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Homework Help: How Much Kinetic Energy is Lost

  1. Nov 17, 2013 #1
    1. The problem statement, all variables and given/known data
    A 2.00-kg object traveling east at 20.0 m/s collides with a 3.00-kg object traveling west at 10.0 m/s. After the collision, the 2.00-kg object has a velocity 5.00 m/s to the west. How much kinetic energy was lost during the collision? (no image was given).

    2. Relevant equations
    Conservation of momentum, and conservation of energy.

    Also, the correct answer is supposed to be 458J

    3. The attempt at a solution
    What bothers me the most about this problem is that it seems to be neither an inelastic, nor an elastic collision. Basically what I have tried so far involves solving for the objects final velocity (I used conservation of momentum for this), and then plugging that value into conservation of energy (i.e. Ki + Ui = Kf + Uf).

    Am I supposed to be using calculus in some way? (I'm taking a calculus based physics course).

    Any input would be greatly appreciated!

    Thank you in advance to everyone! :smile:
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 17, 2013 #2


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    Science Advisor
    Homework Helper
    Gold Member

    It's always a bit confusing, but the usual definition is that 'inelastic' just means 'not perfectly elastic'. (Personally, it would seem more natural to keep it for completely inelastic, everything else being various degrees of elastic.)
    But you know work is probably not conserved. Use momentum conservation and the other facts you are given to deduce the velocities, and hence KE before and after.
  4. Nov 17, 2013 #3
    Thank you so much, I was able to solve the problem!
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