SAT Subject Physics Practice Question 294: Answer and Explanation

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Question: 294

65.

An object of mass 2m moving with velocity U strikes an object of mass m initially at rest. After the collision, the objects move away with velocities u and v, as shown.

Which one of the following equations correctly relates u and v?

A. 2u cos 30° = v cos 60°
B. u cos 30° = 2v cos 60°
C. 2u sin 30° = v sin 60°
D. u sin 30° = 2v sin 60°
E. u sin 30° = v cos 60°

Correct Answer: C

Explanation:

Momentum is conserved in the collision. Before the collision, the vertical component of the momentum was zero (since there was only one moving object and it was moving horizontally only). Therefore, the total vertical momentum after the collision must be zero also. The vertical component of the momentum of the 2m mass after the collision is (2m)(u sin30°), and the vertical component of the momentum of the other mass after the collision is (m)(−v sin60°). Since the total vertical momentum after the collision is zero, we find that

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