After constructing all of your free body diagrams.

  1. Identify forces that are acting in opposite directions

  2. Of the forces found in step one, find which forces are the same type of force (eg Normal, friction, etc)

  3. Of the forces that are the same type, find the forces that act on the same two bodies. In the notation used in class, they will have the same subscripts, but reversed (eg: F1,2 & F2,1).

These will be force pairs and will have the same magnitude. Example: the force of gravity of the Earth on you is equal in magnitude but opposite in direction than the force of gravity of you on the Earth. You pull up on the Earth just as much as it pulls down on you.

No two Newtons 3rd Law force pairs will be on the same FBD.