Which of Newton's laws is also called the law of inertia?

Newton’s first law is also called the law of inertia. It states that an object remains at rest or continues moving in a straight line at constant speed unless a net external force acts on it.

Inertia is an object’s resistance to changes in its motion. Mass measures this resistance: a more massive object requires more force to start, stop, or redirect than a less massive object under comparable conditions. A passenger lurching forward when a bus brakes is a familiar example of the body tending to preserve its existing motion.

The law does not mean that objects naturally stop moving. They stop because forces such as friction and air resistance act on them. In ideal conditions with no net force, motion continues indefinitely at constant velocity. The law is properly applied in an inertial reference frame, where unforced motion is uniform and straight.

Galileo developed important ideas about inertia before Newton incorporated them into the three laws published in the Principia in 1687. Newton’s second law quantifies how net force changes motion, while the third law describes paired interaction forces.