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Homework Help: Conservation of momentum

  1. Oct 5, 2004 #1
    If I have a solid disc with a diameter of 30 cm and a mass of 700 grams with a speed of 45 rpm, and a mass of 350 grams falls 6 cm from center what is the speed of the disc. There are also no external torques.

    I have no problem solving the angular momentum of the disc before the mass falls on it.

    L=Iబ which is L=1/2(.7 *.15^2)*4.9

    From there I do not know how to incorporate the 350 grams or the 6 cm into the problem.
  2. jcsd
  3. Oct 5, 2004 #2
    Never mind I found out what I was doing wrong.

    Instead of: L initial = L final I used the: I initial + I gum=I final

    I then did: బ initial * I initial + బ gum*I gum=బ final*I final

    Since బ gum had no angular speed I ended up with:
    బ initial * I initial =బ final * I final
    and since I figured up I final I was able to divide it out and get బ final by itself.

    బ initial * I initial/I final=బ final

    and arrived at the answer. :rofl:
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