Newton's first law

A body remains at rest or moves at constant velocity unless acted on by a net external force. It identifies inertial motion and introduces the idea of an inertial reference frame.

Inertia: The tendency of matter to resist changes in its state of motion. The law describes inertia as continued rest or uniform motion when no net force acts.
Velocity: The rate and direction of change of position. Constant velocity is the law's precise description of unforced motion.
Isaac Newton: English mathematician and natural philosopher who formulated the laws of motion and universal gravitation. Newton stated the first law as part of the framework published in the Principia.
Seat belt: A vehicle restraint that limits occupant motion during sudden deceleration or collision. A belt supplies the force needed to change an occupant's motion with the vehicle.
Newton's laws of motion: Three classical principles relating forces, mass, and motion. The first law is one part of this broader system of mechanics.
Inertial reference frame: A reference frame in which force-free bodies move at constant velocity. The law holds in inertial frames, making the choice of frame part of its meaning.
Acceleration: The rate of change of velocity over time. The law rules out acceleration when the net external force is zero.
Aristotelian physics: A natural philosophy in which motion is classified by distinct principles governing earthly and celestial bodies. Its account of motion differed from Newton's claim that uniform motion needs no continuing cause.
Centrifugal force: An apparent outward force used to describe motion in a rotating reference frame. Rotating-frame examples reveal why the law requires an inertial frame or an added apparent force.
Principle of least action: A variational principle that characterizes physical paths through stationary action. It offers a different formulation of mechanics, in which free motion follows from an action principle.

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