Angular Displacement

If the body of Fig. 11-3 rotates about the rotation axis as in Fig. 11-4, changing the angular position of the reference line from to , the body undergoes an angular displacement given by

This definition of angular displacement holds not only for the rigid body as a whole but also for every particle within that body because the particles are all locked together.

If a body is in translational motion along an axis, its displacement is either positive or negative, depending on whether the body is moving in the positive or negative direction of the axis. Similarly, the angular displacement of a rotating body is either positive or negative, according to the following rule:

An angular displacement in the counterclockwise direction is positive, and one in the clockwise direction is negative.