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Statics, vectors

  1. Nov 4, 2009 #1
    1. The problem statement, all variables and given/known data

    A weight of 10 N is hanging from the top of a three-legged support. The legs are attached to the horizontal floor and form equal angles with each other. Find the forces in the legs, if they form an angle of 30 degrees with the rope.


    2. Relevant equations
    [tex]\sum[/tex]Fx=0
    [tex]\sum[/tex]Fy=0
    [tex]\sum[/tex]Fz=0

    3. The attempt at a solution
    I'm lost. Maybe I should just give up on the whole topic of statics, I've never had this much difficulty with any other subject in physics. I understand the examples and the stuff in the book but I can't seem to solve any problems on my own. I'm not sure if these are even correct. Sum of the projections of the vectors-
    BAsin30+BCsin30+BDsin30=0 The projections of P and BE are zero, since they are at a 90 degree angle with x.
    BAcos30+BCcos30+BDcos30+BE+10=0
    BAcos15+BCcos15+BDcos15+BEcos45+10cos45=0
    The 15 is there, because I figured they were at a 15 degree angle with z.
    I don't know if BA, BC and BD are all supposed to be equal:S
    Anyways, help would be very much appreciated:)
     

    Attached Files:

  2. jcsd
  3. Nov 4, 2009 #2
    Yes, they are all the same.
     
  4. Nov 4, 2009 #3

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi starryeyed! :smile:
    Yes, of course they're supposed to be equal … don't be paranoid!

    From symmetry, the forces will obviously be equal, and since they're equal, any horizontal component will be zero …

    so just take the vertical components

    (which I think you've done, but I don't understand what BE is supposed to be :redface:)

    (and why are you talking about projections? :confused:)
     
  5. Nov 7, 2009 #4
    If you isolate one leg as a free body and put all the forces acting on it, and then make an equilibrium statement about it, the answer will appear more easy. Isolating one leg implies a downward force at its top of P/3. Don't forget you must put ALL the forces on the free body diagram.
     
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