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Homework Help: Question about tension and deflection (Mechanics of Material)

  1. Feb 12, 2010 #1
    Hello everyone, I have some difficulties to solve this question in my homework.

    1. The problem statement, all variables and given/known data
    http://img714.imageshack.us/img714/7953/picz.png [Broken]

    Rigid bar ACE connected to rod AB, CD, EF which have same material and surface area A. Calculate tension caused by force P in three rods.

    2. Relevant equations
    Equation :
    sigma Fy = 0
    sigma M =0
    comparison between deflection in A, C, and E

    3. The attempt at a solution
    I try to solve this question but I can't find sufficient equation to solve it. It have 4 equations and I only find 3. Please help me solve this.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 12, 2010 #2


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    Have you tried summing the moments around different points?
  4. Feb 12, 2010 #3
    You haven't shown your working, but have you considered the property of the bar that it is rigid? What does that imply for the displacements of A, C and E?
  5. Feb 12, 2010 #4


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    these are your 3 relevant 'equations'
    there are 3 relevant equations (as you note; the last is concerned with geometric compatability) and 3 unknowns (the tensions in each rod). What do you mean by four ?(the x direction is irrelevant here). Note that the bar is rigid, but that the rods can extend or compress (which should have been more clearly stated in the problem). Hint: This problem is best solved by replacing the force P with an equivalent force-couple system at midpoint.
  6. Feb 13, 2010 #5
    When I try a few times again, I can solve it at last ! The unknown P I change to one so I have 3 unknowns and 3 equation then revert it back to P. Thanks all^^
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