gyroscope

A rapidly-spinning symmetrical top usually used to maintain direction or to demonstrate the principles of angular momentum. Often one treats it with the "gyroscopic approximation" which assumes that the angular momentum is parallel to the direction of spin (i.e. the contribution from precession is assumed negligible). Practical gyroscopes usually consist of a disc or spoked wheel transfixed by an axial rod about which it rotates, along with a cage-like structure to both support and protect the components. Gyroscopes used for demonstrations often have no motors, relying upon low-friction mounting to allow them to keep spinning for a long time. Gyroscopes used for measurement purposes will generally have some motor to provide constant uniform rotation.

The pedagical value of the gyroscope lies in its demonstrating the way that the laws of angular momentum and torque violate our naive expectations based upon everyday experience. A gyroscope tipped at an angle will not fall over, but will tend to precess about the vertical axis. Attempts to change the direction of the main axis by hand meet resistance and produce unexpected motions.