.5\text{ m}$$
• Initial linear speed ($$u$$) = $$1\text{ m s}^{-1}$$
• Angle of inclination ($$\theta$$) = $$30^\circ$$
• Acceleration due to gravity ($$g$$) = $$9.8\text{ m s}^{-2}$$ (standard value used for precise decimal options)

For a solid uniform sphere, the moment of inertia about its center is given by:

$$I = \frac{2}{5}mR^2$$

2. Calculate Retardation During Pure Rolling Up the Incline

When an object rolls up an inclined plane without slipping, both gravity and static friction act on it. The linear acceleration ($$a$$) of a rolling body down an incline is given by the formula:

$$a = \frac{g \sin\theta}{1 + \frac{I}{mR^2}}$$

Substitute the ratio $$\frac{I}{mR^2} = \frac{2}{5}$$ into the acceleration formula:

$$a = \frac{g \sin 30^\circ}{1 + \frac{2}{5}} = \frac{g \left(\frac{1}{2}\right)}{\frac{7}{5}} = \frac{5}{14}g$$

Plugging in $$g = 9.8\text{ m s}^{-2}$$:

$$a = \frac{5}{14} \times 9.8 = 5 \times 0.7 = 3.5\text{ m s}^{-2}$$

3. Determine Time to Return to Point A

Let $$t_1$$ be the time taken by the sphere to come to rest at the highest point of its climb. Using the first equation of motion ($$v = u - at_1$$) where final velocity $$v = 0$$:

$$0 = 1 - 3.5 \cdot t_1 \implies t_1 = \frac{1}{3.5} = \frac{2}{7}\text{ s}$$

Since the system is conservative and rolls without slipping in both directions, the time of ascent ($$t_1$$) equals the time of descent ($$t_2$$). Therefore, the total round-trip time ($$T$$) to return to point $$A$$ is:

$$T = 2 \cdot t_1 = 2 \times \frac{2}{7} = \frac{4}{7}\text{ s} \approx 0.571\text{ s}$$

Rounding to two decimal places closest to the calculated metric options yields approximately $$0.56\text{ s}$$.

Conclusion

The total time the sphere takes to return to the starting point is 0.56 s, which corresponds to Option C.

We need to determine the torque acting at point $$P$$ with respect to point $$O$$ and point $$Q$$ on an equilateral triangular plate.

1. Identify the Geometry and Vector Positions

From the diagram on the, the plate forms an equilateral triangle with side lengths of $$10\text{ cm} = 0.1\text{ m}$$ and interior angles of $$60^\circ$$. Let's establish the coordinate positions by placing the origin at point $$O(0,0)$$:

• Point O: $$(0, 0)$$
• Point Q: $$(10, 0)\text{ cm} = (0.1, 0)\text{ m}$$
• Point P: Located using trigonometric components along the side $$OP$$:

  $$x_P = 10 \cos 60^\circ = 5\text{ cm} = 0.05\text{ m}$$

  $$y_P = 10 \sin 60^\circ = 5\sqrt{3}\text{ cm} = 0.05\sqrt{3}\text{ m}$$

  Therefore, the position vector of $$P$$ relative to $$O$$ is: $$\vec{r}_{P/O} = 0.05\hat{i} + 0.05\sqrt{3}\hat{j}$$

The applied force vector given in the problem is:

$$\vec{F} = 4\hat{i} - 3\hat{j}$$

2. Calculate Torque with respect to Point O ($$\vec{\tau}_O$$)

Torque is defined by the cross product $$\vec{\tau} = \vec{r} \times \vec{F}$$:

$$\vec{\tau}_O = \vec{r}_{P/O} \times \vec{F} = \det \begin{bmatrix} \hat{i} & \hat{j} & \hat{k} \\ 0.05 & 0.05\sqrt{3} & 0 \\ 4 & -3 & 0 \end{bmatrix}$$

$$\vec{\tau}_O = \left[ 0.05 \times (-3) - 0.05\sqrt{3} \times 4 \right] \hat{k}$$

$$\vec{\tau}_O = \left( -0.15 - 0.20\sqrt{3} \right) \hat{k}\text{ N}\cdot\text{m}$$

