SAT Subject Physics Practice Question 249: Answer and Explanation

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Question: 249


A block is moving counter-clockwise in a circular path on a flat table. If the speed of the block is increasing at the moment it is at the position shown, which one of the five arrows best illustrates the direction of the acceleration on the block?

A. A
B. B
C. C
D. D
E. E

Correct Answer: B


Because the block moves in a circular path, it must experience a centripetal acceleration (that is, an acceleration directed toward the center of the circle). Now, since the speed of the block is changing, it must also be experiencing a tangential acceleration; in particular, because the block’s speed is increasing, this tangential acceleration must be in the same direction as the block’s velocity. Therefore, the total acceleration of the block is the sum of the centripetal acceleration ac and the tangential acceleration at. The figure below shows that this sum points upward and to the left, so arrow B is best.

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