A spinning top (The point of contact of the top with the ground, its tip O, is fixed.) Fig. . (b) An oscillating table fan with rotating blades. The pivot of the fan, point O, is fixed.
The blades of the fan are under rotational motion, whereas, the axis of rotation of the fan blades is oscillating. Fig. . A rigid body rotation about the z-axis ( Each point of the body such as P or P describes a circle with its centre (C or C ) on the axis of rotation.
The radius of the circle ( r or r ) is the perpendicular distance of the point ( P or P ) from the axis. A point on the axis like P remains stationary). Axis of oscillation Axis of rotation from blades In some examples of rotation, however, the axis may not be fixed. A prominent example of this kind of rotation is a top spinning in place [Fig.
. (a)]. (We assume that the top does not slip from place to place and so does not have translational motion.) We know from experience that the axis of such a spinning top moves around the vertical through its point of contact with the ground, sweeping out a cone as shown in Fig. .
(a). (This movement of the axis of the top around the vertical is termed precession . ) Note, the point of contact of the top with ground is fixed . The axis of rotation of the top at any instant passes through the point of contact.
Another simple example of this kind of rotation is the oscillating table fan or a pedestal fan [Fig. . (b)]. You may have observed that the axis of rotation of such a fan has an oscillating (sidewise) movement in a horizontal plane about the vertical through the point at which the axis is pivoted (point O in Fig.
. (b)). While the fan rotates and its axis moves sidewise, this point is fixed. Thus, in more general cases of rotation, such as the rotation