Newton's third law & interaction diagrams
Talking sense about force-pairs
Drawing an interaction diagram provides a way to generate a representation of all the interacting objects in a process, connected as interacting pairs. Each interaction results in a force acting on each of the pairs of objects.
Newton's third law provides a way to switch views between each member of a pair of linked objects. Suppose you've carefully worked out a force on one such object in an interacting pair. Newton's third law then functions as a kind of short-cut, to allow you to immediately write down the corresponding force on the other member of the pair.
This law supports the facility to switch your point of view, and to focus on the objects in turn, allowing one to form the environment for the object you're focusing one, or vice versa.
Fully fledged interaction diagrams are one way to analyse a situation. Isolating an object from its environment and identifying the forces acting on it was the approach adopted in the SPT: Forces topic. Now you can move around amongst the objects in the situation without having to start from scratch each time: Newton's third law enables you to carry some information over from one point of view to another.
Alternatively, you could approach Newton's Third Law as the principle that underlies the construction of interaction diagrams – a way of gluing together the interaction diagram, as all forces appear in pairs and the necessary connections between these force-pairs (equal in magnitude, but opposite in direction: →FAB = -→FBA) are given by the law.
