A flywheel is a heavy rotating disk used as a repository for angular momentum. Flywheels resist changes in their rotation speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source (such as a piston-based engine) or when the load placed on it is intermittent (such as a piston-based pump). Flywheels can also be used by small motors to store up energy over a long period of time and then release it over a shorter period of time, temporarily magnifying its power output for that brief period. Recently, flywheels have become the subject of extensive research as power storage devices; see flywheel energy storage.
The kinetic energy stored in a rotating flywheel is