Conservation of Angular Momentum with Slightly Modified Commercial Apparatuses (Journal Article)

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Abstract: A spinning ice-skater who speeds up when she pulls in her arms is a common textbook illustration of how changing a rotating object’s moment of inertia affects its angular speed. An analogous classroom demonstration involves sitting on a rotating stool while moving handheld weights in and out. Given that students have difficulties understanding conservation of angular momentum when the moment of inertia changes,1 we wanted to add a quantitative experiment that closely resembled the typical textbook example and the in-class demonstration. A few such experiments have been published: an apparatus that moves masses radially relative to a person rotating on a stool,2 a rotating track along which model cars move radially inward or outward,3 and a modified, centripetal-force apparatus with masses that slide outward.4 The first two were more elaborate than we wanted, but the third one seemed to be a suitable experiment for an introductory lab. However, as described, that experiment required substantial modifications to a relatively expensive apparatus.5 We have found two ways to perform the experiment by making simple modifications to less expensive commercial apparatuses from two manufacturers.