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Forces, friction, and gravity

  1. Dec 2, 2007 #1
    1. What does W=? ?
    [​IMG]



    2. Not really too sure, but here are some I know:
    Fa=Fb*Cos(Theta)b
    Fg=Fa*Sin(Theta)a+Fb*Sin(Theta)b


    3. The attempt at a solution
    I'm completely clueless... We've never done anything in class with Friction and pulling on something involved in one Problem.

    EDIT:
    I just found out this was posted in the Wrong Physics section... If a moderator or Admin. could be kind enough to move it that would be great:)
     
    Last edited: Dec 2, 2007
  2. jcsd
  3. Dec 2, 2007 #2

    Dick

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    Regardless of what forum the question was posted in the question still isn't very clear. W=mg for whatever m the hanging mass is. The tension T on all of those strings is equal, if they are strings.
     
  4. Dec 2, 2007 #3

    nrqed

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    The only way I could make sense of the question is if it asks "what is W" if the block on the surface is just about to start sliding". In that case, the friction force on the block on the surface would be [itex] \mu_s n [/itex] where n is the weight of that block which is 80 N. This will also be the tension in the horizontal string.

    I can't quite make sense of your equations since I don't know what "Sin(theta)a" means.
    But consider the knot where all three strings are connected and impose that the net force along x and along y are both zero. That will give you two equations for two unknowns which are F_a and W.
     
  5. Dec 2, 2007 #4

    nrqed

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    Hi Dick. Why do you say that the tension in all strings is the same?
     
  6. Dec 3, 2007 #5

    Dick

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    Hi nrqed. Meant to say that the tension forces balance at the knot, not that they are equal.
     
  7. Dec 3, 2007 #6
    This equation has to remain in equilibrium if that helps, but I just found out the answer today in class and it's 23.3.
     
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