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Homework Help: Determining the work done on the puck using conservation of angular momentum? Help!

  1. Nov 19, 2012 #1
    Determining the work done on the puck using conservation of angular momentum?? Help!

    Number20.png

    This is what I did...

    1) Given

    mpuck = 0.300 kg
    rinitial = 0.4 m
    vinitial = 0.6 m/s

    mpuck = 0.300 kg
    rfinal = 0.15 m
    vfinal = ____ m/s

    ƩW = KEfinal - KEinitial

    2) KEinitial = (1/2)mv2
    KEinitial = (1/2)(0.300)(0.62)
    KEinitial = 0.054 J

    3) KEinitial = (1/2)mv2
    KEinitial = (1/2)(0.300)v2
    KEinitial = 0.15v2

    4) Find v - Angular momentum is conserved due to a lack of friction. The puck goes from 40 cm to 15 cm, so it has a different angular momentum.

    Linitial = Lfinal
    initial = Iωfinal
    [STRIKE](0.300)[/STRIKE](0.4)(0.6)2 = [STRIKE](0.300)[/STRIKE](0.15)v2
    (0.4)(0.6)2 = (0.15)v2
    0.144/0.15 = v2
    0.96 = v2
    0.979 = v

    5) Plug v back into Net work equation

    ƩW = KEfinal - KEinitial
    ƩW = (0.979) - (0.054)
    ƩW = 0.925 J

    Which is wrong...
    :(
    Help!
     
  2. jcsd
  3. Nov 19, 2012 #2

    TSny

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    Re: Determining the work done on the puck using conservation of angular momentum?? He

    You want those squares?
     
  4. Nov 19, 2012 #3
    Re: Determining the work done on the puck using conservation of angular momentum?? He

    Oh...

    But even without them I still get the wrong answer.


    (0.4)(0.6) = (0.15)vfinal

    1.6 = vfinal

    4) KEfinal = 0.15(1.62)
    KEfinal = 0.15(2.56)
    KEfinal = 0.384

    So,

    W = (0.384) - (0.054) = 0.33

    Still wrong...
     
  5. Nov 19, 2012 #4
    Re: Determining the work done on the puck using conservation of angular momentum?? He

    brb in an hour!!
     
  6. Nov 19, 2012 #5

    TSny

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    Gold Member
    2017 Award

    Re: Determining the work done on the puck using conservation of angular momentum?? He

    Is 0.15 m correct for the final radius? Another reading of the question might help.
     
  7. Nov 19, 2012 #6
    Re: Determining the work done on the puck using conservation of angular momentum?? He


    Final radius would be... .4 m - .15 = .25!

    Ohhhhh thank you!
     
    Last edited: Nov 19, 2012
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