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What Force?

The THIRD LAW is a real ringer. It looks so trivial, yet it warns us of a leading cause of confusion in mechanics problems: There are always two forces for every interaction! When A exerts a force $\vec{\bf F}_{AB}$ on B there is always an equal and opposite force $\vec{\bf F}_{BA} = - \vec{\bf F}_{AB}$ exerted back on A by B. The latter is arbitrarily designated the ``reaction force,'' but of course this is only because we first started talking about the former; both forces have equal intrinsic status. So if you say, ``The force between A and B is....'' I don't know which force you are talking about! Never talk about ``the force'' unless you mean ``the Force'' from Star Wars. Always make up a sentence describing the action taking place: ``The force exerted on [A] by [B] is....''