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(ME Statics) Method of joints

  1. Oct 19, 2014 #1
    1. The problem statement, all variables and given/known data
    So this is more of a general question as I'm trying to follow the book's way of solving the problems and it's not working.
    This is what the book says:

    Always assume the unknown member forces acting on the joint’s
    free-body diagram to be in tension ; i.e., the forces “pull” on the pin.
    If this is done, then numerical solution of the equilibrium equations
    will yield positive scalars for members in tension and negative scalars
    for members in compression. Once an unknown member force is
    found, use its correct magnitude and sense (T or C) on subsequent
    joint free-body diagrams.

    This is my example problem:
    Hibbler.ch6.p12.jpg
    Find the internal forces in B.

    2. Relevant equations


    3. The attempt at a solution
    Skipping a couple of steps ahead I know that the correct answers for GB=27.5 (Tension) and AB=28 (compression).

    Now drawing the FBD according to the convention laid out by the book:
    Untitled.jpg
    I realize that I could infer that the force going in the Y direction HAS to be positive in this case, but I chose a simple example for simple calculations. Lets pretend like we didn't know which direction it should go and simply follow the book's suggestion that we draw all unknowns as tensile forces.

    Now for my math:

    ∑Fy=0=-28Sin(63.4)-BF
    BF=-25.04

    This is NOT correct. The correct answer is 25.04 as we can tell by inspection.
     
  2. jcsd
  3. Oct 20, 2014 #2
    Why did you draw AB in tension but state it is compression?
     
  4. Oct 20, 2014 #3
    I was under the impression that if it was in compression at AB the arrow would go the same way at point B. My bad. I figured out what I was doing wrong last night, thanks though :).
     
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