# A body of 1 kg moving at 4 m/s strikes a stationary body of 750 g. The first body stops and the other moves at 5 m/s. What is the coefficient of kinetic friction.

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### 1 Answer

A body of mass 1 kg is moving at 4 m/s and it collides with a stationary body with a mass of 750 g or 0.75 kg. After collision, the first body comes to a stop and the second moves at 5 m/s.

If friction could be ignored here, the law of conservation of kinetic energy would ensure that total kinetic energy is conserved. As the total initial kinetic energy is 1*4^2 = 16 J, the total final kinetic energy would be 16 J too. But the total final kinetic energy is 0.75*5^2 = 0.75*25 = 18.75 J. It is seen that due to the collision the total kinetic energy in the system has increased by 2.75 J.

This is not possible in an isolated system.

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