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PIRA Code(s)
Set up time
5 minutes
Physics and Science Concepts
Mechanics, Angular Momentum, Rotational Dynamics, Angular Momentum, Relative Motion
Place the train on the track. Plug the track into the variac and start the train by moving the dial.
Watch for sparks from the train. Maintain a cautious distance from the wheel.

An electric model train runs on a circular track which is supported so that it is free to turn about its center. The moment of inertia of the train is comparable to that of the track and supports. Thus, if the train is started in the forward direction, the track begins to move in the opposite direction, and if the train is brought to a halt, the whole system stops.