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Homework Help: Simple Pulley System with Two Masses

  1. Jun 20, 2012 #1
    1. The problem statement, all variables and given/known data


    r = 0.2m
    I = 1.4kg m^2
    m1 = 2kg
    m2 = 5kg
    h = 4m

    2. Relevant equations

    Δ(Gravitational potential energy) = m1*g*h - m2*g*h
    Δ(Kinetic energy) = (1/2)*m1*v^2 + (1/2)*m2*v^2 + (1/2)*I*(v^2/r^2)

    3. The attempt at a solution

    System is isolated; no change in the internal energy of the system so all the lost gravitational potential energy must go into increasing the kinetic energy of the system.

    Therefore, ΔKE = -ΔGPE → (1/2)*v^2(m1 + m2 + I*r^(-2)) = -ΔGPE
    → v = √((-2ΔGPE)/(m1 + m2 + I*r^(-2))) = 2.36643191m/s is what the final answer works out to be.

    Problem is, the answer sheet says that the answer should be 0.014. I'm kind of rusty with rotational motion, but even after analyzing this problem to death I still have no clue why my answer is nearly 170 times too big... I must be making a stupid mistake somewhere.

    As always, any help is much appreciated.

    P.S. sorry for the awful formatting, but these university computers are still running IE7 for some reason and the only part of the submission form that's actually responsive is the text box.
  2. jcsd
  3. Jun 20, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    I get the same answer as you. :bugeye:
  4. Jun 20, 2012 #3


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    Homework Helper

    Your solution is correct. The answer sheets do have mistakes.

  5. Jun 21, 2012 #4
    Huh. Well then. Thanks for the confirmation.
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