N3L states that when body A is exerting a force on B, body B must also be exerting a force on A. Furthermore, these two forces are always equal in magnitude but opposite in direction. This means that every force has a buddy force. And they come in and out of existence simultaneously, always as an equal-but-opposite pair. Isn’t that sweet?
Caution: not all equal-but-opposite forces are N3L pair. For example, a man is standing still so the gravitational pull W and the normal contact force N are equal-but-opposite. But W and N are not an N3L pair. The tell-tale signs include:
(1) They are different in nature. W is gravitational but N is electrical.
(2) They do not act on “each other” or “opposite bodies”. In fact both W and N are acting on the man.
(3) If the man is not at rest but accelerating, W and N will no longer be equal in magnitude.
When you truly understand N3L, you will realize that the term action-reaction is a misnomer because, really, the pair of forces must happen together. They are equal partners and neither force is more “active” nor more “reactive” than the other.