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Homework Help: Statics question

  1. Jun 10, 2007 #1
    1. The problem statement, all variables and given/known data


    This is the diagram of the question. There are 3 wooden beams nailed together and supported at A,D, and F. Assuming only vertical forces, what are the vertical reactions at A, D, and F?
    The solution uses this diagram to make three basic equations:
    (1) for BEF, F=2/3(P+E)=2B
    (2) for ECD, D=2/3C=2E
    (3) for ABC, A=2/3B=2C

    For (2), why is it D=2/3C=2E and not D=2/3C=2(P+E)?
    Why doesn't P come into the equation? Isn't it acting at point E? I understand the solution completely except for this one point. Thank you.

    Using algebra I can get the solution of D=1/13P, F=9/13P, A=3/13P
    But only if D=2E rather than D=2(P+E)
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 10, 2007 #2
    Someone suggests that force P may be only applied to beam BEF, not on beam DCE. Would that make it work?
  4. Jun 10, 2007 #3


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    Gold Member

    That is not quite correct, force P is applied as shown, to the system of beams, at the intersection of the 2 beams BEF and DCE. Draw a Free Body Diagram of beam DCE. Isolate it such that you cut the beam with your Free Body Diagram immediately to the right of joint E, such that P, the applied force, does not enter into the diagram, and therefore the given solution appears correct.
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