Inertia

Meet Inertia, a puck gliding across ice who sees no reason to stop.

Newton's first law says that an object stays at rest, or keeps moving at constant velocity in a straight line, unless a net force acts on it. Motion does not need a force to keep it going; only changes in motion do. Galileo argued this in the early 1600s, against the old idea that moving things naturally come to rest, and Newton made it his first law in 1687.

A hockey puck sliding at 10 m/s on smooth ice faces very little friction, so it keeps almost the same speed for a long way. On a rough floor the same puck stops within metres, because friction is a net force acting against its motion.

Seatbelts exist because of inertia: when a car stops suddenly, your body keeps moving forward until something applies a force. Spacecraft coast for years between planets with their engines off.

The law holds in inertial reference frames. In an accelerating car or a rotating merry-go-round, objects appear to move without any force, which is why fictitious forces are introduced there.