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Moments about a point

  1. Sep 5, 2005 #1
    mass of e is 100 kg
    mass of bar is 20 kg and acts at midpoint
    sume of moments about point a due to weight of bar and forces on b by bc, bd, and be, are 0, determine tensions in bc and bd

    so i continued like this.
    The moment about point A from all those forces is 0.
    0 = (Raf x Wab) + (Rab x Wbe) -(Rab x Tbc) - (Rab x Tbd)

    I can figure out the first two terms of this eq, but i have to find magnitude of the vectors of Tbc and Tbd. But i only have this equation and these two equations:

    Tbc= |Tbc|*Ebc where Ebc is a unit vector in BC direction
    Tbd= |Tbd|*Ebd where Ebd is a unit vector in the BD direction

    Did i set this up incorrectly or how do i proceed?

    a picture has been included.

    Thanks for your help

    Pt A is at the origin
    Pt D is (0,5,5)
    Pt C is (0,4,-3)
    Pt B is (4,3,1)
    Pt E is where the 100 kg weight is
    AB is the bar where 20 kg acts down the midpoint
    any help is APPRECIATED!

    Attached Files:

  2. jcsd
  3. Sep 5, 2005 #2
    bump, anyone?
  4. Sep 6, 2005 #3


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    Homework Helper

    You know the position vectors of all the points.
    Use vector algebra to work out the vectors AB, DB, CD.
    Thes structure is in static equilibrium, therefore
    [tex]\sum F_i = 0[/tex] [tex]\sum F_j = 0[/tex] [tex]\sum F_k = 0[/tex]

    [tex]\mbox{If}\ \bf{AB} = (x_1,y_1,z_1)\ \mbox{then}\ F_{AB} = \lambda(x_1,y_1,z_1)[/tex]

    You should be able to use the above two lines to solve the problem.
    Last edited: Sep 6, 2005
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