# SAT Subject Physics Practice Question 262: Answer and Explanation

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**Question: 262**

**33.** A mover, exerting a steady force of 200 N, pushes a box of mass 50 kg across a flat wooden floor. If the velocity of the box does not change while he pushes, what is the coefficient of kinetic friction between the box and the floor?

A. 0.2

B. 0.4

C. 0.5

D. 0.6

E. 0.8

**Correct Answer:** B

**Explanation:**

Since the velocity does not change, the acceleration of the box is zero, so the net force the box feels must also be zero. Since the mover pushes on the box with a force of 200 N, the force of kinetic friction must also be 200 N (in the opposite direction) to give a net force of zero. Because the strength of the force of kinetic friction, *F*_{friction}, is equal to μ*F*_{N}, where *F*_{N} = *mg* (since the floor is flat), we find that