Probably here we need some explanation to all of those are not familiar with the science of construction.
The second graphic is called “free body diagram”. It represents a cantilever beam with fixed support like the one where Mr D. Tail is standing on enjoying the sunset but strip out from all insignificant and irrelevant detail (insignificant for an engineer).
Any geometric structure is effected by loads, in our case by Mr D. Tail. These loads are manifested as external forces and represented to the free body diagram with arrows (F).
This simple diagram is used by the structural engineers as method of calculation to analyse how a force effecting the structure in order to dimension each element.
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