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Equilibrium under the action of concurrent forces

  1. Feb 21, 2013 #1
    1. The problem statement, all variables and given/known data
    Hello all, I just finished a question on equilibrium. I wish I could upload a diagram or something but I don't know how to get it from my Microsoft oneNote to here. But basically there is a mass held In equilibrium by two ropes. on the left is mass 2 and on the right is mass 3. mass 3 = 200N


    ........0..............0
    ......../\50°.|.30°/\
    ......./...\....|...../...\
    .....m2....\...|.../.. m3(200n)
    ...............\..../
    .................V
    ................m1
    2. Relevant equations
    ƩF = 0

    3. The attempt at a solution
    I have solved everything and got everything correct. But that isn't the problem
    what I did was I got the Y-components and X components

    so here is my question
    Fx2 - Fx3 = 0 ------> these are the x-components and I know the sum must be zero to be in equilibrium.( my question applies to y-component as well)
    why is it that we must Subtract the two?

    is it because they are in opposite directions? i.e. <----(-)----0------(+)------->
    or is it because weight cannot be negative? cause if I add them together I get - newtons

    Thanks for the help!
     
  2. jcsd
  3. Feb 21, 2013 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    The net force on m1 has to be zero in equilibrium. That is valid for both the x and y components. If F2x and F3x are the x components of the forces exerted by the ropes, F2x+F3x=0. But these forces are opposite, and their magnitude is equal to the tension multiplied by the cosine of the given angle. The tension itself is positive. So F2x=-T2 sin(50°) and F3x=T3 sin(30°)

    To upload picture, go to "go advanced" "manage attachment" "browse" and "upload"

    ehild
     

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  4. Feb 22, 2013 #3
    aahhhh makes sense now. thanks a lot!
     
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