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A body of mass 5 kg moving with velocity 10 m/s collides with another body of mass 20 kg at rest and comes to rest. The velocity of the second body after the collision is:

1. 2.5 m/s
2. 5 m/s
3. 7.5 m/s
4. 10 m/s

Correct answer: 2.5 m/s

Solution

The initial momentum is $5\times10=50$ kg m/s. Since the first body comes to rest, the second body must have momentum 50 kg m/s, giving speed $50/20=2.5$ m/s.

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