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Forces on a Truss

  1. Mar 11, 2007 #1
    1. The problem statement, all variables and given/known data

    Ok, you'll have to wait for my picture. Basically we are doing trusses, and I'm not sure where I'm going wrong here.

    We are to find the tension/compression in each member(line) of the truss.

    Ok, from my picture. F1 is directly at B(in neg y direction), and is 2800N. F2 is at point D(in the neg x direction) and is 2100N.

    The distances between A&B, and B&C are 4m. A is a pin system(basically has x and y forces).....and C is held up by a cable(I believe only supports in the y direction).

    I also drew another picture, which consists of little circles on each joint, and arrows pointing in the directions of the tensions(and forces acting on the "circles").

    2. Relevant equations

    3. The attempt at a solution

    Ok, I'm only going to post some of this, to see if I started correctly(where I think I messed up).

    So, from my picture, I took overall equilibrium. So, I chose to sum moments about point A=0(with a counter-clockwise rotation I chose).

    moments at pt A = 0= 8Cy -2800(4) - 2100(3.4641)
    Cy = 1259.33N up

    Sum Forces in +x direction=0

    Ax - 2100 = Ax=2100N to the right

    Sum forces in +y direction = 0

    Ay + Cy - 2800 = Ay = 1540.67N up

    If this is correct, then I'll post more. Next, I drew that other picture, and started at points and found the tension(as I draw them all in tension to begin with) at each point.


    View attachment problem 6.bmp
  2. jcsd
  3. Mar 11, 2007 #2


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    Looks okay to me.
  4. Mar 11, 2007 #3
    For point A:

    Sum Forces in +x=0:

    Tab + Tadcos(60) =0 Tab = -3558.02N
    Sum Forces in +y=0:

    Ay(or 1540.67) - Tadsin(60) = 0 Tad = 1779.01N

    I'll stop here again too.

    The book says Tab = 2.38kN and Tad = 0.567kN

    So, what did I do wrong?
  5. Mar 11, 2007 #4
    Is this portion correct?
  6. Mar 11, 2007 #5
    trying to do another one of these now.....so any help would be nice:)
  7. Mar 12, 2007 #6
    Is my drawing or the last post not clear.....I'd like to know where I've went wrong. I'll do the calculations, I can't find any other way to do them, as of now...need to know if I'm right up to this point.
  8. Mar 12, 2007 #7


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    You have a math error up front when you calculated C_y. Right equation, careless math.

    Then you left out the A_x reaction when you summed forces in x direction at A.

    And one of those members at A will not be in tension.
  9. Mar 12, 2007 #8
    Ok.....I figured it out.....I guess I didn't go over my work as carefully as I thought I did. Thanks for the help....I'm sure by the end of the day, I'll have another one of these posted...ha ha......Thanks again!!!
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