Internal Forces in the Method of Sections

In the method of sections, which is commonly used for analyzing complex truss structures, the internal forces within the members of the structure are determined by applying the principle of equilibrium to a section of the truss. This allows us to calculate the internal forces such as axial forces, shear forces, and bending moments in the members.

External Forces
The external forces are the forces acting on the truss that are not located within the section under consideration. These forces include applied loads, reactions at supports, and any external moments. The external forces are applied to the truss as a whole and are not specifically related to the section being analyzed.

Method of Sections
The method of sections involves cutting the truss into two sections by passing an imaginary line through the desired section. This imaginary line is called the section line. By isolating one of the sections and considering the equilibrium of forces acting on that section, we can determine the internal forces within the members of the truss.

Calculation of Internal Forces
To calculate the internal forces in the method of sections, we consider the equilibrium of forces acting on the isolated section. This includes both external forces and internal forces within the members. By applying the equations of equilibrium, such as the sum of forces in the x-direction and sum of forces in the y-direction, we can solve for the unknown internal forces.

Choice of External Forces
In the method of sections, we only consider the external forces acting on the truss as a whole. This is because the internal forces within the members are a result of the equilibrium of these external forces. By neglecting the internal rotational forces and considering only the external forces, we can simplify the analysis and focus on determining the internal forces within the members.

Therefore, the correct answer is option 'D' - External forces.