Converting the units into $$\text{N}\cdot\text{cm}$$ to match the option formats (by multiplying the values by $$100$$):

$$\tau_O = -15 - 20\sqrt{3}$$

3. Calculate Torque with respect to Point Q ($$\vec{\tau}_Q$$)

First, find the position vector of $$P$$ relative to $$Q$$:

$$\vec{r}_{P/Q} = \vec{r}_P - \vec{r}_Q = (0.05 - 0.1)\hat{i} + 0.05\sqrt{3}\hat{j} = -0.05\hat{i} + 0.05\sqrt{3}\hat{j}$$

Now, compute the cross product for torque about $$Q$$:

$$\vec{\tau}_Q = \vec{r}_{P/Q} \times \vec{F} = \det \begin{bmatrix} \hat{i} & \hat{j} & \hat{k} \\ -0.05 & 0.05\sqrt{3} & 0 \\ 4 & -3 & 0 \end{bmatrix}$$

$$\vec{\tau}_Q = \left[ (-0.05) \times (-3) - 0.05\sqrt{3} \times 4 \right] \hat{k}$$

$$\vec{\tau}_Q = \left( 0.15 - 0.20\sqrt{3} \right) \hat{k}\text{ N}\cdot\text{m}$$

Converting the units into $$\text{N}\cdot\text{cm}$$ gives:

$$\tau_Q = 15 - 20\sqrt{3}$$

Conclusion

The torque values at point $$P$$ relative to $$O$$ and $$Q$$ are $$-15 - 20\sqrt{3}$$ and $$15 - 20\sqrt{3}$$ respectively. This corresponds perfectly to Option A.

We begin by recalling the definition of moment of inertia. For any rigid body about a given axis we write the relation

$$I = M\,k^{2}$$

where $$I$$ is the moment of inertia, $$M$$ is the total mass of the body, and $$k$$ is the radius of gyration about that axis. The radius of gyration therefore depends directly on the numerical value of the moment of inertia about the chosen axis.

Now we consider a uniform thin circular disc of mass $$M$$ and radius $$R$$. Its centre is the origin, its plane is the X-Y plane, so the Z-axis passes through its centre perpendicular to the plane. The X- and Y-axes are two mutually perpendicular diameters lying in the plane of the disc.

First, we write down the standard results for the moments of inertia of such a disc (these results can be obtained by integration, but are quoted here as standard):

$$$I_{Z} = \frac12\,M R^{2} \qquad\text{(about the Z-axis)}$$$

$$$I_{X} = I_{Y} = \frac14\,M R^{2} \qquad\text{(about any diameter in the plane, e.g. X or Y axis)}$$$

We next find the radii of gyration corresponding to each axis by substituting these values into the defining equation $$I = M\,k^{2}$$.

For the Z-axis we have

$$$k_{Z}^{2} = \frac{I_{Z}}{M} = \frac{\dfrac12\,M R^{2}}{M} = \frac12\,R^{2}$$$

so

$$k_{Z} = \sqrt{\frac12}\,R = \frac{R}{\sqrt2}\,.$$

For the X-axis we write

$$$k_{X}^{2} = \frac{I_{X}}{M} = \frac{\dfrac14\,M R^{2}}{M} = \frac14\,R^{2}$$$

and therefore

$$k_{X} = \sqrt{\frac14}\,R = \frac{R}{2}\,.$$

The Y-axis expression is identical because $$I_{Y} = I_{X}$$, giving

$$k_{Y} = \frac{R}{2}\,.$$

So we finally have

$$k_{X} = k_{Y} = \frac{R}{2}, \qquad k_{Z} = \frac{R}{\sqrt2}\,. $$

These three values are clearly not equal because

$$\frac{R}{2} \neq \frac{R}{\sqrt2}\,. $$

Therefore the assertion “the respective radii of gyration about all the three axes will be the same” is false.

