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A man is in a moving train. Then, with respect to the train:

1. his momentum must not be zero
2. his kinetic energy is zero
3. his kinetic energy is not zero
4. his kinetic energy may be zero

Correct answer: his kinetic energy may be zero

Solution

Kinetic energy depends on speed in the chosen frame. With respect to the train, a man standing still has zero speed and hence zero kinetic energy, but if he is walking inside the train, it is non-zero. Therefore it may be zero.

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