Newton's law

Newton's law refers to any of three fundamental principles of classical mechanics formulated by Isaac Newton.

The first law states that objects remain at rest or in uniform motion unless acted upon by a force.

The second relates force to mass and acceleration: F = ma.

The third holds that every action produces an equal and opposite reaction.

In casual use, Newton's law often means any one of these, typically whichever applies: Newton's law says what goes up must come down—though that's a loose paraphrase.

Newton's law of universal gravitation, describing gravitational attraction between masses, is sometimes included under the umbrella.