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Homework Help: Work and Energy of a car

  1. Sep 6, 2005 #1
    How much work is required to stop a 1000 kg car traveling at 100 km/hr?

    What I was thinking was that I would use the formula for KE,
    KE=1/2mv2 (last two of course is for squared
    since the KE would equal work. I have gotten as far as:

    KE= 1/2(1000 kg)(100km/hr)2

    I am assuming that somewhere there is a conversion needed, I'm just not getting the next steps.
  2. jcsd
  3. Sep 6, 2005 #2


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    Which next steps? Just convert the km/hr into m/s and you're done.
  4. Sep 6, 2005 #3
    Then maybe I'm not setting up the conversion correctly, I was using dimensional analysis and set it up

    (100 km/hr) (1000m) (3600 sec) = 3.6 x 10^8
    (1 km) (1 hr)

    but I know this is wrong, by the time you square it and finish the equation, its way too big. The answer in the book is 386,000J, but I keep getting answers with 360 b/c of the sec. I'm sure this is a simple mistake I'm making, but I'm still stuck. Sorry.
  5. Sep 6, 2005 #4


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    the hour is on the bottom!
    your conversion factors should be
    (1000 m)/(1 km) * (1 hr)/(3600 s)

    I suppose here, 100% of the Work removes KE.
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