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Kinetic Energy

  1. Oct 9, 2007 #1
    1. The problem statement, all variables and given/known data
    Our homework set today was to find out why is it that when you hit someone at 30mph they have a 20% chance of dying. Yet when you hit someone at 40mph they have a 90% chance of dying.


    2. Relevant equations
    KE = 1/2 M V^2


    3. The attempt at a solution
    First of all i found out that 1mile = 1.6km; which means 1 mile = 1600m
    I then did 60x60 to find out the amount of seconds in 1 hour: which was 3600 seconds
    I then divided 30 by 3600 to get the meters/second; which was 8.333 [reoccuring] m/s
    Now I found out the Kinetic Energy. We were allowed to choose which car we used, and I used my fathers. This weighs 2090kg (Mazda 5)

    KE = 2090 x (8.333^2) = 145127.278
    145127.278 / 2 = 72563.63901 J

    This is where the problem came

    I then tried it with 40mph

    40 / 3600 to get the m/s; which turned out as 0.0111 [reoccuring]
    I now thought how the heck could that be right? But I continued & tried to do the equation.

    Ke = 2090 x (0.011^2) = 0.25289
    0.25289 / 2 = 0.126445

    Okay now I'm very confused. Can anyone help me out?

    When 30mph KE is 72563.63901 how is 40mph 0.0111?

    Thanks,
    Phykid
     
  2. jcsd
  3. Oct 9, 2007 #2

    Doc Al

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    Staff: Mentor

    The calculation methods for 30 mph and 40 mph are identical, so recheck your arithmetic. Redo both conversions from mph to m/s.

    You forgot the 1.6 factor in going from miles to km.
     
    Last edited: Oct 9, 2007
  4. Oct 9, 2007 #3
    I'm confused, sorry :uhh:

    EDIT: Ignore that, someone posted same time as me
     
  5. Oct 9, 2007 #4

    Doc Al

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    Staff: Mentor

    Redo your conversion of 30 mph to m/s. Do it carefully, step by step.
     
  6. Oct 9, 2007 #5
    So how would I go about including the 1.6 into it?

    Thanks,
    Phykid
     
  7. Oct 10, 2007 #6
    Bump :O
     
  8. Oct 10, 2007 #7

    learningphysics

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    Homework Helper

  9. Oct 10, 2007 #8
    Brilliant!
    Thanks for all your help
     
  10. Oct 10, 2007 #9

    Doc Al

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    Staff: Mentor

    I hope you realize that in your first post you correctly described how 1 mile = 1.6 km = 1600 m (approx). But then for some reason you forgot to use that bit of info when you did your conversion from mph to m/s.
     
  11. Oct 10, 2007 #10
    I didnt until you pointed it out in your first post :)
    thanks
     
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