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Gravitational / Magnetic Potential Energy Question

  1. Jun 13, 2010 #1
    To get energy out of an object falling down to earth or a magnet slamming into another one, you have to lift the object up or pull the one magnet away from the other, which makes sense with Newton's laws.

    I was wondering, if an object just "magically" appeared out of nowhere in the air without anyone lifting it up first, would it fall down? After all, nobody put any energy into lifting it up.

    Thanks, and sorry if this seems like a silly question, lol.
  2. jcsd
  3. Jun 13, 2010 #2
    The problem with these types of questions is that if you presume magic, then you can conclude any thing you want. That is, there can be no conclusion from a nonsensical premise.

    If you want something to magically appear, then that would violate conservation of energy. After it appeared then it would still be influenced by the gravitational field, unless the magic somehow disrupted that. :)
  4. Jun 14, 2010 #3
    Haha, you're right I suppose!

    How about a situation in which a baseball is placed steadily, unmoving, about as far from the earth as common man-made satellites. Disregard the motion of the Earth and Moon. Would the baseball fall to earth, being well within its gravitational field? Or would it simply stay stationary regardless, because it received no potential energy like a baseball launched up there would get from being lifted up?
  5. Jun 14, 2010 #4
    The baseball would have potential energy by virtue of being at a particular position in the gravitational field. This potential energy would (in part) come from the energy it takes to lift it there.
  6. Jun 14, 2010 #5
    But what if it came from space? I mean, to be realistic, what if it was an alien baseball? Lol, I can't think of anything else. Basically I mean, what if it came from deep space and just was placed in the Earth's gravitational field? It didn't come from Earth. What then?
  7. Jun 14, 2010 #6
    Then it has potential energy due to it being in a gravitational field.
  8. Jun 15, 2010 #7
    I see, thanks for the answers!
  9. Jun 16, 2010 #8
    I have been thinking about that for a while though ..

    the gravitational potential is directly proportional to the mass of the two involved objects, if a planet kept attracting meteorites for example, they will gain kinetic energy as they fall, as soon as the hit the surface thermal energy will be released, AND the total mass of the planet will increase too, so the gravitational potential will increase and other meteorites get attracted and so on ..

    so the result is an increase in thermal energy, that can escape the planet in the form of radiation, without a decrease in the field strength, which seems controversial for me.
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