Newton's Third Law
Forces and Motion

Developing diagrams

Teaching Guidance for 14-16 Supporting Physics Teaching

Comprehensive diagrams

Wrong Track: The forces acting are obvious. There's always gravity and friction, and a bit of buoyancy, which we nearly always ignore – even if I've no idea why.

Right Lines: To build a model of a situation where there are a number of interacting objects takes skill and care. You'll need to be thinking about the interactions between each of the objects, then assign linked pairs of forces to each object – linked by Newton's third law, that is.

Thinking about the learning

There's a tendency to promote and encourage a rush to the answer. This often relies on cutting out several stages of the modelling process, which can lead students off down the wrong path. If they're ever to see what the Newtonian world view buys them, they'll need to engage with the modelling.

Thinking about the teaching

In the Physics Narrative, and again in the Teaching Approaches, we've encouraged the use of interaction diagrams for those who want to develop a full description of a situation. If these seem over-complex for the students you teach (we think that's likely, for pre-16 students), you can revert to use the approach suggested of isolating the object from its environment, and drawing only that object and the forces acting on it. That'll be enough to find the resultant force and so predict the changes in the motion of the object. This is the approach developed and suggested in this topic and in the SPT: Forces topic.

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