inertia

Inertia is the property of matter that keeps a body doing whatever it is already doing. A body at rest stays at rest. A body in motion keeps moving in a straight line at constant speed. Nothing happens to the motion unless a force acts. The word itself means 'sluggishness' in Latin, and in a literal sense that is what inertia is — a reluctance on the part of matter to have its motion changed.

Newton's first law is the formal statement of inertia. It sounds obvious today, but it replaced two thousand years of Aristotelian physics in which motion was assumed to require a continuous cause. Galileo was the first to see past this picture. He rolled balls on polished planes and noticed that the smoother the surface, the further they travelled. In the limit of no friction, the ball would never stop.

Mass is the quantitative measure of inertia. A heavy body is hard to start moving and hard to stop once moving. In Newton's second law, F = ma, the mass is precisely the constant that tells you how much acceleration a given force will produce.

Galileo glimpsed inertia in his inclined-plane experiments in the early 1600s. Descartes gave a clear statement of rectilinear inertia in his Principles of Philosophy (1644). Newton finally codified it as the first of three laws in the Principia (1687).