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Homework Help: Pulley problem with definite mass rope

  1. Nov 14, 2008 #1
    Hi, i've been working on this problem for more than 4 hours now. and I haven't got a clue on how to finish the problem. ANY help would be tremendously appreciated!

    1. The problem statement, all variables and given/known data
    a pulley has a circumference of 1.20m and mass of 2.25kg. pulley is a solid uniform disk. a heavy rope, 8.00m in length with a mass of 4.80kg hangs over the pulley with one end of the rope 0.600m lower than the other end of the rope. the pulley is initially at rest. when the pulley is released, the pulley turns as the lower end of the rope accelerates downward. assume no slippage. what is the angualr velocity of the pulley at the moment the end of the rope leaves pulley?

    2. Relevant equations

    3. The attempt at a solution

    ok so we know L1+L2+1/2(circumference)=8.00m; therefore, L1=3.4m, and L2=4.0m.
    the initial potential energy is mg(4.00m) because the rope will leave pulley after it moves 4.00m more. The problem is that I don't know which mass to use. Do i use the mass of the whole rope? the mass changes as the rope falls over the pulley, as well as its acceleration.

    ignoring the ambiguity of mass, our KE(trans)+KE(rotat)=1/2(mass1)(v^2)+1/2(mass of pulley)R^2)(w^2)=1/2(m)(rw)^2+1/2(mR^2)(w^2). In this equation, I don't know what is mass1. is it the mass of entire rope?

    I am not even sure if my PE is right. So any help would be truly appreciated! Thanks in advance, Physics Forum members :)
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 14, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Hopelessmin! Welcome to PF! :smile:

    (have an omega: ω and a squared: ² :wink:)
    All PE is relative!!

    So starting with "the initial potential energy is …" is wrong … just state what your "base" of PE is (in this case, choose "base" PE as being when the two ends are of equal length), and just use the difference in PE from that. :smile:

    (and once you do that, it is obvious how much of the rope you use! :wink:)
    How much of the rope is moving? That's what has KE. :wink:

    Then solve the equation KE + PE = constant. :smile:
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