SAT Subject Physics Practice Question 42: Answer and Explanation

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Question: 42

8. A wooden block of mass M is moving at speed V in a straight line.

How fast would the bullet of mass m need to travel to stop the block (assuming that the bullet became embedded inside)?

A. mV/(m + M)
B. MV/(m + M)
C. mV/M
D. MV/m
E. (m + M)V/m

Correct Answer: D


The linear momentum of the bullet must have the same magnitude as the linear momentum of the block in order for their combined momentum after impact to be zero. The block has momentum MV to the left, so the bullet must have momentum MV to the right. Since the bullet’s mass is m, its speed must be v = .

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