AP Physics 1 Question 79: Answer and Explanation
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9. Which of the following best describes the relationship between the magnitude of the tension force, FT, in the string of a pendulum and the radial component of gravity that pulls antiparallel to the tension, Fg, radial? Assume that the pendulum is only displaced by a small amount.
- A. FT > Fg, radial
- B. FT ≥ Fg, radial
- C. FT = Fg, radial
- D. FT ≤ Fg, radial
Correct Answer: B
B Think of the pendulum as undergoing circular motion. The centripetal force would be equal to the difference between tension and the radial component of gravity.
So long as the pendulum is in motion (as long as v > 0), tension must have a larger magnitude than the radial component of gravity. However, when v = 0 (at the maximum displacement), the two will be equal. Therefore, (B) is the correct answer.