Explanation
Centre of mass lies within or outside the material of the body as position of it depends upon the shape of the body and the distribution of mass.
Speed of centre of mass is constant as both the particles are initially at rest.
In case of rotational motion of rigid body centre of mass moves in a circular path uniformly.
At centre of mass, if any force is applied then torque about centre of mass will be zero but acceleration will be non zero. So, velocity will change.
Motion of centre of mass is result of all the external forces applied to it.
From equation of motion of centre of mass
MaCM = Fext if Fext = 0
aCM = 0
Therefore, vCM = Constant
So, centre of mass may move if external force acts on a system but move with constant velocity without accelerating.
Moment of Inertia about any diameter of the ring I = 1/2 Mr2
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