inertia

Inertia is the property of matter that describes the resistance to any change in its state of motion or rest. An object at rest tends to stay at rest, and an object in motion tends to continue moving at a constant velocity in a straight line unless an external force acts upon it. Mass is a quantitative measure of inertia: more massive objects resist changes in motion more than less massive ones.

Historically, inertia was observed by Galileo and formalized by Isaac Newton as part of his first law of motion. Newton's first law states that in the absence of a net external force, a body remains at rest or moves with constant velocity.

Is inertia a force? No; it is a property of matter. It interacts with external forces through Newton's second law, F = dp/dt, where momentum p = mv. Inertia is related to mass: inertial mass determines how strongly an object resists acceleration.

In everyday terms, inertia explains why seatbelts and airbags are needed to change motion during collisions. In celestial mechanics, inertia combined with gravity shapes orbits: objects tend to move in straight lines, while gravity continually redirects their paths.

Relativity extends the concept but preserves its essence: all matter has rest mass that resists acceleration, and momentum increases with speed according to relativistic formulas. In general relativity, inertia is related to spacetime geometry via the equivalence principle.