Once again, for any truss member that is either in tension or compression, if we run an imaginary cut through it, 
the internal force can be imagined to have been exposed and it can be considered as external.
So now we have a way now to apply rigid body equilibrium method to analyze the internal reaction. 
We will learn more about the internal reactions later. 
Since, if you recall, one 2D rigid body free body diagram enables us to solve for a maximum of three unknowns, so if we only need to solve for 
the forces in no more than three truss members in the structure, we can imaginarily cut the structure open and apply 
rigid boy equilibrium analysis. This is known as the method of sections. 
Let’s look at this example. 
For this structure, we are not asked to solve for all members, just BC, CF and CG. Therefore instead of using the method of joints, 
we use the method of sections, because this way there is fewer steps involved.
Here is the plan: since we need to solve for the forces in members BC, CF and CG, 
we can run an imaginary cut through these three members.
And now the internal forces in these three members are exposed. 
Note how they are actions and reactions on the left and right segment. 
So we can choose either side for the calculation and we will get the same results. 
Don’t forget, we can only solve for three unknowns using one rigid body free body diagram, 
therefore either side we choose, we need to determine the support reaction first.
Now, the left segment has only one support reaction associated with the roller while the right segment has two support reactions associated 
with the pin. So for convenience, we should pick the left segment for further analysis. 
with the pin. So for convenience, we should pick the left segment for further analysis. 
So the first step, for this problem, it is necessary to determine the support reaction at point A. 
To do so, we draw the free body diagram for the entire structure, 
and summarize the resultant moment
about point D, and from this single equation we are able to solve for the force A_y.
Keep in mind that for some problems it might not be necessary to determine the support reaction, so you should use your own judgment. 
Step two, cut the structure 
and we’ve already decided to choose the left segment for our analysis. 
Step three, apply rigid body equilibrium to solve for our unknowns. We start with a free body diagram of the left segment and since we 
have already solved for the support reaction at point A, we have only three unknowns: F_BC, F_CF and F_FG, 
and we can write three equilibrium equations. Remember normally we would write two force equations and one moment equation. 
However, note that how force BC and force CF are concurrent 
and force CF and force FG are also concurrent, therefore it will be convenient to write 
two moment equilibrium equations about point C and point F. 
See, when the moment is summarized about point F, only F_BC shows up and you can solve it easily from this equation.
And when moment is summarized about point C, you only have force FG, and can easily solve it as well. 
Lastly, let’s write one force equilibrium equation along the x direction, 
and from it we can solve for F_CF.
Hopefully from this example you have learned how to apply the method of sections for your simple truss analysis.
