Newton's Third Law
Forces and Motion

Newton's third law

Teaching Guidance for 14-16 Supporting Physics Teaching

Stating the law helpfully

Wrong Track: Action and reaction are equal and opposite.

Right Lines: An interaction between two objects can be replaced by two forces, one acting on each object. These forces are equal in magnitude, but opposite in direction.

Thinking about the learning

The neat and seemingly concise and precise statement:

Jo: Action is equal to minus reaction

is easily memorised, but has proved hard to use well as a way of thinking about situations.

One line of thinking that it sets in train is first action, then reaction, which leads to reasoning in chains rather than seeing the forces as replacing an interaction. The former view tells stories as a linear sequence; the latter emphasises that the forces on both objects arise simultaneously. There is no first one force, then the other force. It's not action, then reaction.

To use a form of words, however memorable, that encourages this line of thinking is to send students off down the wrong tracks.

Thinking about the teaching

Learning to think of complex interactions as being processes that happen all at once is not easy. Our natural causal explanations tend to be narrations in time, involving a temporal sequence (this leads to that, that leads to the other). We'd suggest that working with a longer version of the third law helps this transition.

Then you'd be well advised to think about the role that the third law might play in your students' thinking. If you've followed the development in the SPT: Forces topic and in the SPT: Force and motion topic, you'll already be able to predict motions of isolated objects, and have learnt how to isolate objects and to find the resultant force. So what does the third law add?

Some will prefer to start with the third law (and we think this might be appropriate if our teaching of forces started with older children), but met here, at this late stage, it looks like a short cut. That is, once you've worked out the force on one of a pair of interacting objects, you can simply write down the force on the other, using the third law. No further analysis is necessary.

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