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What is the kinetic energy?

  1. Dec 10, 2013 #1
    1. The problem statement, all variables and given/known data A physics book weighing 20 N is dropped from a position 2.0 meters above the floor. a) what is the potential energy of the book before it is dropped? b) what is the kinetic energy of the book when it is 1.0 meter from the floor



    2. Relevant equations Ek = 1/2mv^2 , Ep= mgh



    3. The attempt at a solution a) Ep = mgh = 20N X 2.0 = 40N

    B) I dont know what to do ):
     
  2. jcsd
  3. Dec 10, 2013 #2

    vela

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    What forces act on the book? Are any non-conservative?
     
  4. Dec 10, 2013 #3
    Non-Conservative forces acting on the book are , air resistance, sound and heat. The force acting on the book are gravity
     
  5. Dec 10, 2013 #4
    How far has the book traveled?

    And what are the equations that tell you velocity under constant acceleration after a certain distance?
     
  6. Dec 10, 2013 #5
    book has traveled 1.0 meters? ac= v^2/r ?
     
  7. Dec 10, 2013 #6

    vela

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    Sound and heat aren't forces. They don't push or pull on the book, right? Air resistance, however, is a force, but you can neglect it. So gravity is the only force acting on the book, and it's conservative. So you can use energy conservation. That means the energy at the initial point is equal to the energy at the final point.

    What's the total mechanical energy at the initial point? You already found the potential energy there. What is the book's kinetic energy initially? Add the two together to find the total energy.

    At the final point, the book is one meter off the floor. What does the total energy consist of there?
     
  8. Dec 10, 2013 #7
    So if im right, Ep2 = 20N x 1.0 m = = 20N

    Ep1 + Ep2 = 40N + 20N = 60N

    Ek= 60N?
     
  9. Dec 10, 2013 #8

    vela

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    You got the potential energy at the end almost correct. You have the wrong units.

    Energy conservation says that
    \begin{align*}
    E_i &= E_f \\
    KE_i + PE_i &= KE_f + PE_f
    \end{align*} So you have the potential energies. What are the kinetic energies?
     
  10. Dec 10, 2013 #9
    Ek= 60 J?
     
  11. Dec 10, 2013 #10

    vela

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    No. You have a formula for kinetic energy. Use that. And there are two, one at the beginning and one at the end.
     
  12. Dec 10, 2013 #11
    Thanks boss
     
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