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Homework Help: Finding kinetic energy retained by an object after an elastic head-on collision

  1. Nov 7, 2008 #1
    Hi, Im new to the forums & I have a question about the following problem:

    A 0.10 kg object moving initially with a velocity of 0.20 m/s makes an elastic head-on collision with a 0.15 kg object initially at rest. What percentage of the original kinetic energy is retained by the 0.10 kg object?

    m1=0.10 kg
    vi1=0.20 m/s
    v1f=-0.0025 m/s

    m2=0.15 kg
    vi2=0.00 m/s
    v2f=0.198 m/s

    I calculated the final velocities of both objects after the collision, but can not decide how to find the kinetic energy retained by m1. I tried finding the kinetic energy lost by m1 (0.002) but then I didnt know where to go from there. I know this must be a simple problem, but Im just not seeing it? Thanks for any guidance :)
  2. jcsd
  3. Nov 7, 2008 #2
    You must use conservation of energy AND conservation of linear momemtum. Once you have calculated the final velocities correctly (your's appear to be incorrect :cry:) then you can calculate their respective kinetic energies (1/2*mv^2). You can then compare each final kinetic energy to the initial system energy (K1/Kinitial and K2/Kinitial) to obtain your percentages (note that they should add to 100%). Also, since energy is conserved the sum of the final kinetic energies will equal the initial kinetic energy (of the incoming object). Hope this helps! Good luck!
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