Newton's first law

Newton's first law is often summarised as "an object in motion stays in motion," but the more useful half is the part people forget: an object at rest stays at rest, too, and both halves hold only as long as no net force interferes.

This is harder to see on Earth than it should be, because friction and air resistance are always quietly acting, making moving objects slow down and stop even when nothing else pushes them. A puck on frictionless ice, or a spacecraft coasting through empty space, shows the law in its pure form: no net force, no change in velocity, forever.

The property behind this resistance to changing motion is called inertia, and it scales with mass — a loaded shopping trolley takes more effort to start moving and more effort to stop than an empty one, for exactly the same reason.