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Homework Help: What force do ropes exert on a sign

  1. Nov 21, 2008 #1
    1. The problem statement, all variables and given/known data
    A sign that weights 648 N is suspended by two ropes that make an angle of 38o with the horizontal. What force does each rope exert on the sign?

    2. Relevant equations
    A + B + W= 0
    Fh= F cos x
    Fv= F sin x

    3. The attempt at a solution
    This is how my textbook says to do it:

    The direction of W is down, so the direction of A + B is up. The sum A+B has no horizontal components, so the horizontal components of A and B, Ah and Bh have equal magnitudes. Now, Ah = Acos38o and Bh=Bcos38o. Since Ah=Bh, the magnitudes of A and B must be equal. The magnitude of the sum of the vertical components of A and B equals the magnitude of the weight of the sign, 648 N. That is, Av + Bv =648 N Since Av = Asin38 and Bv = Bsin38 and A=B, Av=Bv.
    Av=Bv =1/2(648N)= 324N
    And.. A=Av/sin38 = x
    B=A= x

    They don't tell me what x should be, nor what the 324N is for. Please help
  2. jcsd
  3. Nov 21, 2008 #2


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    Science Advisor

    Start by drawing a free body diagram (FBD). Then uses Newton's second law (Fnet = ma) in both the x and y directions. You'll end up with two equations that you can solve simultaneously.

    HINT: The force that the ropes exert on the sign is equal to the tension in the ropes.

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