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Pully with 2 blocks friction question

  1. Mar 19, 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
    http://ScrnSht.com/ftjykb
    figure^^
    3. The attempt at a solution
    .4 x cos30 x 9.81 x 3kg = 10.2 =all forces down hill
    ften = block b mass* 9.81
    ftension must be equal to this to get the block started
    10.2 = m x 9.81
    m= 1kg
     
  2. jcsd
  3. Mar 19, 2012 #2
    This part is wrong. F[itex]_{gravity}[/itex] = mgsin30 and F[itex]_{friction}[/itex] = μmgcos30.
     
  4. Mar 19, 2012 #3
    As pointed out by Tal444,
    Tension will not be equal to 10.2

    Tension is acting against two forces.
    Force of friction and force of gravity (component along plane )
     
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