force

A force is anything that pushes or pulls on a body. In the formal language of mechanics, force is whatever can change a body's momentum. Newton's second law, F = ma, is simultaneously the definition of force and the recipe for computing motion. The SI unit is the newton.

Forces come in many varieties — gravity, normal force, tension, friction, drag, electric and magnetic forces, the nuclear forces. Each is an independent physical law that specifies a particular force in terms of the state of the bodies involved. Forces add as vectors: the net force on a body is the vector sum of all the forces acting on it, and it is the net force that enters Newton's second law.

Crucially, Newton's laws do not tell you what forces exist in the universe. They tell you what forces do once you know them. Newton himself supplied the first great example beyond the three laws: universal gravitation.

The word 'force' had been used loosely for centuries, but Newton made it a precise, quantitative concept in the Principia (1687). The newton was named in his honour and adopted as the SI unit in 1948.