$$\require{cancel}$$
A particle of mass $$m$$ is attached by a light (i.e. zero or negligible mass) arm of length $$r$$ to a point at O, about which it can freely rotate. A force $$F$$ is applied, and the mass consequently undergoes a linear acceleration $$a = \frac{F}{m}$$. The angular acceleration is then $$\ddot{\Theta} = F.mr$$. Also, the torque is $$\tau = Fr$$ . The ratio of the applied torque to the angular acceleration is therefore $$mr^{2}$$. Thus the rotational inertia is the second moment of inertia. Rotational inertia and (second) moment of inertia are one and the same thing, except that rotational inertia is a physical concept and moment of inertia is its mathematical representation.