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Homework Help: Two blocks and a pulley (please help, im confused)

  1. Mar 26, 2007 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 = 2 kg rests on a table with which it has a coefficient of friction µ = 0.56. A string attached to the block passes over a pulley to a block of mass m3 = 4 kg. The pulley is a uniform disk of mass m2 = 0.6 kg and radius 15 cm. As the mass m3 falls, the string does not slip on the pulley. With what acceleration does the mass m3 fall?


    2. Relevant equations
    F=ma
    Torque=I*alpha


    3. The attempt at a solution
     
  2. jcsd
  3. Mar 26, 2007 #2
    For a problem like this its always helpful to draw a free body diagram.

    You have a weight force acting on m3 equal to (m3)g, and a tension in the rope -T.

    m3(g)-T=(m3)a(net)

    On m2, you have a force of friction -uFn and the same tension force T, such that -uFn+T=(m2)a(net)

    If you add the two together, you will have a net tangential force (Ftan)acting on the pulley. And you know that t=RFtang=I*alpha=R(m3*g-um2g), and you have hte numbers to find I for the pulley, so you can find alpha which is a/r where a is the linear acceleration.

    I'm pretty sure I did that right.
     
  4. Mar 27, 2007 #3
    thanks a lot, that helped. i figured it out.
     
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