AP Physics 1 Question 205: Answer and Explanation
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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 do which of the following?
- 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
Correct Answer: A
The force pulling the block down the ramp is mg sin θ, and the maximum force of static friction is μsFN = μsmg cos θ. If mg sin θ is greater than μsmg cos θ, then there is a net force down the ramp, and the block will accelerate down. So, the question becomes, "Is sin θ greater than μs cos θ?" Since θ = 30° and μs = 0.5, the answer is "yes."