SAT Subject Physics Practice Question 3: Answer and Explanation

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Question: 3

3. Which of the following must always be true?

I. If an object’s acceleration is constant, then it must move in a straight line.

II. If an object’s acceleration is zero, then its speed must remain constant.

III. If an object’s speed remains constant, then its acceleration must be zero.

A. I and II only
B. I and III only
C. II only
D. III only
E. II and III only

Correct Answer: C


I is false since a projectile experiencing only the constant acceleration due to gravity can travel in a parabolic trajectory. II is true: Zero acceleration means no change in speed (or direction). III is false: An object whose speed remains constant but whose velocity vector is changing direction is accelerating.

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