ROTATIONAL MOTION
In general ,there are two types of motion:
(i) translatory (ii) rotatory
We may, in any particular caseof an object in motion, find that motion is purely translational or purely rotational or a combination of both.For example
(a) A square shaped scale slides of an object on an inclined plane, its motion is purely translational.
(b) The motion of a fly wheel,fixed position,is only rotational(rotation about its own axis)
(c)When sphere rolls on the inclined plane,it slides as well as rotates and thus its motion is a combination of the two types of motion.
Translational motion: A rigid body performs a pure translational motion if each particle of the body undergoes the same displacement in the same direction in any given time-interval.
Pure rotational motion: Fig(1)shows a rigid body of arbitrary shape in pure rotation around a fixed axis say
Z-axis.Let us consider two particles p1 and p2 of the body.These particels move in the circles of a radii r1 and r2 respectively, with centres c1 and c2 lying on Oz axis .The circles lie in X-Y plane.So,in general every point of the body moves in a circle whose centre lies on the fixed axis of motion.It should be remembered that particles lying on the axis of rotation remain stationary.So,a rigid body performs a pure rotational motion if every particle of the body moves in a circle ,and centres of all circles lie on a straight line called the axis of rotation.
The pure rotation of a rigid body can be described by any one of the particles of the body except the particles on the axis of rotation.