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Homework Help: F=Ma Problem Help

  1. Sep 19, 2009 #1
    Hi, I'm practicing for the physics olympiad qualifier and I came across this problem:

    A spaceborne energy storage device consists of two equal masses connected by a tether and rotating
    about their center of mass. Additional energy is stored by reeling in the tether; no external forces
    are applied. Initially the device has kinetic energy E and rotates at angular velocity wEnergy is
    added until the device rotates at angular velocity 2w What is the new kinetic energy of the device?

    I know that E=.5Iw^2, but why does Ioda decrease by a factor of two?

  2. jcsd
  3. Sep 19, 2009 #2


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    Welcome to PF :smile:

    I will decrease because the masses are drawn in closer together.

    Also, conservation of angular momentum could be helpful here.
  4. Sep 21, 2009 #3
    Wow! Only one hour! Impressive ~ And the solution is accurate too (conservation of momentum)

    My first post was just a "test" question, so to speak (I wouldn't be trying for Physics Olympiad if I didn't know about conservation of momentum =P) and I'm really impressed with the speed. More (unfortunately harder) questions to come.

    Thanks Redbelly.
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