SAT Subject Physics Practice Question 46: Answer and Explanation

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Question: 46

3.

In the figure above, what is the torque about the pendulum’s suspension point produced by the weight of the bob, given that the mass is 40 cm below the suspension point, measured vertically, and m = 0.50 kg ?

A. 0.49 N × m
B. 1.15 N × m
C. 1.7 N × m
D. 2.0 N × m
E. 3.4 N × m

Correct Answer: B

Explanation:

From the diagram, sinθ = 40 cm/L, so Lsinθ = 40 cm. That is, the question gave the lever arm already. (The given 60° angle was irrelevant.) Thus, from τ = Fℓ, the torque is (0.40 m)(0.50 kg)(10 m/s2) = 2.0 N • m.

The torque is (0.40/sqrt(3) m)(0.50 kg)(10 m/s2) = 1.15 N • m

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