Precession

If you have ever played with toy gyroscopes, you know that they can perform all sorts of interesting tricks. They can balance on string or a finger; they can resist motion about the spin axis in very odd ways; but the most interesting effect is called precession. This is the gravity-defying part of a gyroscope. The following video shows you the effects of precession using a bicycle wheel as a gyro:

Click here to download the 30-second full-motion video showing precession at work. (1.7 MB)

The most amazing section of the video, and also the thing that is unbelievable about gyroscopes, is the part where the gyroscopic bicycle wheel is able to hang in the air like this:

The ability of a gyroscope to "defy gravity" is baffling!

How can it do that?

This mysterious effect is precession. In the general case, precession works like this: If you have a spinning gyroscope and you try to rotate its spin axis, the gyroscope will instead try to rotate about an axis at right angles to your force axis, like this:

In figure 1, the gyroscope is spinning on its axis. In figure 2, a force is applied to try to rotate the spin axis. In figure 3, the gyroscope is reacting to the input force along an axis perpendicular to the input force.

So why does precession happen?