Method of Sections: Ignored Forces?

[influx]

In the above example, the method of sections is used to solve the truss. However, they do not consider the forces in other beams (the beams that are not being cut; e.g. AB, AH). Is this always the case with this method?

 

[mfb]

There are no external forces coming from those beams.

 

[influx]

There are no external forces coming from those beams.

I understand but why are we not considering the internal forces eg force AH (like we are considering the internal force HG)

 

[mfb]

HG is not an internal force in this part. That's the idea - split the whole constructions in smaller parts. Some splittings work, some do not, this is a working one.