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Kinetic energy help

  1. Apr 15, 2008 #1
    1. The problem statement, all variables and given/known data

    This is not homework, I am too old for school. I am trying to teach myself accident reconstruction. The equation I have for impact utilizes crush points. The crush points reveals an energy of 479609.6 to crush the car, so what I am trying to figure out is the speed of the car in km/h.. Any help with this is appreciated greatly, thank you.

    Is it possible to find km/h from K.E

    2. Relevant equations

    I know that KE = .5mv²

    lets say the mass = 1165 kg and the K.E = 479609.6

    3. The attempt at a solution

    411.682 = 479609.6 / 1165

    20.289 = sqr (479609.6)

    10.144 m/s² = 20.289 / 2

    36.532 km/h = 10.144 / .2777
    Last edited: Apr 15, 2008
  2. jcsd
  3. Apr 15, 2008 #2
    It's better to do the problem with variables, and not substitute values until the end.

    start with E = (1/2)mv^2 and solve for v. Then put in the numbers.

    I can't really tell what you're calculating here. You never use your first intermediate result, and the end result is wrong.

    you need to supply units for the Kinetic Energy. I suppose they are Joules?
  4. Apr 15, 2008 #3

    Chi Meson

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    Close. Notice the way I worked through the information. You gave yourself a bit of extra work by solving each little piece bit by bit. Somewhere you dropped a number.

    K= 479,600 joules (additional digits are insignificant here)
    m= 1165 kg

    K= .5 mv² v=√(2K/m)
    v=√(2•479,000 J/1165 kg)= 28.69 m/s = 103.3 km/h
  5. Apr 16, 2008 #4
    Thank you very much Chi Meson, I was not even close, man did I make a mess. I really appreciate you pointing me in the right direction. Now that I see how it is done, I will not forget this lesson. Thanks again ~ Probie
  6. Apr 22, 2008 #5
    This thread can be marked as solved. Thank you very much again Chi Meson. You people are the greatest in the world. Thanks.
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