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Homework Help: Kinetic engery

  1. Oct 4, 2006 #1
    Im wondering how do we know when to substract Kinetic engery from potential engery or when do we add them?

    Is that making sense?


    question is asking:
    a women skis downhill at a constant speed of 8.0 m/s when she reaches an icy patch on which her skis move with negligible friction. If the icy patch is 10 m high, what is the skier's speed at its bottom?

    the book says to use this formula:
    KE2=KE1 + PE


    why are adding kinetic and potential together? I understand how to manipulate the equaiton to solve for speed but why are we using this formula?
     
  2. jcsd
  3. Oct 4, 2006 #2
    The sum forms the total (mechanical) energy. In the absense of friction, it's a contant (conservation of energy). Since it doesn't matter where the "zero" of potential energy is, assume the skier has P.E = 0 at the bottom of the icy patch.
     
  4. Oct 4, 2006 #3
    still lost...

    When does it matter about the ZERO potential engery?
     
  5. Oct 4, 2006 #4
    Did you read the page I linked to?
     
  6. Oct 4, 2006 #5
    DUhhh... sure didnt.. I thought that was an underline.. BRB :)
     
  7. Oct 4, 2006 #6

    arildno

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    ALWAYS START OUT WITH CONSTANT MECHANICAL ENERGY.
    On a given side, you will have the potential&kinetic energy for the same instant summed together.
     
  8. Oct 4, 2006 #7
    ahhhh.... its amazing what happens with the light bulb comes on!
     
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