AP Physics 1 Question 132: Answer and Explanation

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Question: 132


A rigid rod is pivoted at its right end. Three forces of identical magnitude but different directions are applied at the positions 1, 2, and 3 as shown. Which of the following correctly ranks the torques τ1, τ2, and τ3 provided by the forces F1, F2, and F3?

  • A. τ1 > τ2 > τ3
  • B. τ3 > τ2 > τ1
  • C. τ2 > τ1 > τ3
  • D. τ2 > τ1 = τ3

Correct Answer: A


A-Torque is force times distance from a fulcrum; but that force must be perpendicular to the rod, so in this case the force used in the equation will be the vertical component, which includes a sin 45o term for F1 and F3. The sine of 45o is 0.7; call the length of the rod L, so F2 is a distance L/2 from the pivot, and F3 is about L/4 from the pivot. So τ1 = 0.7FL. τ2 = 0.5FL. τ3 = (0.7·0.25)FL.