The motion of a rigid body which takes place in such a way that all of its particles move in circles about an axis with a common angular velocity; also, the rotation of a particle about a fixed point in space. Rotational motion is illustrated by (1) the fixed speed of rotation of the Earth about its axis; (2) the varying speed of rotation of the flywheel of a sewing machine; (3) the rotation of a satellite about a planet, in which both the speed of rotation and the distance from the center of rotation may vary; (4) the motion of an ion in a cyclotron, where the angular speed of rotation remains constant, but the radius of the circular motion increases; and (5) the motion of a pendulum, in which case the particles describe harmonic motion along a circular arc.