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Homework Help: Torque in a Crane

  1. Jul 22, 2009 #1
    1. The 12.2m crane weighs 18 kN and is lifting a 67-kN load. The hoisting cable (tension T1) passes over a pulley at the top of the crane and attaches to an electric winch in the cab. THe pendant cable (tension T2), which supports the crane, is fixed to the top of the crane. Find the tensions in the two cables and the force Fp at the pivot.

    There is a picture with this, but I can't upload it right now. The crane makes and angle of 40degrees with the cord holding the load (67 kN). T1 on the other side of the crane makes an angle of 5 degrees and T2 makes an angle of 10 degrees with the crane. I can upload a picture of this later, but this will only be possible after probably 8 tonight.

    2. Torque=rT

    3. I think I might have T1, 67 kN.

    Please help me!
  2. jcsd
  3. Jul 22, 2009 #2
  4. Jul 22, 2009 #3


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    Homework Helper

    You need to show us your work. I assume that the pivot point is the upper right corner of the yellow rectangle (where the boom attaches to the cab).

    Hint: you probably need a more general version of the relevant equation that you listed. Hint: there are more relevant equations.
  5. Jul 22, 2009 #4
    I drew a line of action through the point of application (the pulley at the top) and a lever arm.

    lever arm = r sin 5 = 12.2m * sin 5 = 1.063m
    1.063m * 67 kN = 71.24 kN*m

    That's my work, but I don't know if I did it right. I'm not sure I drew the line of action right because I don't know which direction the torque is going in. Also, I don't know the sign of the torque.

    As for T2, is it simply 67 kN+ 18kN?
  6. Jul 22, 2009 #5


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    Homework Helper

    OK, I will make more specific hints.

    So far, you are showing a torque calculation, but it is not clear to me that you know why this calculation will help you answer the question. What principles (i.e. laws) of physics should you apply to solve this problem, and what is your reasoning for this? The important thing here is to justify why they are important, because this will tell you how to solve the problem. The rest is details.

    How many forces are applied to the boom? (I count five.) Can you count them up? Where, and in what directions, are these forces applied to the boom?

    BTW, I believe that you are correct about the value of T1.
  7. Jul 22, 2009 #6
    I'm not sure I follow, but please reconsider, keeping the following in mind.

    Does a force with a specified line of action, have a moment about a point on that line of action?
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