Newton's first law states that an object stays at rest or keeps moving at a constant velocity unless acted on by an unbalanced force — often called the law of inertia.
This law explains why objects resist changes to their motion. A book on a table stays still until you push it; a hockey puck sliding on frictionless ice would keep moving forever in a straight line if nothing slowed it down.
Inertia is why passengers lurch forward when a car brakes suddenly — their bodies were in motion and resist the sudden change, even though the car itself has stopped.
Last reviewed: September 2026