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Homework Help: Work and Rotational Kinetic Energy

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data
    (a) If R = 12 cm, M = 360 g, and m = 20 g (below), find the speed of the block after it has descended 50 cm starting from rest. Solve the problem using energy conservation principles. (Treat the pulley as a uniform disk.)

    2. Relevant equations
    KEi + PEi = KEf + PEf

    3. The attempt at a solution
    0 + mgh = (1/2)mv^2 + (1/2)Iw^2 + 0

    mgh = (1/2)mv^2 + [(1/2)(1/2)mr^2](v/r)^2

    mgh = (1/2)mv^2 + [(1/4)mr^2](v/r)^2

    4mgh = 2mv^2 + mr^2(v/r)^2

    but I don't this has got to be wrong somewhere because the r's will cancel out and I know that they need to be in the equation for velocity..can anyone help? I would greatly appreciate it!
  2. jcsd
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