SAT Subject Physics Practice Question 47: Answer and Explanation

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Question: 47


A uniform meter stick of mass 1 kg is hanging from a thread attached at the stick’s midpoint. One block of mass m = 3 kg hangs from the left end of the stick, and another block, of unknown mass m, hangs below the 80 cm mark on the meter stick. If the stick remains at rest in the horizontal position shown above, what is m ?

A. 4 kg
B. 5 kg
C. 6 kg
D. 8 kg
E. 9 kg

Correct Answer: B


The stick will remain at rest in the horizontal position if the torques about the suspension point are balanced.

Note that the center of mass formula could also be used to find m2, though it would require algebra. Typically, the center of mass forma is useful for determining where the balance point is. If the balance point is known, then torque is usually easier.

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