SAT Subject Physics Practice Question 18: Answer and Explanation

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Question: 18

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.
E. None of the above

Correct Answer: D

Explanation:

The maximum force that static friction can exert on the crate is μsFN = μsFw = μs mg = (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. [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.]

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