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Homework Help: Tension of a cable?

  1. Mar 24, 2009 #1
    1. The problem statement, all variables and given/known data

    Two weights attached to a uniform beam of mass 56 kg are supported in a horizontal position by a pin and cable as shown in the figure.The acceleration of gravity is 9.8 m/s2 .

    What is the tension in the cable which supports the beam? Answer in units of kN.


    2. Relevant equations

    3. The attempt at a solution

    here's what I have thus far... I'm sorry, I know it's not much!

    Mbeam= 56k
    Mass 1 = M1 = 22kg
    Mass 2 = M2 = 59kg
    Force of M1 = F1 = M1g = 215.6N
    Force of M2= F2 = M2g = 578.2N
    ΣFy = 0
    ΣFx = 0
    ΣFτ = 0
    If ΣFy = 0, then:
    0 = Fcy – mg – F1 – F2
    If ΣFτ = 0, then:
    0 = -mg(4.2m) – F1g(2.6m)

    I'd really love it if someone could solve this for me, but it would be even better if someone could walk me through how to get the answer because I honestly have no clue!

    Thank you in advance!
  2. jcsd
  3. Mar 24, 2009 #2


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    Homework Helper

    It is a mistake to use ΣFy = 0 because you don't know what force acts at the pivot point on the wall. Instead, use the sum of the torques = 0. Take the torques about that pivot point marked with a big black dot.
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