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The motion of the flywheel of an engine and of a pulley on its axle are examples of an important type of motion of a rigid body, that of the motion of rotation about a fixed axis. Consider the motion of a uniform disk rotating about a fixed axis passing through its center of gravity C perpendicular to the face of the disk, as shown in Figure 11-1. The motion of this disk may be described in terms of the motions of each of its individual particles, but a better way to describe the motion is in terms of the angle through which the disk rotates. Calling two successive positions of a point in the plane of the disk P1 and P2 , we find the angle of rotation by drawing radial lines from C to P1 and to P2 . The angle Ө between these two lines is the angle through which the disk has rotated; every point in the plane of the disk has rotated through the same angle Ө in the same interval of time. The angle Ө is called the angular displacement of the body. Both the angle Ө and the direction of the axis of rotation must be given in order to specify properly a rotational displacement.