The principle of gyroscopic motion is an application of Newton's law which states that a massive, rapidly spinning body strongly resists being disturbed and tends to react to a disturbing torque by precessing (rotating in a direction at right angles to the direction of the torque).
The principle of precession has been used to develop the familiar dime-store gyroscope that will stand upright on the rim of a glass or walk a taut string.
The gyroscope, is an interesting toy, but it is a highly useful instrument. Elmer A. Sperry exploited its principles when he developed the gyroscope as a navigational device for ships and airplanes. The gyroscope can point to true North, regardless of its surroundings. It is widely used in various aircraft instruments as a turn indicator, artificial horizon, attitude or position indicator, and in automatic pilots.
(Source: http://history.nasa.gov/SP-401/ch13.htm )
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