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Momentum/Kinetic Energy Problem

  1. Nov 11, 2013 #1
    1. The problem statement, all variables and given/known data
    A car moving with an initial speed v collides with a second stationary car that is 52.9 percent as massive. After the collision the first car moves in the same direction as before with a speed that is 31.8 percent of the original speed. Calculate the final speed of the second car. Give your answer in units of the initial speed (i.e. as a fraction of v).

    2. Relevant equations

    3. The attempt at a solution
    I've attempted this solution using the conservation of momentum and the conservation of energy. I seem to get different answer with both though. I don't understand why I can't use the conservation of energy law to figure out this problem instead of using the conservation of momentum. Help is much appreciated

    Thanks, ClassicRock
  2. jcsd
  3. Nov 11, 2013 #2


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    In a collision, momentum is always conserved; however, kinetic energy is conserved only in elastic collisions. Your calculations suggest the collision isn't elastic.
  4. Nov 11, 2013 #3
    The collision is elastic. That is actually the second part of the question.
  5. Nov 11, 2013 #4
    it's not stated that the collision is elastic.So you cannot use conservation of energy in this case.

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  6. Nov 11, 2013 #5


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    My calculations show the collision is nearly elastic but not quite.
  7. Nov 11, 2013 #6
    In the second part of the question it states that this is an elastic collision. Maybe that's why when I use the conservation of energy my number is within .01 of the conservation of momentum answer.
    conservation of momentum gives me 1.29
    conservation of energy gives me 1.30
  8. Nov 11, 2013 #7
    Thank you for the help. Maybe the problem set has the wrong answer
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