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Homework Help: Kinetic energy lost during a collision?

  1. Dec 8, 2015 #1
    1. The problem statement, all variables and given/known data
    A 1kg object traveling at 1 m/s collides head-on with a 2 kg object initially at rest. If the collision is completely inelastic, determine a) the velocity of the objects after impact and b) how much kinetic energy was lost during the collision.

    2. Relevant equations
    Part a) m1v1 + m2v2 = (m1+m2)v'
    Part b) ke before =1/2m1v1^2, ke after= 1/2 (m1 + m2)v2^2

    3. The attempt at a solution
    Part a)
    m1v1 + m2v2 = (m1+m2)v'
    1 + 0 = 3(v')
    v' = 1/3

    Part b)
    KE before: 1/2 m1v1^2
    .5 (1)(1^2)
    KE after: 1/2 (m1+m2)v2^2
    .5(3)(0) = 0??????

    KE Lost:
    (kinetic before - kinetic after)/kinetic before
    (.5-0)/.5 = 1???????
    Last edited: Dec 8, 2015
  2. jcsd
  3. Dec 8, 2015 #2

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    You have the system gaining energy! That can't be right. Check your figures.

    Plus, you found the fractional loss, and you were asked for just the loss.
  4. Dec 9, 2015 #3


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    Homework Helper

    Explain what are v1, v2 and v'.
  5. Dec 9, 2015 #4
    The velocities you're using for kinetic energy are wrong; try to use the same variable names as with your momentum equation.
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