Inertia

Inertia is the tendency of matter to resist changes in its velocity. In classical mechanics, mass measures translational inertia.

Newton's first law: An object maintains its velocity unless a net external force acts on it. It states inertia’s central consequence: motion does not change without a net force.
Velocity: The rate and direction of change of an object's position. Inertia preserves velocity, not merely speed or position.
Weight: The gravitational force acting on an object. Weight depends on gravity, while inertia is measured by mass and does not require a gravitational field.
Seat belt: A vehicle restraint that limits occupant motion during sudden deceleration or collision. It restrains occupants whose bodies would otherwise continue moving as the vehicle slows.
D'Alembert's principle: A mechanics principle that reformulates dynamics as a balance of applied and inertial forces. It incorporates resistance to acceleration into a force-balance method.
Mass: A physical quantity that measures an object’s inertia and its contribution to gravitational interactions. In classical mechanics, an object’s mass quantifies its resistance to acceleration.
Net force: The vector sum of all forces acting on an object. Only an unbalanced total force changes an object's velocity.
Gravitational mass: A measure of how strongly an object couples to gravitational fields. It describes gravitational interaction, unlike inertial mass’s role in acceleration.
Inertial navigation system: A navigation system that estimates motion using accelerometers and gyroscopes without external position references. It infers changing motion from measurements against an inertial baseline.
Equivalence principle: The principle that inertial and gravitational effects are locally equivalent in general relativity. It links inertial mass to gravity, beyond inertia’s role in ordinary motion.

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