Changing the spin of an object involves its mass, its shape, its mass distribution, and its axis of rotation. The moment of inertia I, accounts for all these factors.

The moment of inertia, I, is the
*rotational* equivalent of mass.

For a simple object such as a ball on a string, where all the mass is the same distance away from the axis of rotation:

I = mr^{2}

If mass is distributed at different distances from the rotation axis, the
moment of inertia is determined by:

I =∫
r^{2} dm