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Homework Help: Static Equilibrium ( easy?)

  1. Sep 6, 2009 #1
    Hello! I'm a student from Norway, and I've been struggling with a static equilibrium for a few hours now. The terms I use might not be "right", but I'll try to explain the problem to the best of my ability.

    1. The problem statement, all variables and given/known data

    Find the forces Fa and Fb on this figure:

    http://img44.imageshack.us/img44/4505/321k.jpg [Broken]

    2. Relevant equations

    [tex]\Sigma F_X = 0[/tex]
    [tex]\Sigma F_Y = 0[/tex]
    [tex]\Sigma \tau = 0[/tex] (sum torque = 0)

    3. The attempt at a solution

    I solved it by drawing the forces on the figure:
    http://img41.imageshack.us/img41/7513/321graph.jpg [Broken]

    After much fiddling with the "relevant equations" I came across the following:

    20kN *3,5m - Fb * sin(30) * 5,5m = 0
    => Fb = 37,21kN


    20kN * 3,5m - Fb * cos(70) * 5,5m = 0
    => Fb = 37,21kN

    Fa * cos(11,75) * 2m - 20m * 3,5m = 0
    => Fa = 35,74kN

    I know from my measurements and by checking the solution in the book, it's pretty damn close. By which formula, or by what method I got no clue whatsoever.

    I'd appreciate if someone could help me out by pointing me in the right direction :smile:

    Thanks in advance

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 6, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Hi urk.nono, welcome to PF. I am not sure what the second drawing you posted shows. I am used to force vectors represented as arrows, not line segments. Anyway, if you write the x and y components of the force at A as [FSUB]x[/SUB] (to the right) and Fy (up), can you find three equations saying

    ΣFi,x = 0
    ΣFi,y = 0
    Σtorques = 0

    Note: Your work will be much easier if you calculate torques about point A.
    Last edited: Sep 6, 2009
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