1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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

  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?

  2. jcsd
  3. Oct 9, 2007 #2

    Doc Al

    User Avatar

    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

    User Avatar

    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?

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


    User Avatar
    Homework Helper

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

    Doc Al

    User Avatar

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