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Moment and equilibrium

  1. May 10, 2010 #1
    1. The problem statement, all variables and given/known data

    See attached image file

    2. Relevant equations

    law sine
    law cosine

    3. The attempt at a solution

    Find the reactions about two points from the applied force at point B

    Bx = 200Cos60 = 100N
    By = 200Sin60 = 173.20N

    May = -173N(.4m) + CyN(.15m) = 0
    May = -461.86Nm

    Max = -100N(.3) = -30Nm

    Ma = -491.86Nm

    I tried solving to find reaction at a, but not 100% if my result is correct. Any help would be appreciated
     

    Attached Files:

  2. jcsd
  3. May 10, 2010 #2

    PhanthomJay

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    these are not relevant; what are the equilibrium equations?
    yes, direction?
    Are you trying to solve for Cy (in Newtons) by summing moments about A? If so, your equiibrium equation and units are incorrect.
    What point are you summing moments about?
    You must first note that the support at A can provide only a vertical force, since it is a roller joint free to slide horizontally. The pinned support at C can provide both a horizontal and a vertical force. You have 3 unknowns, Ay, Cx, and Cy, and 3 equilibrium equations. Try summing forces in the x direction first, then try again to sum moments about A. Do not forget to include the moments from all force or force components.
     
  4. May 10, 2010 #3

    Redbelly98

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    EDIT: PhanthomJay beat my response. Gotta learn to type faster :smile:

    Those are not relevant at all here. Instead, something involving the sum of the forces and sum of the moments would apply.

    Okay, though By will point downward and that is usually indicated as a negative value.

    Is this supposed to be the torque about point A? It's not correct, for one thing you are saying it is both 0 and -462 Nm.

    Before trying again, make a list of all x and y forces that act at points A and C. (You already have done this for point B.)

     
  5. May 11, 2010 #4
    So Cx + Bx = 0
    Cx + 100 = 0
    Cx = -100N?
     
  6. May 11, 2010 #5

    Redbelly98

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    Yes, that's right.
    Next do the same thing for:
    • the y-forces
    • the moments about a point of your choice.
    (Hint: either points A or C are good choices for setting up the moment equation.)
     
  7. May 11, 2010 #6
    I'm confused trying to find Cy and Ay.
    But this is what I tried doing.


    0 = Fay + FBy
    Fay -173.2 = 0
    Fay = 173.2

    Fay + Fby + Fcy = 0
    Fcy = 0?

    or is it

    250mm(-173.2N) = Fay(150mm)
    Fay = 288.66N (I think this is the right way)
     
    Last edited: May 11, 2010
  8. May 11, 2010 #7

    PhanthomJay

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    No
    Yes
    No. If you are summing torques (moments) about C (are you?), you must include the torque from Bx (the horizontal component of the force at B). Solve for Ay, then use your other equation to solve for Cy.
     
  9. May 11, 2010 #8
    I'm really frustrated with this and I give up, If someone explains this to me I will unlock whatever it is that's blocking mi mind to get through it. Can someone please explain it to me.
     
  10. May 11, 2010 #9

    PhanthomJay

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    You have already calculated Cx correctly, by noting that the sum of the x forces must equal zero, for equilibrium . Since Ax = 0 due to the roller support at A, Cx is 100N acting to the left on the bracket. That is to say,
    Ax +Bx +Cx =0
    0 + 100 + Cx =0
    Cx =-100N

    Now you look at forces in the y direction. They also must sum to zero.
    Ay +By + Cy =0
    You know the value of By, but you don't know Ay or Cy yet, so you need one more equation of equilibrium, which is
    "Sum of Moments of all Forces about any Point =0."

    If you choose point C as your point about which to sum moments, then calculate the moment about C from By (which you did), the moment from Bx (which you did not do), and the moment from Ay (which you did, in terms of Ay, an unknown), and add them all up, set them equal to zero, and solve for Ay. When you sum moments, watch your plus and minus signs; if you choose a clockwise moment as minus, then a counterclockwise moment is plus. Note that the forces at C do not contribute any moment when you sum moments about C. Once you solve for Ay, then you can get Cy from your Ay +By + Cy =0 equation.
     
  11. May 11, 2010 #10
    Ok I drank some cheap wine and my light bulb went off. I feel stupid now, but I got it figured out.

    Ma = 0
    (-100Nx300mm)+Cy(150mm)-(173.21Nx400mm)
    = Cy = 661.89N

    Mc = 0
    (Ay150mm) - (100Nx300mm)-(173.21Nx250mm)
    Ay = 488.7N

    Thanks to all for your help
     
  12. May 11, 2010 #11

    PhanthomJay

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    Always check your answer by summing forces in the y direction
    Ay + By + Cy =0
    It's a good check for the proper direction of the y forces.
    Then celebrate with a tall glass of Cabernet Sauvignon.
    And almost never ever give up.
     
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