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Angular Momentum(Turntable and blob collision)

  1. Mar 26, 2012 #1
    A small blob of putty of mass m = 7.1 falls from the ceiling and lands on the outer rim of a turntable of radius R and mass M = 7.5 kg that is rotating freely with an angular speed of 1.4 rad/s about its axis. The postcollision angular speed of the turntable-putty system is ____ rad/s.

    L = r x p = Iω
    this is the only equation i think i need for this problem

    As far as i can tell all i would need to do is...
    since R is unknown but will be a constant through out the whole problem, i give it a value of 1 for simplicity sake.
    the momentum of the turntable before would be, L = (7.5 x 1^2) x 1.4
    L= 10.5
    since momentum as to be conserved in this problem the end momentum should also be 10.5
    10.5 = [(7.5 x 1^2) + (7.1 x 1^2)] x ω
    10.5 = 14.6 x ω
    10.5/14.6 = ω
    .719 = ω

    and since this is a online assignment i can try and answer as many times until i get it correct, but i can't think of where i might be going wrong cause it says i have the wrong answer.
    I don't need the problem worked out for me per say...just maybe hint at the error or explain where im going wrong, and thank you for any help
  2. jcsd
  3. Mar 26, 2012 #2


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    Welcome to PF!

    Hi MRMooneyham! Welcome to PF! :smile:

    (try using the X2 button just above the Reply box :wink:)
    nooo … wrong moment of inertia ! :redface:
  4. Mar 26, 2012 #3
    I figured that's where I was wrong at, but I don't understand how, so another hint pot favor :)?
  5. Mar 26, 2012 #4


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    you've used MR2 for the moment of inertia of a disc about its axis

    it isn't

    look it up, you need to learn the common moments of inertia :smile:
  6. Mar 26, 2012 #5
    so then it would be? im still using the ^ symbol cause i dont understand the x^2 above the reply
    L = (1/4 * 7.5 * 1^2) * 1.4
    L = 2.625
    so after the collosion the equation would look like
    2.625 = [(1/4 * 7.5 * 1^2) + (7.1 * 1^2)] * ω
    so 2.625 = 8.975 * ω
    and ω = .292479?
  7. Mar 27, 2012 #6


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  8. Mar 27, 2012 #7
    ok thanks, i got confused on that :/
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