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Homework Help: Kinetic energy and potential energy of a baseball

  1. Feb 16, 2009 #1
    1. The problem statement, all variables and given/known data

    can the kinetic energy of a baseball be negative.
    can the potential energy of a baseball be negative.

    2. Relevant equations
    no relevant equations.

    3. The attempt at a solution
    i believe the potential energy can be negative if it is falling and the kinetic energy can be negative if the ball is hit with a greater force over a longer distance than it was thrown in the opposite direction
    Am i right?
  2. jcsd
  3. Feb 16, 2009 #2
    What is the equation for kinetic energy?

    What is the equation for potential energy?

    Remember that when using potential energy we can use ANY reference level, (i.e. we can choose the ground to be h=0, or halfway up into the atmosphere to be h=0). We are not able to give an exact value of the potential energy, we can only ask : "What is the potential energy change of this particle with respect to this point?" ie the potential energy of a baseball that is about to be dropped on your head from 6' is a lot smaller and will hurt a lot less if you are standing up at 5'5" just under the ball then laying down! :-)
  4. Feb 16, 2009 #3
    sorry i worded the questions wrong:

    Can the kinetic energy of a baseball ever be negative.
    Can the gravitational potential energy of a baseball ever be negative.

  5. Feb 16, 2009 #4
    So, what are the possible values for those two equations, given that m is always possitive, g is possitive. The kinetic energy of an object is a measure of the work that would be done if it was brought to a stop, i.e. if I throw a baseball at a watermelon it will crack open.

    I think the negative kinetic energy example you gave above was saying that if you throw a baseball at me, and then I hit it with my bat and send it flying in the other direction then the kinetic energy is now negative, but if the ball comes back and hits you it will hurt! This means it had energy! If there is any energy form that has negative energy, it would mean it has a LACK of energy, i.e. it would take work to give it any energy at all.

    Kinetic and potential energy can be thought of as bank accounts, raising a ball is synonymous to putting money in the bank (potential energy) that you can use later to buy stuff (do work!)

    For kinetic energy, you can think of the speed of the ball as the bank account balance, the higher the speed, the higher the kinetic energy. If you stop it, all the kinetic energy is used up.
  6. Feb 16, 2009 #5
    thank you very much it all makes sense now
  7. Feb 16, 2009 #6
    You are very welcome, what answers did ya get? I just wanna make sure! :D
  8. Feb 16, 2009 #7
    the kinetic energy cant be negative while the gravitational potential energy can be negative
  9. Feb 16, 2009 #8
    That's right ;).
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