Newton's Three Laws

Classical mechanics — the physics of everyday moving things, from thrown balls to orbiting planets — rests almost entirely on three statements Newton wrote down in 1687. They sound simple. What they actually claim is sharper than it first looks.

First law: objects don't change their motion on their own

An object stays at rest, or keeps moving in a straight line at constant speed, unless a force acts on it. This wasn't obvious historically — it took real effort to separate "why does motion continue" from "why does motion change," because everyday friction makes moving things look like they naturally slow down. They don't. Friction is a force too; remove it (space, ice, an air table) and constant-velocity motion just continues, unprompted.

Second law: force causes acceleration, in proportion to mass

F = m·a

A given force accelerates a light object more than a heavy one. This is the working equation of the whole subject: if you know the forces acting on something, you know its acceleration, and from acceleration you can work out its entire future position and velocity.

Third law: forces come in pairs

If object A pushes on object B, object B pushes back on A, equally and in the opposite direction. Push on a wall and the wall pushes back on you — that's why you don't fall through it, and why you can walk at all (you push backward on the ground; it pushes you forward).

What's next

These three laws don't yet tell you what the forces are — gravity, tension, friction, a spring. The next lessons work through those one at a time, always coming back to F = m·a to find out what actually happens.