Demos: 1Q-21 Conservation of Angular Momentum (Bicycle Wheel & Turntable)

The demonstrator, holding a bicycle wheel, sits on a stool in the center of the turntable. The wheel is spun and then held with the axis vertical. The angular momentum vector is also in the vertical direction (whether it is up or down depends on how the wheel is spinning). If the wheel is suddenly inverted, the turntable (and demonstrator) acquire an angular momentum in the opposite direction such that the original angular momentum of the system is conserved.

If L is the original angular momentum of the wheel, the turntable angular momentum is L_t, so that

L_t - L = L

Directions: The bicycle wheel can be turned by hand fast enough to do the demonstration, but a motor is available for spinning it up, if needed. Hold the spinning wheel in both hands by the axle in a vertical position as you sit on the turntable. Draw attention to the direction of the angular momentum by use of the right hand rule. (For example, if the wheel is spinning counterclockwise (from above), the angular momentum vector points vertically upward. Then invert the wheel, causing the wheel’s angular momentum vector to point downward. The turntable will start rotating in a counterclockwise direction. Invert it again and it should stop (usually not completely because of instabilities in the system).

Using a gloved hand (or some kind of hand protection), you can bring the wheel rotation to a stop (but keep the axis vertical) and nothing will change, because this action is internal to the system.

Suggestions for Presentation: As can be seen from the diagram, the angular momentum of the turntable is twice that of the wheel. Then why the turntable is turning so much more slowly than the wheel? (The turntable plus demonstrator plus bicycle wheel has much more mass.) You should also ask the students to explain why the turntable rotates in the direction it does. Focus on the angular momentum vectors.

Applications: None that are readily apparent.

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