SAT Subject Physics Practice Question 215: Answer and Explanation
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.