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Statics of this crane

  1. Sep 13, 2012 #1
    ]1. The problem statement, all variables and given/known data[/b]

    The problem is attached in the picture

    3. The attempt at a solution

    The free-body diagram is in the second picture. However, when I try to do

    1)Sum of horizontal forces = 0
    2)Sum of Vertical forces = 0
    3)Moments about A, B, C, = 0

    I get nonsensical answers...

    Could it be that at the pins my forces are in the wrong direction?

    Attached Files:

    Last edited: Sep 13, 2012
  2. jcsd
  3. Sep 13, 2012 #2
    Why do you compute moments about A, B and C? You should compute moments about ONE point.
  4. Sep 13, 2012 #3
    What I meant was, form 3 equations, like do it 3 times.
  5. Sep 13, 2012 #4


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    Perhaps show us the working that gave you non-sensical answers so we can try and find the mistake.

    In your second diagram you appear to have omitted forces on the winch.

    Are you assuming T is vertical?
    Last edited: Sep 13, 2012
  6. Sep 13, 2012 #5
    Again, you don't need to compute the moments about different points. Choose just one, and the sum of the moments of all the forces about this one point must be zero.
  7. Sep 13, 2012 #6
    I'm trying to solve the forces on the crane first..

    Letting the joint A horizontal and vertical forces be F1 and F2, joint D F3 and F4, we have:

    Sum of forces in x-direction

    F1 = F3

    Sum of forces in y-direction

    T + 800 = F2 + F4

    Moments about A

    T(1) + 1600 = F3 (1)
    T + 1600 = F3

    Moments about B

    800 + F2 + F4 = F3

    Moments about C

    T + 2F3 = 2F2 + 2F4
    Last edited: Sep 13, 2012
  8. Sep 13, 2012 #7
    Yes, I'm doing it by the simplistic approach first before considering T tilted at 30 deg..
  9. Sep 13, 2012 #8
    any input would be appreciated!
  10. Sep 14, 2012 #9


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    Do not do it. Write up the horizontal (x ) and vertical (y) components of all forces acting at crucial points. You have the load first. What forces act on it? What about the pulley in the middle?
    What are their torques acting on the beams?
    I suppose the beams are considered massless, so the tension is the same in them along their length.

  11. Sep 14, 2012 #10


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    What he said.

    This bit...

    is missing forces because of the approach you took ignoring the winch.
  12. Sep 14, 2012 #11
    Ok how about this now...

    I am only considering all the forces acting on the crane....hence the free-body diagram is only of the crane and not inclusive of the winch

    Attached Files:

  13. Sep 14, 2012 #12
    I think you'd find it easier in the long run to consider all forces now rather than having to back-track and recalculate. Trust me, looking at things too simply is a hard habit to break and can lead to things being overlooked.
  14. Sep 14, 2012 #13


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    T1 acts at an angle so it has components in both x and y. If you show it verticaly you will (and did) forget about the x component.

    Take a look at the forces on pulley C. There will also be T2 acting to the left there.

    In short by ignoring the winch you appear to have forgoten some of the forces acting on the crane.
  15. Sep 14, 2012 #14
    is the horizontal component at B the same as the horizontal component at C?? In my opinion I don't think so..

    Then the result would be too many unknowns and too little equations..
  16. Sep 14, 2012 #15


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    Not clear what you mean by "the horizontal component at B".

    Pulley B has two forces acting it. Neither are vertical.
  17. Sep 14, 2012 #16
    Ok, I still can't see what's wrong with my free-body diagram. Point B has a vertical and horizontal component. Point C I shall assume only vertical component for simplicity. Point A and D are shown in the picture.
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