SAT Subject Physics Practice Question 169: Answer and Explanation

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Question: 169

15. It refers to the collision of two blocks on a frictionless table. Before the collision, the block of mass m is at rest.

What is the total momentum of the blocks just AFTER the collision?

A. 12 kg-m/s
B. 16 kg-m/s
C. 18 kg-m/s
D. 24 kg-m/s
E. 32 kg-m/s

Correct Answer: E


Using the equation p = mv, we can figure out that before the collision, the momentum of the left-hand block was (4 kg)(8 m/s) = 32 kg-m/s, and that of the right-hand block was zero (since it was at rest), so the total momentum before the collision was 32 kg-m/s. Since total momentum is conserved in the collision, the total momentum after the collision must also be 32 kg-m/s.

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