Physics Narrative for 14-16
Interactions modelled as pairs of forces
Pairs of forces replace each interaction. The interaction diagram, introduced earlier in this episode, is a thorough way to keep tabs on all the force pairs (remember: one per interaction).
You can identify the interactions by paying close attention to the physical situation, noticing certain changes, and using your developed skills as a force-spotter to identify certain forces acting on objects according to their environment. The difference now is that you're also paying attention to the
other end of the interaction, so treating the
environment as a second object.
Now you have a universe of a pair of interacting objects, rather than an object isolated from its environment (as you did in the SPT: Forces topic and in episode 01 of this topic).
Reduce multiple interactions to sets of pairs
Where there are multiple interacting objects, you simply consider them in pairs. One interaction per pair. Two forces per interaction.
Once you've finished noticing all the interactions, and recording all the pairs, then simply inspect each object. Identify all the forces acting on it and then add these to calculate the resultant force.
The resultant force, acting on that single object, changes the motion of the object.
Ramming the point home
Research shows you can't say it enough, so we'll risk that error here.
The two forces in the force-pair of Newton's third law each act on two different objects.
Or, if you really must:
In identifying force-pairs,
action is a force acting on one object and
reaction is a force acting on a different object.