Newton's Third Law

Statement. Whenever an object 1 exerts a force $\vec F_{21}$ on an object 2, object 2 simultaneously exerts an equal and opposite force $\vec F_{12}$ back on object 1:

Both forces act along the line joining the two objects.

Key points:

• The action and reaction forces act on two different bodies — never on the same body — so they can never cancel each other when analysing the equilibrium of just one of the two bodies.
• Either force may arbitrarily be called the "action" and the other the "reaction" — there is no cause-and-effect time order between them; both exist simultaneously.
• The law guarantees that an isolated, single, partner-less force cannot exist in nature.
• Unlike the first and second laws, the third law is valid in both inertial and non-inertial frames.

Everyday illustrations: a hammer striking a nail (hammer-on-nail force paired with nail-on-hammer force); a ball bouncing off a wall (ball-on-wall paired with wall-on-ball); and a person walking, where the foot pushes the ground backward ("action") and, by reaction, the ground pushes the foot forward — which is precisely what propels the walker forward. …