The method of sections finds selected truss-member forces without solving every joint in the structure.

Imagine cutting the truss through the members whose internal axial forces are of interest. The cut exposes those internal forces, which become external forces on either part of the cut structure.

Choose the cut

In a planar truss, three independent equilibrium equations are available, so a useful cut usually passes through no more than three members with unknown forces.

Isolate one side

Draw a free-body diagram of whichever side of the cut contains fewer loads or simpler geometry. Replace each cut member by an axial force along its member direction.

Apply equilibrium

Use

$$\sum F_x=0,\qquad \sum F_y=0,\qquad \sum M=0.$$

Taking moments about the intersection of two unknown member-force lines can eliminate both of them and reveal the third directly.

The method of sections is particularly efficient when only a few member forces are needed.