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Homework Help: Two Blocks and a Pulley

  1. Mar 13, 2009 #1


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    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 = 0.75. 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 = 0.5 kg and radius 15 cm. As the mass m3 falls, the string does not slip on the pulley.
    With what acceleration does m3 fall?

    2. Relevant equations
    Frictional Force = mew x normal
    F = MA
    Newtons' second law

    3. The attempt at a solution
    If I could treat the pulley as a point and didn't have to pay attention to it this would be a cinch. To be honest, I don't know what to do with it at all, and I'm looking for some conceptual help here.
  2. jcsd
  3. Mar 13, 2009 #2


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    Much time later, I finally got the acceleration to be 3.9. The next question in this set is: what is the tension in the string above the table, and below the pulley. (They will be separate.)
    Now, since F=MA, I thought this would be easy. 3.9 x 4 = 15.6. It itsn't that easy, it turns out. Any ideas?
  4. Mar 13, 2009 #3


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    Staff Emeritus
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    Well, 15.6 N is the net force on the 4 kg mass.

    This force is due to the string tension and what else?
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