(a) The inability of a body to change its state of uniform rotation about an axis is called rotational inertia or M.I. of the body. Vi
It plays the same role in rotatory motion as is being played by the mass in translational motion, i.e., it is rotational analogue of mass.
(b)M.I. of a body about a fixed axis of rotation is defined as the sum of the products of the masses and square of the perpendicular distance of various constituent particles from the axis of rotation.