<P> A change in the position of a particle in three - dimensional space can be completely specified by three coordinates . A change in the position of a rigid body is more complicated to describe . It can be regarded as a combination of two distinct types of motion: translational motion and rotational motion . </P> <P> Purely translational motion occurs when every particle of the body has the same instantaneous velocity as every other particle; then the path traced out by any particle is exactly parallel to the path traced out by every other particle in the body . Under translational motion, the change in the position of a rigid body is specified completely by three coordinates such as x, y, and z giving the displacement of any point, such as the center of mass, fixed to the rigid body . </P> <P> Purely rotational motion occurs if every particle in the body moves in a circle about a single line . This line is called the axis of rotation . Then the radius vectors from the axis to all particles undergo the same angular displacement in the same time . The axis of rotation need not go through the body . In general, any rotation can be specified completely by the three angular displacements with respect to the rectangular - coordinate axes x, y, and z . Any change in the position of the rigid body is thus completely described by three translational and three rotational coordinates . </P> <P> Any displacement of a rigid body may be arrived at by first subjecting the body to a displacement followed by a rotation, or conversely, to a rotation followed by a displacement . We already know that for any collection of particles--whether at rest with respect to one another, as in a rigid body, or in relative motion, like the exploding fragments of a shell, the acceleration of the center of mass is given by </P>

What is the rotary effect of a force about an axis of rotation