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Homework Help: Two boxes connected to a string over an ideal pulley?

  1. Mar 13, 2012 #1
    1. The problem statement, all variables and given/known data
    Two blocks are connected by a string that goes over an ideal pulley as shown in the figure and pulls on block A parallel to the surface of the plane. Block A has a mass of 3.00 kg and can slide along a rough plane inclined 30.0° to the horizontal. The coefficient of static friction between block A and the plane is 0.400. What mass should block B have in order to start block A sliding up the plane?

    2. Relevant equations
    box A- sum of x & sum of y
    box B- sum of y
    plug into each other

    3. The attempt at a solution

  2. jcsd
  3. Mar 13, 2012 #2
    First of all. What are all the forces acting upon block A?
  4. Mar 13, 2012 #3
    The forces acting on block A are static friction, fnormal, gravity, and ftension.
  5. Mar 13, 2012 #4
    Yes. So the friction force and force of gravity are preventing the block from sliding up the ramp. What's next?
  6. Mar 15, 2012 #5
    ffriction = -.4*9.81*3
    fgrav= -sin30*9.81*3
    ftension =m*9.81

    ffriction +fgrav = ftension

    Not sure which part i messed up
    Last edited: Mar 15, 2012
  7. Mar 15, 2012 #6
    Friction force is equal to μF[itex]_{normal}[/itex].
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