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Simple Tension Problem

  1. Apr 21, 2013 #1
    1. The problem statement, all variables and given/known data
    I am reviewing for a test tomorrow and cannot figure this problem out. A sign with the mass of 100kg hangs from the ceiling. If the two cables that are attach the sign to the ceiling are at a 50 degree angle from the horizontal, what is the tension in each cable? The answer is 640N, but i don't know why?

    2. Relevant equations
    w=m*g


    3. The attempt at a solution
    I understand that w=m*g, and i found that w=931N, However i have no idea what to do from here.
     
    Last edited by a moderator: Apr 21, 2013
  2. jcsd
  3. Apr 21, 2013 #2

    Doc Al

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    Staff: Mentor

    Recheck your math. What did you use for g?

    Draw yourself a free body diagram of the sign including all the forces acting on it.

    If the sign is in equilibrium, what must be the net force on it?
     
  4. Apr 21, 2013 #3
    first the calculations was a typo i meant to say 980. secondly i understand that the net force should be zero. This problem still makes no sense to me.
     
  5. Apr 21, 2013 #4

    Doc Al

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    Staff: Mentor

    Good.

    Did you draw a free body diagram of the sign? What forces act on the sign? What are their vertical components?
     
  6. Apr 21, 2013 #5
    ok, so i drew the free body diagram and the only things i was the angles of the two and the weight.
     
  7. Apr 21, 2013 #6

    Doc Al

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    Staff: Mentor

    I assume you mean that the only forces acting are the two tensions, which act upward at an angle, and the weight.

    So if you call the tension in each cable T, what is the vertical component of that tension force?
     
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