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Compute the distance x and tension of each cable

  1. Feb 9, 2014 #1
    1. The problem statement, all variables and given/known data
    The thin steel plate, weighing 82lb/ft^2, is being lifted slowly by the cables AC and BC. Compute the distance x and find the corresponding tension in each of the cables.
    [See attachments for Figure]

    2. Relevant equations
    Summation of X = 0
    Summation of Y = 0
    Moment=Rd

    3. The attempt at a solution
    So i tried to get the weight of the steel plate. here's the solution..
    2(21) + 4(12) + 1/2(9)(4) = 108ft^2
    now i multiply the given 82lb/ft^2 to cancel out the ft^2
    i get 8856lb.. After that i don't know where to start next. Can you guys help me out?
    I really think the problem is easy but i got lost on where to start...
     

    Attached Files:

  2. jcsd
  3. Feb 10, 2014 #2

    CWatters

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    I've not done many of these but I imagine it will swing so that the centre of gravity is below the hook. So where is the centre of gravity?
     
  4. Feb 10, 2014 #3

    ehild

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    What do you mean by
    "Summation of X = 0
    Summation of Y = 0
    Moment=Rd"

    The plate is moving with a small constant speed upward. What is the acceleration then? It is kept horizontal, it does not turn. What do these mean on the forces and torques (moments) acting on it?



    ehild
     
  5. Feb 10, 2014 #4
    so you mean forces along x-axis is = to 0 and also the torque.
    And how would i find the two angles there.
    Should I Sum all the torque along A?
     
  6. Feb 10, 2014 #5

    ehild

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    The sum of the x components of the forces is 0, and also the sum of the y components of the forces.

    Also the sum of all torques about any point is zero. You can choose A.

    You need to write up three equations in terms of x and the the unknown angles.

    AS CWatters wrote, x is related to the position of the CM.

    ehild
     
  7. Feb 10, 2014 #6

    haruspex

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    The information doesn't seem to be complete. It looks like the top of the plate is horizontal, and it looks like AC and BC have the same length. But these cannot both be true, and neither is clearly given as true. At a guess, AB is horizontal. At the least, that's the easier version of the problem.
     
  8. Feb 13, 2014 #7

    CWatters

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    I've looked at it again and I think haruspex has a point. I think they are asking you to work out what value of x is needed for the top edge of the plate to stay horizontal (and not as I suggested above that it will rotate). This implies AC and BC are different lengths but the lengths aren't specified so that's ok.
     
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