The movie below illustrated the way in which each of the loads on the structure (in this case a bench) are resolved into single loads. Each and every physical load that acts on the structure must be represented. This means that all of the laods are replaced by vectors. Even the supports are replaced by single loads.
Reactions of a Beam
Horizontal Components of a Reaction
Everything that is needed to solve a force system is included on the FBD. Free body diagrams may not seem necessary in the relatively simple current applications, but as problems become more complex, their usefulness increases.
The following is the process for determining the reaction at the wall for a cantilever beam. A FBD is first drawn of the beam. Next, cut the beam free from the wall and replace the wall with the forces that were supporting the beam at the wall before it was cut free. These forces are unknown, but they are the only forces that can keep the beam in equilibrium. They are identical to the internal forces in the beam at that point before it was cut. The internal forces in the beam before it was cut free from its support are also determined when the forces which will keep, or put, the FBD in equilibrium are found.
A fixed support will resist translation in all directions and rotation (moment). The FBD must show all of these directions. The principles of equilibrium can always be used to solve a FBD. In the FBD above Sum Fy = 2K and Sum Fx = 0. The 2K forces (load and vertical reaction force) cause a counter-clockwise couple of 10 K-FT which must be resisted by a moment on the end of the cut section of 10 K-FT acting in a clockwise direction.
The following are three different systems which all have one 100 pound load and one 150 pound load actin on them at exactly the same point. They are also supported in the same manner.
This is a Free Body Diagram of these three systems which has been drawn to represent the force system. Note how the internal arrangements of the three has been removed. The internal arrangement does not matter for the determination of the supporting reactions ! AND, if the supporting and loading geometries are the same, the external reactions will be the same.
Creating a Free Body Diagram