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Which rotational system would be best to use as a model to measure how angular momentum changes when forces on the system are changed?
  1. a fishing reel
  2. a planet and its moon
  3. a figure skater spinning
  4. a person's lower leg
Question by OpenStax is licensed under CC BY 4.0.
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OpenStax College Physics for AP® Courses Solution, Chapter 10, Problem 11 (Test Prep for AP® Courses) (1:27)

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This is College Physics Answers with Shaun Dychko. So we want to choose the best system for using as a model to see how angular momentum changes when forces on the system are changed. Angular momentum is moment of inertia multiplied by angular velocity so we need to have a system in which we can measure the angular velocity and we need a system in which we can change the forces on it. So the planet and the moon is not a good system because we can't change any of those forces, those things are too big. A figure skater spinning is not ideal because we can't really put a force on a figure skater. You know, he or she might fall over and also there is also a lot of scraping against the ice and so the system is not very isolated. Then part D, a person's lower leg, well, they might flex their muscle in which case they are applying a torque that's difficult to notice if you were to apply some external force to their lower leg. So that system isn't very good and so a fishing reel turns out to be our best option here because we have a little handle on the fishing reel that we can apply a force to and then notice how the rate of spinning of the fishing reel changes.