Newton's Third Law
Forces and Motion

Active and passive

Teaching Guidance for 14-16 Supporting Physics Teaching

When I act, and when I don't

Wrong Track: I push on the table, and then it pushes back.

Right Lines: Isolating the table, there's a force exerted by you, acting on the table. Isolating you, there's a force exerted by the table, acting on you.

Equal, but symmetrical descriptions

Thinking about the learning

The idea that one thing pushes, and then another pushes back might grow naturally from the suggested introduction to forces in the SPT: Forces topic, where the force exerted by an inanimate object is likened to the actions that you can take to effect changes in the world.

Teacher: You can push and pull, and so can other things


Thinking about the teaching

We'd suggest that you develop a forces acting on object description from the interactions of an object with its environment for each occasion. Newton's third law is useful as you switch focus from one interacting object to another, but it applies to all forces equally.

In some ways that's the whole point of the forces-plus-masses world view. Once you have simplified to forces acting on a mass, then you can find the resultant and predict the motion without further reference to the environment.

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