SAT Subject Physics Practice Question 258: Answer and Explanation

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Question: 258


A small box slides down a frictionless track in the shape of a quarter-circle of radius R. The box starts from rest at the top of the track, a height equal to 2R above a horizontal surface. At the moment the box leaves the bottom of the track, a ball of the same mass as the box is dropped from the same height at the bottom of the track.

Which of the following quantities must decrease as the box slides down the track?

A. The normal force on the box
B. The net force on the box
C. The kinetic energy of the box
D. The potential energy of the box
E. The total mechanical energy (kinetic + potential) of the box

Correct Answer: D


As the box slides down the track, it loses height, so it loses gravitational potential energy. The quantities in A, B, and C increase as the box slides down the track, and the quantity in E remains constant.

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