Newton's first law: an object at rest tends to remain at rest, and an object that is spinning tends to spin with a constant angular velocity, unless it is acted on by a nonzero net torque or there is a change in the way the object's mass is distributed.

The net torque is the sum of all the torques acting on an object.

The tendency of an object to maintain its state of motion is known as inertia. For straight-line motion mass is a good measure of inertia, but mass by itself is not enough to define rotational inertia.