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Homework Help: Conceptual Question

  1. Oct 23, 2008 #1
    A ball is thrown into the air with 100J of kinetic energy. When it returns to its original level after encountering air resistances what is it kinetic energy?

    I have KE = 0.5m * v^2
    I am not sure how to approach this problem as I don't have the mass or the velocity or the height of the ball.

    am I using the wrong formula?
    once it starts falling down isn't it really gravitation potential energy? is there really any kinetic energy there once its heading downwards? doesn't kinetic energy depend on the frame of reference?

    I am not sure what the teacher wants here in this answer. I think I am not really understanding the question. help?
  2. jcsd
  3. Oct 23, 2008 #2


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    Welcome to PF!

    Hi netrunnr! Welcome to PF! :smile:

    Does the question say how much the air resistance is? :confused:
  4. Oct 23, 2008 #3
    Re: Welcome to PF!

    nope :( doesn't say anything about air resistance.

  5. Oct 24, 2008 #4


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    But you said it said "after encountering air resistances" …

    did you mean "without encountering air resistances"? :confused:
  6. Oct 24, 2008 #5


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    The problem doesn't ask for either m or v so this is irrelevant.

    You titled this "conceptual problem" so I expect that you are not asked for specific values but just a general statement. The total energy is "Kinetic energy plus Potential Energy" and potential energy is always relative to some initial value. Initially, we know that the kinetic energy is 100J and we can take the potential energy at that height to be 0. When the ball returns to its initial height, its potential energy is again 0. What has happened to its kinetic energy? Is it the same, or is more, or is it less than before? What would be your answer if there were no air resistance?
  7. Oct 25, 2008 #6
    I SEEE
    okay so the kinetic energy decreases going up and increases going down. at the point of origin it recaptures all the kinetic energy it lost on the way up....
    so then after encountering air resistance its kinetic energy is less than what it was when it started? right? so <100J ?

    yah its conceptual so I think that answer will be acceptable.
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