SAT Subject Physics Practice Question 162: Answer and Explanation

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Question: 162

8. If an object’s speed is changing, which of the quantities could remain constant?

A. Displacement
B. Velocity
C. Acceleration
D. Linear momentum
E. Kinetic energy

Correct Answer: C


If an object’s speed is changing, it must be moving, so its displacement is changing. Since velocity, v, is speed plus direction, a change in speed automatically means a change in velocity. Now, since the velocity is changing, the linear momentum, p, must be changing also, since p = mv. Finally, because kinetic energy is equal to times the square of the speed, a changing speed implies a changing kinetic energy. The answer must be C. An object undergoing a change in speed may certainly be subject to a constant acceleration (gravitational acceleration, for example).

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