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Kinetic and gravitational potential energy

  1. Jan 31, 2013 #1
    1. The problem statement, all variables and given/known data
    Explain why the kinetic energy of the skier at the bottom of the hill is not equal to the gravitational potential energy of the skier at the top of the hill.


    2. Relevant equations
    n/a


    3. The attempt at a solution
    I think the answer is because the bottom of the hill is not at surface level, and therefore there is still gravitational potential energy. I just wanted to double check and make sure I'm not missing anything.
    Thanks!
     
  2. jcsd
  3. Jan 31, 2013 #2

    berkeman

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    I don't think that is the answer. If you define the GPE to be zero at the bottom of the hill and the skier starts from rest there, what else would keep the KE from being equal to all of the GPE when the skier reaches the bottom of the hill?
     
  4. Jan 31, 2013 #3

    haruspex

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    Potential energy is almost always a relative matter. I.e. it's the difference between two energy levels, rather than an absolute measure. (Sometimes the PE at infinity is considered zero, so the PE anywhere else is negative. This is commonly used in cosmological contexts.)
    This makes the question a little unclear. You have interpreted it as taking zero PE to be at the centre of the Earth, right? Possibly, but I doubt that's what's intended. Why might the skier's KE not equal the difference in PE between top and bottom?
     
  5. Jan 31, 2013 #4
    Oh okay. So does that mean that there is another kind of energy contributing to the total energy? So the kinetic energy is not equal because some of the energy was lost to thermal energy?
     
  6. Jan 31, 2013 #5

    berkeman

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    Yes. What caused that loss?
     
  7. Jan 31, 2013 #6
    Friction?
     
  8. Jan 31, 2013 #7

    berkeman

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    Bingo!
     
  9. Jan 31, 2013 #8
    Great, thank you both very much!!
     
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