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B Work and Kinetic Energy

  1. Feb 11, 2017 #1
    I am confused of how Work basically equals to Kinetic Energy...I will give an example just as way to help you help me get an idea about how it works....the example I thought of is... '' James (100 kg) jogs on a straight road at a constant velocity of 2.2 m/s for 600 seconds, how much energy does Jame consume?''...How I thought of it is....since he has constant velocity..his Force is 0 (F= ma), therefore work done is 0 (W= Fs), but KE=0.5mv^2....so his kinetic energy is 242J.

    What did I do wrong?
  2. jcsd
  3. Feb 11, 2017 #2


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    Staff: Mentor

    If James is sitting on a rolling chair without any friction and air resistance, moving at a constant velocity of 2.2 m/s, then there is indeed no work done and the energy difference equals zero. His kinetic energy is positive (242J I assume), but he started already moving and ends moving in your example at a constant speed, so there is no energy gained and no energy lost, because no work is done.

    If you let him start at zero speed, you have to calculated the acceleration (a > 0) and the energy balance is a different one.
  4. Feb 12, 2017 #3
    Oh, I get it now...thank you very much :)
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