Common JEE Mistakes in Work, Energy and Power

Common JEE Mistakes in Work, Energy and Power

### Mistake 1

Considering displacement direction incorrectly.

### Correct Concept

$$W=Fs\cos\theta$$

---

### Mistake 2

Using conservation of mechanical energy with friction.

### Correct Condition

$$W_{friction}=0$$

must be satisfied.

---

### Mistake 3

Confusing force and energy units.

Force:

$$N=kg\,m\,s^{-2}$$

Energy:

$$J=kg\,m^2s^{-2}$$

---

### Mistake 4

Assuming negative work means negative energy.

### Correct Concept

Negative work only indicates energy removal from the system.

# 37. Instantaneous Work and Power Relations

### Formula

$$dW=\vec{F}\cdot d\vec{s}$$

### Power Relation

$$P=\frac{dW}{dt}$$

### Terminologies

- dW → Small amount of work done.

- ds → Small displacement.

- P → Instantaneous power.

### Conditions / Special Cases

For variable force:

$$P=\vec{F}\cdot\vec{v}$$

### Usage

- Used when force or velocity changes continuously.