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Homework Help: What is the tension in the cable?

  1. Mar 31, 2010 #1
    1. The problem statement, all variables and given/known data

    Problem's pic:
    http://i92.photobucket.com/albums/l18/bonbons06/steelBeam01.png

    Steel construction beams, with an industry designation of W12 × 22, have a weight of 22 lb per foot. A new business in town has hired you to place its sign on a 4.0 m long steel beam of this type. The design calls for a beam to extend outward horizontally from the front brick wall (see figure). It is to be held in place by a 5.0 m long steel cable. The cable is attached to one end of the beam and to the wall above the point where the beam is in contact with the wall. During an initial stage of construction, the beam is not to be bolted to the wall, but to be held in place solely by friction.

    What is the tension in the cable? (lb)


    2. The attempt at a solution

    The beam weighs 22 lb./ ft * 13..115 ft = 288.5 lbs
    The friction force between wall and the beam supports 144.25 lbs

    The cable, the beam, and the wall make a 3-4-5 right triangle.
    The tension of the cable pulls at angle with a sin = 3/5 and cos = 4/5.
    The bottom right end of the cable supports ½ the weight of the beam.
    The vertical component of the Tension must support 144.25 lbs

    Tension * sin θ = vertical component of the Tension
    Tension * sin θ = = Tension vertical
    Tension * 3/5 = 144.25
    Tension = 240.4166 lbs => INCORRECT. What's wrong here?

    Please advise! Thanks!
     
  2. jcsd
  3. Mar 31, 2010 #2

    rl.bhat

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    The frictional force between wall and the beam supports 144.25 lbs
    How did you get this value?
    In the equilibrium condition
    Vertical component of the tension + frictional force = Weight of the beam.
     
  4. Mar 31, 2010 #3
    The cable supports ½ the weight of the beam and the wall supports the other half.
     
  5. Mar 31, 2010 #4
    Weight of the beam = (22 * 4) / 0.304 = 289.47 lbs.
     
    Last edited: Mar 31, 2010
  6. Mar 31, 2010 #5
    Still getting the same answer and it's incorrect :(
     
  7. Mar 31, 2010 #6

    rl.bhat

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    Frictional force = μ*Fn = μ*Tcosθ.
    What is the μ value?
     
  8. Mar 31, 2010 #7

    PhanthomJay

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    I think sometimes the problems try to get cute and mix up SI and USA units intentionally, while other times, it may be unintentional. Try assuming the beam length is 4 feet and the cable length is 5 feet, and see what you get.
     
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