AP Physics 1 Question 209: Answer and Explanation
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9. A crate of mass 100 kg is at rest on a horizontal floor. The coefficient of static friction between the crate and the floor is 0.4, and the coefficient of kinetic friction is 0.3. A force F of magnitude 344 N is then applied to the crate, parallel to the floor. Which of the following is true?
- A. The crate will accelerate across the floor at 0.5 m/s2.
- B. The static friction force, which is the reaction force to F as guaranteed by Newton's Third Law, will also have a magnitude of 344 N.
- C. The crate will slide across the floor at a constant speed of 0.5 m/s.
- D. The crate will not move.
Correct Answer: D
The maximum force which static friction can exert on the crate is μsFN = μsFw = μsmg = (0.4)(100 kg)(10 N/kg) = 400 N. Since the force applied to the crate is only 344 N, static friction is able to apply that same magnitude of force on the crate, keeping it stationary. Choice (B) is incorrect because the static friction force is not the reaction force to F; both F and Ff (static) act on the same object (the crate) and therefore cannot form an action/reaction pair.