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A bullet of mass 10 g is moving with a speed u when it enters a bunch of a certain identical fixed wooden blocks kept in sequence in line. The velocity of the bullet drops to zero as it just leaves the third plank. How many such planks will the same bullet can penetrate when the initial speed of the bullet is doubled (assume all the planks to be fixed on the floor and the bullet travels horizontally)
Each plank removes the same amount of kinetic energy, so the number of planks penetrated is proportional to $$\frac{1}{2}mu^2$$. Doubling the speed makes the kinetic energy 4 times larger, so the bullet can penetrate $$4 \times 3 = 12$$ planks.
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