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Homework Help: Potential + Kinetic Energy Question

  1. Jun 21, 2007 #1
    1. The problem statement, all variables and given/known data
    A 15.0 kg. model plane flies horizontally at 12.5m/s. Part a) asks for kinetic energy. I got 1172J. Part b) says the plane goes into a power dive and levels off 20.4 m closer to the earth. How much energy did it lose during the dive. I got 2998.8J. Doesn't really make sense.

    Then part c) asks how much kinetic energy did the plane gain during the dive. I'm lost. Please help.
  2. jcsd
  3. Jun 22, 2007 #2


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    the first question is right.

    the second one, "how much do it loose"
    well since it is falling, it must loose some potential energy. But energy can not be destroyed; so it does not loose any total energy.

    Think of this:
    When you let a rock fall free from a buildning, an ignore air resistance, what is happening with the rock from an energy point of view?
  4. Jun 22, 2007 #3
    It is transfering potential to kinetic. So for part b) it loses no energy. For part c) i found out the potential energy it would lose to be 2998 J. So the kinetic energy gained 2998 J. Part D asks what the new kinetic energy would be, so I added 1172 and 2998. Is this better? PS, thank you for your help. I appreciate how no matter how basic some of the question I ask are, no one is sarcastic or demeaning.
    Last edited: Jun 22, 2007
  5. Jun 23, 2007 #4
    for part (b) i guess the question should have been, how much potential enrgy the plane has lost? then you use the equation to find the potential energy converted to kinetic enrgy.

    using teh law of conservation of energy, potential energfy lost = kinetiv energy gained.

    i think you're right for part (d)
  6. Jun 23, 2007 #5


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    Energy is conserved, so NDiggity is correct. No energy is lost, the plane travels faster in the lower orbit. The drop in potential energy is offset by the gain in kinetic energy [disregarding friction].
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