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Homework Help: Energy Loss in Head on Collision of Two Cars w/Same Speed and Mass

  1. Sep 25, 2011 #1
    1. The problem statement, all variables and given/known data

    Two railroad cars, each with a mass of 7,000kg and traveling at 90 km/h collide head on and come to rest. How much mechanical energy is lost in this collision?

    2. Relevant equations

    [tex]KE=\frac{1}{2}mv^2[/tex]

    3. The attempt at a solution

    [tex]KE=\frac{1}{2}(7000)(90)^2=2.835*10^7[/tex]

    Mechanical energy = KE+PE. PE = 0.

    Energy lost = 2KE or [tex]5.67*10^7[/tex]

    Book answer is [tex]4*10^6[/tex]

    Where am I going wrong? Thanks..
     
  2. jcsd
  3. Sep 25, 2011 #2

    lewando

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    Homework Helper
    Gold Member

    If your result is in joules, the velocity units of km/h is not helping.
     
  4. Sep 25, 2011 #3

    Doc Al

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    Staff: Mentor

    Convert the speed to standard units of m/s.
     
  5. Sep 25, 2011 #4
    Ahh... So 90 km/h = 25 m/s.

    [tex]\frac{1}{2}(7000)(25)^2=2.1875*10^6 => 2.1875*10^6(2) = 4.375*10^6 => 4*10^6[/tex]

    Thanks!
     
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