As in translatory motion, the objects oppose any change in their state of rest and state of motion. The property due to which this behaviour is observed, is called ‘Inertia’. Inertia depends on the mass of the body. Therefore the mass of the body is the measure of inertia. Similarly, in rotational motion also, the rotating body about an axis or resting body free to rotate about the axis, opposes any change in their state. This tendency of opposing the change in rotatory motion is called ‘moment of inertia’ or ‘Rotational inertia’ i.e., ‘Rotational inertia of a body about the axis of rotation is called moment of inertia of the body about the axis of rotation,’ It is denoted by I.
Measurement of moment of inertia: Measurement of moment of inertia of a body is done by the product of the mass of the body and square of its distance from the axis of rotation.
∴ Moment of inertia (I) = mass (m) x (normal distance)2 from the axis
or \(I = mr^2\)
∴ Unit of moment of inertia in M.K.S. system = kg m2
Unit of moment of inertia in C.G.S. system = g cm2
Dimensional formula of I = [M1L2T0]
