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Homework Help: Free Body Diagrams: Static Equilibrium

  1. Feb 3, 2010 #1
    1. The problem statement, all variables and given/known data

    Blocks A & B each have mass 5kg, and all contact surfaces are frictionless.

    Determine the force F needed to keep the blocks in static equilibrium, and the forces on all contact surfaces.


    2. Relevant equations

    I know I need to find where Fx= 0 and where Fy= 0.

    3. The attempt at a solution

    Fx = Fcos20
    Fy= Fsin20 - 5 kg

    Am I even close?
  2. jcsd
  3. Feb 4, 2010 #2


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    Homework Helper

    Hi hunter007, welcome to PF.
    When you apply force F to the systemis, the acceleration of the system is
    a = F/(Ma + Mb)..........(1). It is parallel to the ground.
    Since block A is not attached to the block B, due the inertia, it will experience an equal and opposite acceleration. The block A will remain rest when component of the weight of the block A balances the component of the acceleration.
    Equate these components and find a. Put this value in equation(1) to get F.
  4. Feb 4, 2010 #3
    Could you explain a little more on how to get a? Once I find that I could find F.
  5. Feb 4, 2010 #4


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    Homework Helper

    mg*sinθ = a*cosθ.
    Find a.
  6. Feb 4, 2010 #5
    a = 5(9.8)*tan(30)
    = 28.3

    F= 28.3(5+5) = 283 N.

    Did I do it right?
  7. Feb 4, 2010 #6


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    Homework Helper

    The given angle is 20 degrees.
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