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Conservation of momentum?

  1. Nov 14, 2012 #1
    conservation of momentum??

    1. The problem statement, all variables and given/known data

    A 45.0 kg girl is standing on a 156 kg plank. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.51 m/s relative to the plank.

    (a) What is her velocity relative to the ice surface?
    (b) What is the velocity of the plank relative to the ice surface?

    variable:
    girl=45kg
    plank=156kg
    Vi=0m/s
    Vf=1.51m/s

    2. Relevant equations

    mavai + mbvbi = mavaf +mbvbf

    3. The attempt at a solution

    since Vi=0m/s left side of consercation of momentum equation =0
    0 = 45(Vf) + 156(1.51)
    0 = 45(Vf) + 235.56
    -235.56 = 45(Vf)
    -5.23 = Vf

    this answer is not possible because the absolute value of part a and b should add up to 1.51 while 5.23 is already much bigger
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 14, 2012 #2
    Re: conservation of momentum??

    sorry for putting the blank template 2X below...didnt see that before posting
     
  4. Nov 14, 2012 #3

    haruspex

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    Re: conservation of momentum??

    What is your Vf here? How about putting in some variables for speeds of girl and plank relative to the ice?
     
  5. Nov 14, 2012 #4
    Re: conservation of momentum??

    Vf = 1.51 m/s

    and the question is asking what the speeds are... Which I am struggling to solve. How do you suggest I go about soving for the speeds?
     
  6. Nov 14, 2012 #5

    haruspex

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    Re: conservation of momentum??

    No, I mean what does it represent?
    So put in variables to represent them.
     
  7. Nov 14, 2012 #6
    Re: conservation of momentum??

    Vf represents the total velocity at the end. The velocity relative to the ice for the girl and the plank added together should equal the Vf (1.51)
     
  8. Nov 14, 2012 #7

    haruspex

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    Re: conservation of momentum??

    That isn't what you are given: "The girl begins to walk along the plank at a constant velocity of 1.51 m/s relative to the plank."
    Define unknowns for the various velocities relative to the ice and see what equations you can write using them.
     
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