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Homework Help: Simple Pulley Problem With Acceleration

  1. Feb 26, 2015 #1
    1. The problem statement, all variables and given/known data
    Block A in the figure has a mass of 5.00kg , and block B has mass 12.0kg . The coefficient of kinetic friction between block B and the horizontal surface is 0.25.


    What is the mass of block C if block B is moving to the right and speeding up with an acceleration 3.00m/s2 ?

    2. Relevant equations

    Friction = coefficient * normal force
    F = ma

    3. The attempt at a solution

    I found the frictional force heading left (since block is going right). 0.25 * (12 * 9.81) = 29.43 N
    The tension force on B is then just F = ma + friction force, so 12 * 3 + 29.43 = 65.43 N
    And the acceleration of block C, which should also be 3 m/s2, gives the equation Ca = Cg - T, since the acceleration downwards is just gravitational force - tension force. So 3C = 9.81C - 65.43, C = 9.608kg

    But this is wrong!!! And I don't know why :(
    The textbook sucks too...pulleys aren't even so much as mentioned in the chapter we're in.

    Edit: Tried accounting for A.

    A is moving up at 3 m/s2.
    So -3A = Ag - T
    -3(5) = (9.81)(5) - T
    T = 34.05 N
    So then the total forces pulling left are 34.05 + 29.43 = 63.48N (A and friction). So the forces pulling right must equal that + 36 (F = (12)(3)) = 99.48N. So 3C = 9.81C - 99.48, C = 14.608kg

    Still doesn't work :(
    Last edited: Feb 26, 2015
  2. jcsd
  3. Feb 26, 2015 #2
    Maybe I'm misunderstanding your work, but did you consider that the gravitational force of blocks A and C also act on B?
  4. Feb 26, 2015 #3

    Still doesn't work :(
    Last edited: Feb 26, 2015
  5. Feb 26, 2015 #4


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    I don't see where you took into account the tension on the A side.
  6. Feb 26, 2015 #5
    Yeah, realized that from the other post. Edited my post above, but it still is wrong. I was so hoping it would be right too, since it made so much sense...
  7. Feb 26, 2015 #6
    Ahhhh never mind. I'm just bad at math. :D
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