As the body rotates, the point P moves in a circle about O . The radius of the circle is just r , the distance from O to P . The position of the body is specified by the angle which r makes with respect to an arbitrary fixed axis. We usually measure in radians . If s is the arc length that P moves through, then Here, is in radians .
Radians are a unitless angular measure.
Since the circumference of a circle of radius r is , there are radians in a complete circle. Thus there are radians per 360 degrees. One of the more common errors in dealing with angular motion is to forget to use radian measure. There are also average angular velocity and average angular acceleration, which are defined analagously to the corresponding linear quantities. Angular Equations of Motion Keep in mind the translational analogs of the rotational quantities.