(a) (b) Fig. . Rotation about a fixed axis (a) A ceiling fan (b) A potter’s wheel . Let us try to understand what rotation is, what characterises rotation.
You may notice that in rotation of a rigid body about a fixed axis, Fig . Translational (sliding) motion of a block down an inclined plane. (Any point like P or P of the block moves with the same velocity at any instant of time.) Fig. .
Rolling motion of a cylinder. It is not pure translational motion. Points P , P , P and P have different velocities (shown by arrows) at any instant of time. In fact, the velocity of the point of contact P is zero at any instant, if the cylinder rolls without slipping.
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 . Fig. . shows the rotational motion of a rigid body about a fixed axis (the z -axis of the frame of reference).
Let P be a particle of the rigid body, arbitrarily chosen and at a distance r from fixed axis. The particle P describes a circle of radius r with its centre C on the fixed axis. The circle lies in a plane perpendicular to the axis. The figure also shows another particle P of the rigid body, P is at a distance r from the fixed axis.
The particle P moves in a circle of radius r and with centre C on the axis. This circle, too, lies in a plane perpendicular to the axis. Note that the circles described by P and P may lie in different planes; both these planes, however, are perpendicular to the fixed axis. For any particle on the axis like P , r = .
Any such particle remains stationary while the body rotates. This is expected since the axis of rotation is fixed. Fig. .