High School Physics/Rotational Motion

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Precession[edit]

In a classic beginning physics demonstration, the instructor stands on a swiveling platform and holds a spinning bicycle wheel at arm's length. The wheel is vertical and the instructor is standing still. The instructor then tilts the wheel toward horizontal. This causes the instructor to start spinning slowly on the platform. Bringing the wheel back to vertical and tilting it the other way makes the instructor spin the other way. Why?

Imagine the wheel as a collection of small particles. Particles want to move in a straight line. In order for them to move in a circle there must be a force accelerating the particles toward the center of the circle (acceleration is a change in speed or direction or both — in this case just direction). This force is ultimately provided by bonds between the atoms in the wheel and spokes.

What happens when the instructor turns the spinning wheel from vertical to horizontal? Consider a particle somewhere on the wheel. If the wheel weren't being tilted, it would be accelerated around the circle as always. But since the wheel is tilting, it now has to follow a new path. A change in path is an acceleration, which in turn requires force (from the instructor's hands, transmitted through the spokes to the rim). Now consider the particle opposite the first particle on the wheel. It also has to change path, but in the opposite direction. Since the forces on opposite sides are in opposite directions, the result is torque. Each pair of opposite particles on the wheel contributes to the torque that causes the instructor to turn on the platform. The particles that are further away have a longer lever-arm in relation to the professor, so the net torque is non-zero. (Torque=Force*Lever-arm)

Tilting the wheel the other direction produces torque in the opposite direction, slowing the instructor's spin and eventually reversing it.