(1) The force diagram of the problem situation

* Helps identify the most convenient system and the objects in the environment that interact with the system.
* Show connections (e.g. ropes) or contacts between objects.
* Draw forces where they actually are applied.

(2) The free-body diagram

* Isolates the system and each body.
* Helps determine and represent the qualitative behavior of the system in terms of the direction and relative magnitudes of the forces.
* Each object has its own free-body diagram.
* Draw forces acting on center of mass of objects.

(3) The force-vector diagram

* Represents the forces on the system as mathematical vectors that can be conveniently added.
* Each object has its own force-vector diagram.
* Separate "vertical" and "horizontal" components.  Apply Second Law to each.