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Which of the following is not an example of an object undergoing a torque?
  • A car is rounding a bend at a constant speed.
  • A merry-go-round increases from rest to a constant rotational speed.
  • A pendulum swings back and forth.
  • A bowling ball rolls down a bowling alley.
Question by OpenStax is licensed under CC BY 4.0.
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OpenStax College Physics Solution, Chapter 9, Problem 1 (Test Prep for AP® Courses) (1:31)

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This is College Physics Answers with Shaun Dychko. In order for a torque to be applied to an object, there needs to be two things. There needs to be a position where the force is applied, and there needs to be a pivot about which the object will rotate. In part A, there is no such pivot, there's no connection to a pivot point and a place where the force is applied because when you have a car going around a bend there's a center of curvature of its path here, sure, but that's not a pivot point because the car is not connected to it. So there can't be any torque in this case. Whereas with a merry-go-round there's torque to, you know, rotate the whole big circular object. A pendulum experiences a torque, here's the pivot point. It's connected by maybe even a rigid rod or a string. Then with a bowling ball, there's a center here and that will be the -- or there's a point of contact here, I suppose that would be the pivot and then there's gravity you know, being exerted on this side and on this side as well. Then there's friction. In fact the friction would be the most significant source of torque here because friction would be opposing its motion and exerting a torque about the center of the bowling ball. There we go! Answer A.