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Hanging sign forces?

  1. May 7, 2009 #1
    hanging sign forces??

    1. The problem statement, all variables and given/known data

    PIC IS ATTACHED

    A businessman is hanging a sign outside his store in the
    fashion shown in the figure below. The sign has a mass of
    50 kg. For a sign of this mass, what is the tension in the
    wire? The sign is rigidly attached to the wall at the lower
    left corner.

    A. 346 N
    B. 260 N
    C. 490 N
    D. 87 N
    E. 173 N



    3. The attempt at a solution

    i am stuck. only thing i though of was find mg then find the sin of the angle
    so i did mg sin (45)
    490 sin (45) got 370.
    which is not even close to letter E (which is the right answer)
     

    Attached Files:

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  2. jcsd
  3. May 7, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Re: hanging sign forces??

    If it's rigidly attached as it says in the question - then the tension in the wire is zero.
    ie the sign would stay there without the wire

    Normally in these question you assume the weight acts at the centre of the sign and take moments about the point that it joins the wall (assumed to pivot) to find the upward force at the outside end
     
  4. May 7, 2009 #3
    Re: hanging sign forces??

    The question is flawed.
    Answer E is obtained if you assume the sign
    can pivot freely about the bottom left corner.of the sign.
    (hardly "attached rigidly"!)

    Remembor the pivot can exert a force on the sign, but no torque.

    David
     
  5. May 7, 2009 #4
    Re: hanging sign forces??


    so if we assume the sign can pivot, how would you go about getting answer E??
     
  6. May 7, 2009 #5
    Re: hanging sign forces??

    Take moments about the pivot
     
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