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Moment of Inertia Problems and Conservation of Energy

  1. Nov 30, 2009 #1
    I have this situation here:

    GIANCOLI.ch10.p067.jpg

    I'm given the masses for the objects, as well as the radius and mass of the pulley. I have to use conservation of energy laws to find out how fast the mass on the right is going the instant it hits the ground.

    I can find out what the alpha of the pulley is, and therefore the accelration of the system, but that's just kinematics. How can I use conservation of energy to do it?

    How could I do it if the pulley was ideal?
     
  2. jcsd
  3. Nov 30, 2009 #2

    Doc Al

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    Staff: Mentor

    Compare the total mechanical energy of the system before and after the mass falls.
     
  4. Nov 30, 2009 #3
    I don't follow...

    I see that there is none before, and some after, when the block b has zero potential energy, block A has potential and kinetic energy, and the pulley has rotational kinetic energy...but if I knew those values then I could just save the trouble and use kinematics.
     
  5. Nov 30, 2009 #4

    Doc Al

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    Staff: Mentor

    Set up a conservation equation:
    KE1 + PE1 = KE2 + PE2

    Make sure you include the PE and KE of both masses and the pulley. You'll be able to solve for the speed of block B without using force equations or kinematics. Hint: Express all the KE terms in terms of the speed of block B.
     
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