Newton was the first to recognize how the two members of an action/reaction pair of forces are related to each other. Today we know this as Newton’s third law:
We deduced most of the third law in Section 7.2. There we found that the two members of an action/reaction pair are always opposite in direction (see Figures 7.6 and 7.10). According to the third law, this will always be true. But the most significant portion of the third law, which is by no means obvious, is that the two members of an action/reaction pair have equal magnitudes. That is, . This is the quantitative relationship that will allow you to solve problems of interacting objects.
Newton’s third law is frequently stated as “For every action there is an equal but opposite reaction.” While this is indeed a catchy phrase, it lacks the preciseness of our preferred version. In particular, it fails to capture an essential feature of action/reaction pairs—that they each act on a different object.
NOTE Newton’s third law extends and completes our concept of force. We can now recognize force as an interaction between objects rather than as some “thing” with an independent existence of its own. The concept of an interaction will become increasingly important as we begin to study the laws of energy and momentum.