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Inability to calculate kinetic energy if I use SI units

  1. Jun 24, 2011 #1
    1. The problem statement, all variables and given/known data

    If a 2000 lb car with a velocity of 88 ft/s has by my calculations a kinetic energy of 240 kJ. But
    for a 909kg car with a velocity of 26.73 m/s I calculate 324.7 kJ. Same velocity, same mass, different energy calculation.
    2. Relevant equations

    English units: Kinetic energy(Ek)=.5(m)v^2; m= w/g; 2000 lb/32.2= 62.1 slugs:
    5(62.1)(88)^2=240 kJ
    SI units: Ek=.5(m)v^2; kg-mass=kg; 1 kg/2.2lbs; 1 ft= .301 m: .5(909)(26.73)^2=
    324.8 kJ

    3. The attempt at a solution Am I getting my metric mass wrong? I'm having a very hard time wrapping my head around this. Please help me.
     
  2. jcsd
  3. Jun 24, 2011 #2

    Doc Al

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    Staff: Mentor

    If you use English units, your answer will not be in Joules. What's the English unit of energy?
     
  4. Jun 24, 2011 #3
    I believe that the English units for work (force x distance) are lb x ft and horsepower so then they must also be the English units for kinetic energy. Thank you very much!
     
    Last edited: Jun 24, 2011
  5. Jun 24, 2011 #4

    SteamKing

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    To wit:
    1 joule = 0.7376 ft-lbf (foot-pound force) of work
    1 joule/sec = 1 watt (power)
    1 horsepower = 550 ft-lbs/sec (not work, but power) = 746 watts
     
  6. Jun 25, 2011 #5

    Borek

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    BTU?

    (which gets automatically decapitalized when posted without a comment :grumpy:)
     
  7. Jun 25, 2011 #6

    SteamKing

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    BTUs are generally used to express the thermal equivalent or work, or energy.
    1 BTU = 778 ft-lbf = 1055 joules.
     
  8. Jun 25, 2011 #7

    Doc Al

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    In the OP's problem, the calculated energy using English units will be in ft-lbs, not J.
     
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