SAT Subject Physics Practice Question 32: Answer and Explanation

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Question: 32

9. An astronaut drops a rock from the top of a crater on the moon. When the rock is halfway down to the bottom of the crater, its speed is what fraction of its final impact speed?

A.
B. 1/4
C.
D. 1/2
E.

Correct Answer: E

Explanation:

Because the rock has lost half of its gravitational potential energy, its kinetic energy at the halfway point is half of its kinetic energy at impact. Since K is proportional to v2, if Kat halfway point is equal to Kat impact, then the rock’s speed at the halfway point is its speed at impact.

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