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Homework Help: Equilibrium of Coplanar Force Systems HW Problem

  1. Oct 29, 2006 #1
    I've been having trouble with the homework problem attached as an image.

    I need to find the tension in the cable and the reactions at A (Ay and Ax). In previous homeworks, I've been able to find the reactions at similar points A by finding the moment about A around a few other points. Now that there is a pulley and platform involved, I'm drawing a blank.

    In the book it says to find the Moment about the pulley the equilibrium equation is: (sum)Mo = T1(r) - T2(r) = 0 ... from which we get T1 = T2

    This is an odd problem so the answer is in the back of the book and no matter what I do, I cannot get that answer.

    I changed the mass of the boy and platform into units of Newtons as well.

    Can anyone shed any light on this problem for me?

    Thanks alot

    Attached Files:

    • 3-63.jpg
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  2. jcsd
  3. Oct 31, 2006 #2
    Anyone have any suggestions? Really scraping the bottom of the barrel here. Thanks
  4. Oct 31, 2006 #3


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    Homework Helper

    Too bad the attachment doesn't work.
  5. Oct 31, 2006 #4
    Last edited by a moderator: May 2, 2017
  6. Oct 31, 2006 #5


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    The attachment/image seems to working fine.

    The tension in the cable is simply a force, T. The tension applies a force on the platform via the pulley. The resultant force on the pulley introduces a moment on the platform about pivot at A. The weight of the boy and the mass of the platform also produce moments about the pivot at A.

    For a distributed load (such as the platform weight), the moment arm is at the center of gravity.

    Moment producing forces are normal to the respective moment arm.
  7. Nov 1, 2006 #6
    I would love to say that that information was helpful, but it just made me more confused. Does it matter that the rope is at an angle?
  8. Nov 1, 2006 #7


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    Of course, it matters. Check your Static's book for more details, or post an attempt.
  9. Nov 13, 2006 #8
    Are the answers 143.6 and 445 and 71.8?
  10. Nov 14, 2006 #9
    No, but you're close.

    T = 143 N, Ax = 71.5 N to the left, and Ay = 464 N up
  11. Nov 14, 2006 #10
    I got it now, I had left out a cosine. Make a cut through the cable up at where it attaches to the top support. Then sum moments about point A using the vertical weights given in order to find the tension in the cable.
  12. Nov 14, 2006 #11
    I'm not following "make a cut through the cable" part?

  13. Nov 14, 2006 #12
    It is just making a free body diagram with the cable detached from the support. That is how I found the force in the cable. There are 3 steps; sum moments about A with the cable cut to get the cable force (note the cable is conveniently perpendicular to the platform), then sum vertical forces, and then sum horizontal forces to get the reactions at A. This was the easiest way for me to get the answers but it may not match the procedure your book suggests.
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