Newton's inertia

Newton’s first law says an object remains at rest or continues moving at constant velocity unless acted on by a net external force. Inertia is the tendency of matter to resist changes in motion.

How it works

Mass measures inertia in classical mechanics: a larger mass requires a larger net force for the same acceleration. The law applies in inertial reference frames, which are not accelerating or rotating in the relevant sense.

Example or application

A book stays on a table until a force moves it. A rolling object slows because friction and air resistance act on it; without a net force, its velocity would remain constant.

Scope and limits

The law does not say motion always needs a continuing force. It also does not make friction disappear; real situations include forces that change motion.

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