Truss analysis by method of sections for member AB A straight cut passes through members AB, AD, and ED of the given truss. To isolate the internal force in member AB using a single equilibrium equation, the moments should be taken about the joint where the other two cut members (AD and ED) meet, so their unknown forces do not create a moment. Select the correct joint about which moments must be taken to determine the force in AB efficiently.

Introduction / Context:
Truss members are commonly analysed by the method of sections to find internal axial forces quickly. The key idea is to cut through up to three members and write equilibrium for one side of the cut. Choosing a smart moment center can eliminate unknowns and leave one target force solvable in a single step.

Given Data / Assumptions:

• Section passes through members AB, AD, and ED.
• We need the force in member AB only.
• Planar truss, pin joints, axial forces in members, static equilibrium applies.

Concept / Approach:
Pick a moment center at the intersection of two cut members. The lines of action of their axial forces pass through that joint, creating zero moment. This removes two unknowns from the moment equation, leaving the third member force as the only unknown.

Step-by-Step Solution:

Verification / Alternative check:
Had we taken moments about A or E, at least two cut-member forces would contribute to the moment equation, complicating the solution. About D, exactly two vanish, which is optimal.

Why Other Options Are Wrong:

• joint A: Only AD passes near A; AB and ED would still contribute moments.
• joint B: AB passes through B, but AD and ED would still add unknown moments.
• joint C: Not an intersection of the cut members; no special elimination happens.
• None of these: Incorrect because joint D works.

Common Pitfalls:
Forgetting that axial forces act along member centrelines and thus pass through their joint intersections; choosing a moment center that does not eliminate two of the three cut forces.

Final Answer:
joint D