Collier's New Encyclopedia (1921)/Inertia

INERTIA (in-er'shē-ä), the incapability of matter to change its state, whether that be one of motion or rest. If in motion, it will continue so forever unless some counteractive force, like that of gravity, act upon it. If at rest, it will ever remain so unless a counteractive impulse set it in motion. The resistance which, especially at first, it gives to such a force operating to move it is called vis inertiæ—the power of inertia.