AP Physics 1 Question 98: Answer and Explanation

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Question: 98


Bob and Tom hold a rod with a length of 8 m and weight of 500 N. Initially, Bob and Tom each hold the rod 2 m from the its ends, as shown in the figure. Next, Tom moves slowly toward the right edge of the rod, maintaining his hold. As Tom moves to the right, what happens to the torque about the rod’s midpoint exerted by each person?

  • A. Bob’s torque decreases, and Tom’s torque increases.
  • B. Bob’s torque increases, and Tom’s torque decreases.
  • C. Both Bob’s and Tom’s torque increases.
  • D. Both Bob’s and Tom’s torque decreases.

Correct Answer: C


C-The net torque on the rod must be zero, because the rod doesn't rotate. Therefore, whatever happens to Bob's torque must also happen to Tom's torque-these torques must cancel each other out. That eliminates choices A and B. The forces provided by Bob and Tom must add up to 500 N, the weight of the rod. Try doing two quick calculations: In the original case, Bob and Tom must each bear 250 N of weight and are each 2 m from the midpoint, for 500 N·m of torque each. Now put Tom farther from the midpoint-say, 3 m away. For the torques to balance, Fbob(2 m) = Ftom(3 m). The only way to satisfy this equation and get both forces to add up to 500 N is to use 300 N for Fbob and 200 N for Ftom. Now, the torque provided by each is (300 N)(2 m) = 600 N·m. The torque increased for both people.