We now examine the reason. The statement “A rigid body making rotational motion has fixed mass and shape” is always true; the mass and the geometrical size of a rigid body do not change during rotation. Hence the reason is a correct factual statement, although it has no logical connection with the equality (or inequality) of the radii of gyration.

We thus have: Assertion A is not correct, while Reason R is correct but does not explain A.

Hence, the correct answer is Option B.

We recall the standard formula for the moment of inertia of a uniform slender rod of length $$l$$ and mass $$m$$ about an axis perpendicular to the rod:

1. When the axis passes through the mid-point of the rod, the formula is

$$I_{\text{mid}}=\frac{1}{12}\,m\,l^{2}.$$

2. When the axis passes through one end of the rod, the formula is

$$I_{\text{end}}=\frac{1}{3}\,m\,l^{2}.$$

With these two relations ready, we now evaluate each item in List-I one by one and match with List-II.

Term (a): The rod has length $$L$$ and mass $$M$$. The axis is perpendicular to the rod and passes through its mid-point, so we use the mid-point formula.

$$I_{(a)} \;=\;\frac{1}{12}\,M\,L^{2}.$$

Checking List-II, we see that $$\frac{ML^{2}}{12}$$ is entry (iii). So, (a) corresponds to (iii).

Term (b): The rod has the same length $$L$$ but double the mass, i.e. $$2M$$. The axis is at one end, so we use the end-point formula.

First write the formula: $$I_{\text{end}}=\frac{1}{3}\,m\,l^{2}.$$ Now substitute $$m=2M$$ and $$l=L$$:

$$I_{(b)} \;=\;\frac{1}{3}\,(2M)\,L^{2} =\;\frac{2\,M\,L^{2}}{3}.$$

Looking at List-II, $$\dfrac{2ML^{2}}{3}$$ is entry (iv). Therefore, (b) matches (iv).

Term (c): The rod now has length $$2L$$ while the mass is $$M$$. The axis again passes through the mid-point, so we use the mid-point formula.

Formula: $$I_{\text{mid}}=\frac{1}{12}\,m\,l^{2}.$$ Substituting $$m=M$$ and $$l=2L$$ gives

$$I_{(c)} \;=\;\frac{1}{12}\,M\,(2L)^{2} =\;\frac{1}{12}\,M\,(4L^{2}) =\;\frac{4}{12}\,M\,L^{2} =\;\frac{1}{3}\,M\,L^{2}.$$

In List-II, $$\dfrac{ML^{2}}{3}$$ is entry (ii). Thus, (c) corresponds to (ii).

Term (d): The rod now has length $$2L$$ and mass $$2M$$, with the axis at one end. Hence we again use the end-point formula.

Formula: $$I_{\text{end}}=\frac{1}{3}\,m\,l^{2}.$$ Substituting $$m=2M$$ and $$l=2L$$ yields

$$I_{(d)} \;=\;\frac{1}{3}\,(2M)\,(2L)^{2} =\;\frac{2}{3}\,M\,(4L^{2}) =\;\frac{8}{3}\,M\,L^{2}.$$

The expression $$\dfrac{8ML^{2}}{3}$$ appears in List-II as entry (i). Consequently, (d) matches with (i).

Gathering all the matches we have obtained:

(a) → (iii),  (b) → (iv),  (c) → (ii),  (d) → (i).

When we inspect the four options, we see that this exact sequence is given in Option C.

Hence, the correct answer is Option C.

We need to analyze the characteristics of the angular momentum of a mass $$M$$ about two different reference points, $$A$$ and $$B$$, during steady conical pendulum-like circular motion.

1. Analyze Angular Momentum about Point A ($$\vec{L}_A$$)

Point $$A$$ lies at the center of the circular path of the mass $$M$$ along the vertical axis of rotation ($$z$$-axis).

