Newton's Third Law
Forces and Motion

The law's task (Newton's third law)

Teaching Guidance for 14-16 Supporting Physics Teaching

Not empirical

Newton's third law is not empirical. There wasn't an experiment you could do to show that it was wrong in Newton's time. Rather, it's a statement that allows the whole Newtonian description to be precise enough to get a grip on the lived-in world, and to be able to mimic certain elements of that world. It's an essential part of the whole, so can be shown to be helpful by association – the whole apparatus of Newton's laws is certainly reliable.

As you cannot do an experiment to either confirm or falsify the third law, how can you present it?

We'd suggest that you encourage students to see it as a law connecting pairs of interacting objects, that allows you to switch focus from isolating one object, to isolating the other. As you switch focus, so the law enables you, given the forces on one object, to immediately write down the forces on the other.

Teacher Tip: Treat Newton's third law as a way of correlating the descriptions between two different facets of the same process: one facet corresponding to the isolation of each object.

Newton's Third Law
is used in analyses relating to Collisions
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