Comparing with the translation, this equation has the same importance like the Newton's second law for a single particle The moment of inertia is a measure of the rotational inertia of a body, which plays the same role for rotational motion that mass does for translational motion. (p245)Comparing with the translation, this equation has the same importance like the Newton’s second law for a single particle. The moment of inertia I is a measure of the rotational inertia of a body, which plays the same role for rotational motion that mass does for translational motion.(p245)