AP Physics 1 Question 69: Answer and Explanation

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Question: 69

9. A girl of mass m and a boy of mass 2m are sitting on opposite sides of a see-saw with its fulcrum in the center. Right now, the boy and girl are equally far from the fulcrum, and it tilts in favor of the boy. Which of the following would NOT be a possible method to balance the see-saw?

  • A. Move the boy to half his original distance from the fulcrum.
  • B. Move the girl to double her original distance from the fulcrum.
  • C. Allow a second girl of mass m to join the first.
  • D. Move the fulcrum to half its original distance from the boy.

Correct Answer: D


D To balance the see-saw, we need to balance the torques. Since T = Fr sin θ, the boy currently provides double the torque. Choices (B) and (C) would double the torque on the girl's side, and (A) would cut the boy's torque in half. Choice (D) would cut the boy's torque in half, but it would also increase the girl's torque, creating a new imbalance.