Newton's First Law of Motion

Inertia: the tendency of things to resist changes in motion.

Newton's First Law
Every object persists in a state of rest or uniform motion in a straight line unless compelled by an external force to change that state.

Mass: The quantity of matter in an object. It is also the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way.

The inertia of the hammerhead helps tighten it on the handle.