In rotation of a rigid body about a fixed axis, every particle of the body moves in a circle which lies in a plane perpendicular to the axis and has its centre on the axis.
In figure, rotational motion of a rigid body shows about the Z-axis of the frame of reference.
Let `P_(1)` be a particle of the rigid body, arbitrarily chosen and at a distance `r_(1)` from the fixe axis.
The particle `P_(1)` describe a circle of radius `r_(1)` with its centre `C_(1)` on the fixed axis. The circle lies in a plane perpendicular to the axis.
An another particle `P_(2)` of the rigid body, `P_(2)` is at a distance `r_(2)` from the fixed axis. The particle `P_(2)` moves in a circle of radius `r_(2)` and with centre `C_(2)` on the axis.
The circles described by `P_(1)andP_(2)` may lie in different planes, both these planes are perpendicular to the fixed axis.
For any particle on the axis like `P_(3),r_(3)=0`. Any such particle remains stationary while the body rotates.
In rotation of a spinning top, the axis may not be fixed as shown in figure. Assume that spinning top rotates at a fixed place.
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 figure.
This movement of the axis of the top around the vertical is termed as precession. Here, 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 example of this kind of rotation is the oscillating table fan or a pedestal fan.
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 figure).
While the fan rotates and its axis moves sidewise, this point is fixed.
Hence is case of a top or a pedestal fan, one point and not one line of the rigid body is fixed.
In this case, the axis is not fixed though it always passes through the fixed point.
