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Homework Help: 2D Equilibrium

  1. Oct 24, 2008 #1
    This is just a problem from my midterm, and I was wondering if I did it correctly

    1. The problem statement, all variables and given/known data
    205-MT.jpg
    Weight of block B is 15kN, weight of block A is 25kN. Find force P that has to be applied to keep the system in equilibrium in tension in the cable


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


    3. The attempt at a solution
    So first I figured I'd draw a FBD around block B (FBD1 on the image). I also decided I would use a different axis (x' and y'). From this I was able to write the equilibrium equations:
    [tex]\sum[/tex]Fx=3T-15*cos(60)=0, from which T=2.5kN
    (I also wrote the Fy equation, but I won't post it because it serves no real point)

    Next, I drew a FBD around block A (FBD2 on the image). I used a different axis once again. And I got:
    [tex]\sum[/tex]Fx=-25*cos(60)+P(cos30)+2.5kN=0, from which P=11.55kN

    Is that correct? It makes sense in my head, but then again that wouldn't be the first time my gut feeling is wrong.Thanks!
     
  2. jcsd
  3. Oct 24, 2008 #2

    PhanthomJay

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    ...but this time, your gut feeling is right.
     
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