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A body of mass $M$ collides with a wall with velocity $v$ and retraces its path with the same speed. Taking the initial direction of velocity as positive, the change in momentum is: A. zero B. $2Mv$ C. $Mv$ D. $-2Mv$

1. zero
2. 2Mv
3. Mv
4. -2Mv

Correct answer: -2Mv

Solution

Initial momentum is $+Mv$. After rebounding with the same speed, the final momentum is $-Mv$. Therefore, change in momentum is $-Mv - (+Mv) = -2Mv$.

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