3.1.3 Newton’s 3rd Law

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.

Video Explanation

Cheek Punches Glove


Action-Reaction Twins

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