SAT Subject Physics Practice Question 157: Answer and Explanation

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Question: 157

3. For an object traveling in a straight line, its velocity (v, in m/s) as a function of time (t, in s) is given by the following graph.

Which graph best depicts the object’s kinetic energy?

A.
B.
C.
D.
E.

Correct Answer: B

Explanation:

Kinetic energy is proportional to v2. Since the first part of the v versus t graph is a straight line, it must have the form v = at for some constant, a. Squaring this gives us something proportional to t2, the graph of which is parabolic. This eliminates A and D. Next, since v drops to 0 in the original graph, the kinetic energy must also drop to 0, so now E is eliminated. Finally, we can eliminate the graph in C, because if it were correct, it would mean that the object had a constant kinetic energy for the latter part of its motion (since the graph is flat); but the original graph shows us that v is never constant.

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