One of
the most fascinating examples of the effects of angular momentum is in the precession of
tops and gyroscopes. The phenomenon of precession is shown by all spinning objects if a
torque is applied to the axis of rotation, even the planet Earth!
Before we can
attempt to explain the way in which a gyroscope behaves we must first realise that angular
momentum, like linear momentum, is a vector. The way to work out the direction of this
vector is shown in Figure 2.
Imagine that the disc (D) is spinning in an anticlockwise direction about the axis A. The direction of the angular momentum of the spinning disc is found by considering a right hand gripping the axle with the fingers curled round the axle in the direction of spin. The direction of the angular momentum vector is then in the direction in which the right thumb points.
A string is now tied to one end of the axle, the axle is supported and
then set spinning in an anticlockwise direction and the free end is then released (Figure
3).
The weight (W) of the gyroscope acts downwards as shown and this causes a
torque to act on the gyroscope in a direction out of the paper.
This causes a change in the
angular momentum about the vertical axis and so the gyroscope rotates in an anticlockwise
direction about this axis when viewed from above (see Figure 4).
This rotation about the vertical axis is called
precession.
Notice that there is no change in the angular momentum in a vertical
direction.