III.

Precession

When a force applied to a gyroscope tends to change the direction of the axis of rotation, the axis will move in a direction at right angles to the direction in which the force is applied. This motion is the result of the force produced by the angular momentum of the rotating body and the applied force. A simple example of precession can be seen in the rolling hoop: to cause the hoop to turn a corner, guiding pressure is not applied to the front or rear of the hoop as might be expected, but against the top. This pressure, although applied about a horizontal axis, does not cause the hoop to fall over, but causes it to precess about the vertical axis at right angles to the applied pressure, with the result that it turns and proceeds in a new direction.