SAT Subject Physics Practice Question 14: Answer and Explanation

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Question: 14

5. The coefficient of static friction between a box and a ramp is 0.5. The ramp’s incline angle is 30°. If the box is placed at rest on the ramp, the box will

A. accelerate down the ramp
B. accelerate briefly down the ramp but then slow down and stop
C. move with constant velocity down the ramp
D. not move
E. Cannot be determined from the information given

Correct Answer: A

Explanation:

The force pulling the block down the ramp is mg sinθ, and the maximum force of static friction is μsFN = μsmg cosθ. If mg sin θ is greater than μsmg cosθ, then there is a net force down the ramp, and the block will accelerate down. So, the question becomes, “Is sinθ greater than μs cosθ?” Since θ = 30° and μs = 0.5, the answer is yes.

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