SAT Subject Physics Practice Question 261: Answer and Explanation

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Question: 261


The figure above shows a square metal plate of side length 40 cm and uniform density, lying flat on a table. A force F of magnitude 10 N is applied at one of the corners, parallel to one of the sides, as shown. What’s the torque produced by F relative to the center of the square?

A. 0 N-m
B. 1.0 N-m
C. 1.4 N-m
D. 2.0 N-m
E. 4.0 N-m

Correct Answer: D


Since the line of action of the force coincides with one of the sides of the square, the lever arm of the force, ℓ (the distance from the center of the square to the bottom side), is simply equal to s, half the length of each side of the square. This gives us

τ = ℓF = (0.40 m)(10 N) = 2.0 N-m

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