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A rigid body is made up of a number of particles such that the relative distances of constituent particles remain unaffected under the action of force.
The motion of such a body will be mainly of two kinds:

(i) Translational Motion

(ii) Rotational (Rotatory) Motion

In the translational motion, all the constituent particles of a body move along parellel straight lines and undergoes equal distance in equal time.

♦ Translational motion is progressive motion and every particle is equally effective in determining the motion of the body.
♦ The moving body in this case may be treated as single particle.
♦The entire mass being assumed to be concentrated at a point, i.e. the centre of gravity of the body.
♦ Such a motion may be altered or produced by a single force acting on the body or a set of forces equivalent to single force.

fig. an example of rotational motion

    
In the rotational motion on the other hand, the body rotates about a fixed axis. The different constituent particles of the body have different linear velocities although their angular velocity is the same.
♦ Rotational motion in stationary object can be produced or altered by a couple acting on the body or a set of couple equivalent to single couple.