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Homework Help: More quick work and energy questions

  1. Mar 9, 2007 #1
    An object hits a wall and bounces back with half if its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?
    I am not sure I understand the question, is it 1:2?

    A ball drops some distance and gains 30 J of kinetic energy. Do not ignore air resistance. How much gravitational potential did the ball lose?
    I would say more than 30J since we have to take air resistance into consideration.

    Thanks!
     
  2. jcsd
  3. Mar 9, 2007 #2

    radou

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    Not so fast. Write down the kinetic energies and look at the ratio.
     
  4. Mar 9, 2007 #3
    First is 1 and second is 0.5 => 1/0.5 =2. Is this correct?
     
  5. Mar 9, 2007 #4

    radou

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    No, it isn't. The first kinetic energy is 1/2 m v^2, and the second is 1/2 m (1/2 v)^2. What is their ratio?
     
  6. Mar 9, 2007 #5

    AlephZero

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    No - the kinetic energy of the object is (1/2)mv^2.
     
  7. Mar 9, 2007 #6
    I am not sure I get it, but it looks like 1:0.5 to me...
     
  8. Mar 9, 2007 #7

    radou

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    Do you know how to take the ratio of 1/2 m (1/2 v)^2 (final kinetic energy) and 1/2 m v^2 (initial kinetic energy)?
     
  9. Mar 9, 2007 #8
    I guess not... It's ok, I'll figure it out.

    Thanks!
     
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