SAT Subject Physics Practice Question 523: Answer and Explanation

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Question: 523

9. An object at rest is placed into free fall at a height of20 m. What is the velocity of the object when the potential energy equals the kinetic energy?

A. 8 m/s
B. 11 m/s
C. 14 m/s
D. 17 m/s
E. There is not enough information to determine the velocity.

Correct Answer: C

Explanation:

The correct answer is (C). The problem may be solved in several ways. Perhaps the best solution is a little thought. E.g., the potential energy and the kinetic energy are equal to each other when the object has fallen half the distance to the ground, or 10 m. We now treat it as a free fall problem by restating the situation a little. What is the velocity of an object that starts from rest and free falls a distance of 10 m? The solution is found by

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