SAT Subject Physics Practice Question 63: Answer and Explanation

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Question: 63

10. A simple pendulum of length L and mass m aswings about the vertical equilibrium position (θ = 0) with a maximum angular displacement of θmax. What is the tension in the connecting rod when the pendulum’s angular displacement is θ = θmax ?

A. mg sinθmax
B. mg cosθmax
C. mgL sinθmax
D. mgL cosθmax
E. mgL(1 – cosθmax)

Correct Answer: B


First draw a free-body diagram.

The net force toward the center of the bob’s circular path is FTmg cosθmax. This must provide the centripetal force, . But since the speed of the bob at this moment is zero (v = 0), we get FT = mgcosθmax. (The acceleration is purely tangential here, equal to = g sinθmax.)

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