Other forces acting within the truss are internal. A determinate truss will also have internal forces. That is, at every node there are two members joined together. Each member has internal forces acting within them. Using the method of joints, the two forces acting within each member are divided into their x and y components using the known angle at which they are placed.
Different structures have different configurations for angles and forces.
When all the forces are divided into their x and y components, that is when the six equations and six unknowns come into play. Starting at node one, the sum of all the forces acting in the x direction at node one are set equal to zero. The sum of all the forces acting in the y direction at node one are set equal to zero. Then moving on to node two, the sum of all the forces acting in the x direction at node two are set equal to zero and so on until all three nodes are done. The sum of all the forces also includes the known force given in the original configuration of the truss.