**Moment of inertia** is rotational inertia, i.e., moment of inertia is to rotational motion as mass is to linear motion. Rotational versions of Newton's Second Law, momentum, and the formula for kinetic energy use this value (with torque, angular velocity and angular acceleration replacing force, velocity and acceleration, respectively). The moment of inertia for an object depends on its shape and distribution of mass within that shape: the more the mass is on the outside with respect to the axis of rotation, the larger the moment of inertia. For a given mass M and radius r, in order of increasing moment of inertia we have a solid sphere, a solid cylinder, a hollow sphere and a hollow cylinder, namely cMr^{2}, with c=2/5, 1/2, 2/3 and 1, respectively. The general form of the moment of inertia involves an integral.

The moment of inertia is often represented by the letter I.

A rigid body can be considered an infinite number of infinitely small particles, each with mass . If each particle is a distance from a particular axis of rotation, then the moment of inertia of the rigid body about that axis is given by:

is defined by the spatial density distribution &rho.

See also: torque