The Spiderwheel - a new apparatus to demonstrate energy conservation and moment of inertia.

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Mears, M. (2015)
  • Publisher: American Association of Physics Teachers

Rotational and rolling motion often proves to be a difficult topic for undergraduate students in their mechanics courses. A number of demonstrations have been developed but often these rely on switching the rolling object with another in order to vary the mass or radius, and so the students see the entire rolling system change rather than the specific property in question. In this work a new apparatus, known as the spiderwheel, is described which allows large changes in mass distribution (and hence moment of inertia) without anything being added to or removed from the rolling body. A quantitative analysis of the rolling spiderwheel allows students to determine the moment of inertia of the body and compare it with model systems, namely a point-mass (that is, a particle with non-zero mass rotating about a fixed axis), a solid cylinder and a thin rigid hoop. Despite the spiderwheel being a non-ideal system in that it has a complex geometry with less symmetry and multiple components compared to the aforementioned model systems, it is found that the simple point-mass model provides an excellent approximation. Furthermore students in an undergraduate course were asked to predict the effect of moving the masses further from the axis of rotation, and the majority incorrectly predicted a faster roll down the ramp (in line with more comprehensive studies on these misconceptions1. The spiderwheel is a simple yet versatile model for visualising difficult concepts in rotational motion. Students can consolidate their understanding of these concepts by altering the parameters of the spiderwheel and directly observing the effects on rotational and translational velocity.
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