Newton's third law
(M2.1.2) M2.1 · The Three Laws of Motion →If object A exerts a force on object B, then B exerts an equal and opposite force on A.
| force of A on B | N | |
| force of B on A | N |
Any pair of interacting objects, at any separation, whether in contact or not.
Also empirical: the two forces of the pair act on different objects, are equal in magnitude, opposite in direction, and of the same type.
A 60 kg skater throws a 0.5 kg ball forward; by the third law, the force on the skater equals the force on the ball, in the opposite direction, so momentum change is equal and opposite:
This is why the skater recoils — not a separate law, but Newton's third law combined with the impulse-momentum theorem.
The two forces in a third-law pair act on <i>different</i> objects and therefore never cancel each other in a free-body diagram of either object alone.