SAT Subject Physics Practice Question 171: Answer and Explanation

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Question: 171

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

If the blocks had instead stuck together after the collision, with what speed would they move if m = 12 kg ?

A. 2.0 m/s
B. 2.7 m/s
C. 3.2 m/s
D. 4.0 m/s
E. 4.6 m/s

Correct Answer: A


If the blocks stick together after the collision—a perfectly inelastic collision—then conservation of momentum gives us 32 kg-m/s = (4 + m)v, where v denotes the common speed of the blocks after the collision. If m = 12 kg, then v = 32/(4 + 12) = 32/16 = 2 m/s.

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