Gyroscope is a device containing a rapidly spinning wheel that is used to detect the deviation of an object from its desired orientation. Gyroscopes are used in compasses and automatic pilots on ships and aircraft, in the steering mechanisms of torpedoes, and in the inertial guidance systems installed in space launch vehicles, ballistic missiles, and orbiting satellites.

The central topic of this chapter is angular momentum, a quantity that plays a key role in rotational dynamics. Every rotational quantity that we have encountered is the analog of some quantity in the translational motion of a particle. The analog of momentum of a particle is angular momentum, a vector quantity denoted as L. In analogy to the principle of conservation of linear momentum, we find that the angular momentum of a system is conserved if no external torque acts on the system. Like the law of conservation of linear momentum, the law of conservation of angular momentum is a fundamental law of physics.