# 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

**Explanation:**

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.