Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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

    User Avatar
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook