# 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 *v*^{2}. 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 *t*^{2}, 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.