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Homework Help: Uncertainty about answer for Conservation of Momentum Problem

  1. Dec 6, 2007 #1
    1. The problem statement, all variables and given/known data

    A man holding a gun ((mass of man)+(mass of gun)+(mass of bullet)=60kg)) skates on frictionless ice at an initial speed of +2 m/s and fires a 100 gram bullet with a speed of +500 m/s from the gun.

    a) What is the speed of the man after firing the gun? (be sure to include +/- direction)

    b) What is the initial total energy of the man-gun-bullet system (before the bullet is fired)? What is the final total energy of the man-gun-bullet system (just after the bullet is fired)?

    c)is this an example of an elastic, inelastic, or perfectly inelastic collision, and why? I don't understand this concept very well.

    2. Relevant equations


    M1V1i + M2V2i = M1V1f + M2V2f

    Total KE (before)= .5MV^2

    Total KE (after)= .5M1V1^2 + .5M2V2^2

    3. The attempt at a solution

    I think this is correct, but I'm not completely sure.
    I'm not sure if its elastic or inelastic, I have trouble understanding that stuff.

    v=1.17 m/s

    energy before is
    .5*60*4 J
    120 J

    .5*.1*500^2+.5*59.9*1.17^2 J
    12541 J
  2. jcsd
  3. Dec 7, 2007 #2

    Shooting Star

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    Recheck value of energy after.
  4. Dec 7, 2007 #3
    sorry it took me so long to get back on, what did I do wrong?
  5. Dec 7, 2007 #4

    Shooting Star

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    You didn't. I did while calculating. Sorry.

    This is an example of an inelastic collision, because the KE before and after are not the same. The extra KE is coming from the chemical energy of the gunpowder, which was converted to mechanical energy.
  6. Dec 7, 2007 #5
    =) thank you very much
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