In linear motion, mass is the measure of the inertia of an object. Likewise, in rotational motion, the moment of inertia, a quantity derived from the mass of an object, is a measure of inertia. The moment of inertia is found by multiplying each part of the mass by its distance (radius) from the center of mass. It is not easily calculated, as it varies from object to object. The equation for finding I is
The moment of inertia I can be used in place of mass in energy equations governing rotational motion. For example, the equation for rotational kinetic energy is
Likewise, angular momentum l can be defined as
where I is used in place of m and w is used in place of v, and an equation for torque