• The position vector $$\vec{r}_A$$ of the mass relative to point $$A$$ lies entirely within the horizontal plane of rotation.
• The linear velocity vector $$\vec{v}$$ is tangential to the circle at every point.
• By definition, the angular momentum vector is:

  $$\vec{L}_A = \vec{r}_A \times \vec{p} = m(\vec{r}_A \times \vec{v})$$

• Using the right-hand rule, the direction of $$\vec{L}_A$$ points straight up along the positive $$z$$-axis at all times. Since the speed and radius are constant, its magnitude ($$m v r_A$$) and its direction remain completely fixed throughout the motion.

2. Analyze Angular Momentum about Point B ($$\vec{L}_B$$)

Point $$B$$ is located on the vertical rotation axis but at a height above the plane of the circle (the suspension point of the rod).

• The position vector $$\vec{r}_B$$ points from $$B$$ down to the mass $$M$$, tracing out the surface of a cone during motion.
• The angular momentum about $$B$$ is:

  $$\vec{L}_B = m(\vec{r}_B \times \vec{v})$$

• Since $$\vec{r}_B$$ and $$\vec{v}$$ are always perpendicular, the magnitude of $$\vec{L}_B$$ is constant ($$m v l$$).
• However, by the right-hand rule, the vector $$\vec{L}_B$$ is always perpendicular to the rod. As the mass rotates, the vector $$\vec{L}_B$$ sweeps out a cone around the vertical $$z$$-axis. Because its orientation is continuously changing in space, $$\vec{L}_B$$ is not constant in direction.

Conclusion

Therefore, $$\vec{L}_A$$ remains constant both in magnitude and direction, while $$\vec{L}_B$$ changes its direction continuously. This corresponds to the correct choice on the  page:

$$\vec{L}_A$$ is constant, both in magnitude and direction (Option D).

We are asked to find the moment of inertia of the whole system about an axis that is perpendicular to the light rod and passes through the midpoint of the rod. The system consists of two identical solid spheres attached to the two ends of the rod, so we have to calculate the contribution of each sphere and then add the two contributions.

First, recall the two formulas we will need:

1. For a solid sphere about its own centre, the moment of inertia is given by $$I_{\text{cm}}=\dfrac{2}{5}\,mR^{2}.$$

2. The parallel-axis theorem (Steiner’s theorem) states that if an axis is displaced from the centre of mass by a distance $$d,$$ then the new moment of inertia is $$I = I_{\text{cm}} + m d^{2}.$$

Now we insert the numerical data. For each sphere:

Mass: $$m = 1.5\ \text{kg}.$$

Radius: $$R = 50\ \text{cm} = 0.5\ \text{m}.$$

The moment of inertia about its own centre is therefore

$$I_{\text{cm}} = \dfrac{2}{5}\,mR^{2} = \dfrac{2}{5}\,(1.5)\,(0.5)^{2}.$$

We calculate step by step:

$$(0.5)^{2} = 0.25,$$

$$\dfrac{2}{5} = 0.4,$$

$$0.4 \times 0.25 = 0.10,$$

$$0.10 \times 1.5 = 0.15.$$

So $$I_{\text{cm}} = 0.15\ \text{kg m}^{2}.$$

The distance of the centre of each sphere from the given axis: the two centres are 5 m apart, and the axis is at the midpoint of the rod, so each centre is

$$d = \dfrac{5\ \text{m}}{2} = 2.5\ \text{m}$$

away from the axis.

Using the parallel-axis theorem, the moment of inertia of one sphere about the required axis is

$$I_{\text{one}} = I_{\text{cm}} + m d^{2} = 0.15 + (1.5)(2.5)^{2}.$$

Calculating the term $$m d^{2}:$$

$$d^{2} = (2.5)^{2} = 6.25,$$

$$m d^{2} = 1.5 \times 6.25 = 9.375.$$

Adding $$I_{\text{cm}}$$ to this:

$$I_{\text{one}} = 0.15 + 9.375 = 9.525\ \text{kg m}^{2}.$$

Because there are two identical spheres, the total moment of inertia of the system is

$$I_{\text{total}} = 2\,I_{\text{one}} = 2 \times 9.525 = 19.05\ \text{kg m}^{2}.$$

Hence, the correct answer is Option C.