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Homework Help: Statics question

  1. Jun 9, 2006 #1

    Here's a statics problem that I'm unsure about. The system is in static equilibrium, and I'm supposed to find the reaction forces.

    {convention: right and up are taken as positive for x and y respectively, and counter-clockwise is positive for moment}

    [tex]\sum F_{y} = 0[/tex]
    [tex]B_{y} = -1000N[/tex]

    [tex]\sum F_{x} = 0[/tex]
    [tex]A_{x} + B_{x} = 0[/tex]

    Taking the sum of moment at the centre of the circular thingy
    [tex]\sum M_{z} = 0[/tex]
    [tex]0 = -0.05(A_{x}) + 0.05(B_{y}) - 0.25(1000)[/tex]

    [tex]A_{x} = -6000 N[/tex]
    [tex]B_{x} = 6000 N[/tex]

    I solved this problem assuming [tex]A_{y} = 0[/tex], by inspecting the diagram. Is that alright? In cases where it's not as obvious, how do I determine which reactions are not supposed to be considered? I think I must be neglecting something small.

    Thanks in advance.
    Last edited: Jun 9, 2006
  2. jcsd
  3. Jun 9, 2006 #2


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    You can't just randomly consider a reaction force to be 0. I'm not sure how you inspected the diagram to come to that conclusion.

    It comes down to you having 4 unknown reaction forces ([tex]A_x,A_y,B_x,B_y[/tex]) and only 3 equations of equilibrium ([tex]\sum F_{y} = 0[/tex], [tex]\sum F_{x} = 0[/tex],[tex]\sum M = 0[/tex]) Like you said, you have to find some other equation based on other information.

    By looking closer at the diagram it appears as if point B is not physically attached to the circle. That would suggest that it is free to move vertically so [tex]B_y=0[/tex]. It's hard to say from that diagram though.

    Does anyone have a second opinion?

    (By the way, welcome to PF and thanks for showing your work and making it very clear)

    Edit: Also, when you summed the forces in the y direction I think you missed a negative sign.
    Last edited: Jun 9, 2006
  4. Jun 9, 2006 #3


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    I don't think you can assume that, infact I think the opposite, for the system to remain static Ay must be non zero. Perhaps I am wrong, could you post your thought processes.
    I would agree with you there dav. We can say that By = 0 if we assume that the system is frictionless, which seems like a reasnable assumtion as we are not given any coefficents. I am also assuming ofcourse that the spanner is light.

    So now you have three unknowns and three equations. Happy Days :smile: I also would like to comend you on your post, all the required information was provided and it is presented in a clear logical fashion, a pleasure to read.
    Last edited: Jun 9, 2006
  5. Jun 9, 2006 #4


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    Homework Helper

    Definetly agree with the replies, but i will also want to request the problem statement, maybe there's more info there?
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