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Homework Help: Inelastic collision?

  1. Nov 25, 2009 #1
    1. The problem statement, all variables and given/known data

    One hockey puck of mass 0.2 kg strikes an identical stationary puck on a frictionless ice rink. If the first puck had a velocity V0 = 3.8 m/s before the collision and V1 = 0.8 m/s after in the same direction, what fraction of the energy was lost?

    2. Relevant equations
    Conservation of momentum m1v1 + m2v2 = m3v3 + m4v4
    Work energy theorem k = (1/2) mv^2 or (mv)^2/2m

    3. The attempt at a solution
    3.8 + 0 = .8 + 3
    (I left out mass becuase all objects have the same mass.)

    I am pretty much stuck becuase I do not know the final velocity of V2, and if there was energy lost then V2 shouldn't equal 3 m/s.
  2. jcsd
  3. Nov 25, 2009 #2


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    Welcome to PF!

    Hi sirfinklstin! Welcome to PF! :wink:
    Why not?? :confused:

    What is your energy equation? :smile:
  4. Nov 25, 2009 #3
    u almost solved the problem...
    v1= 3.8
    v2=0...bcoz stationary given
    and v4 u have to find out...final velocity of other mass...
    which u got 3

    now jus find difff between initial and final energy
    wats d problm??
  5. Nov 27, 2009 #4
    Ok, thanks for the help with that, i thought a loss of energy would mean less than 3 m/s for V4, here is what I have now -

    puck 1 before collision

    puck 2 before collision

    puck 1 after collision

    puck 2 after collision

    loss of energy = .48J

    is this correct?
  6. Nov 27, 2009 #5
    looks good! just convert that into a fraction...
  7. Nov 27, 2009 #6
    (48/100)j ?
  8. Nov 27, 2009 #7
    "what fraction of the energy was lost?"
  9. Mar 23, 2010 #8
    So, how to calculate fraction of the energy was lost? I'm lost :(
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