SAT Subject Physics Practice Question 215: Answer and Explanation

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Question: 215


One end of a rigid, massless rod of length 50 cm is attached to the edge of the table at point O; at the other end of the rod is a ball of clay of mass m = 0.2 kg. The rod extends horizontally from the end of the table. What is the torque of the gravitational force on the clay ball relative to point O?

A. 0.01 N-m
B. 0.1 N-m
C. 1 N-m
D. 10 N-m
E. 100 N-m

Correct Answer: C


The torque is equal to rF, where F = mg. In this case, then, we find that torque = rmg = (0.5 m)(0.2 kg)(10 N/kg) = 1 N-m.

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