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Gravity ?

  1. Apr 2, 2010 #1
    When I lift a rock up and let it go it always falls back down in the presence of the earths G field . So where is this infinite energy supply coming from . why does it always pull it back , assuming were not talking about escape velocity. Or am I viewing this incorrectly.
  2. jcsd
  3. Apr 2, 2010 #2
    The energy came from you and allowed you to lift the rock. I don't believe your supply is infinite. :)
    This energy was stored in the gravitational field, and released when you let the rock fall.
  4. Apr 2, 2010 #3
    i guess what i am asking is , lets take a rock sitting on the ground will the G field of earth always keeping it there forever , where does the energy come from to distort space-time and make things accelerate towards it and keep it there forever , it seems like gravitational energy is infinite .
  5. Apr 2, 2010 #4
    Let's separate out a couple of things here.
    First, as far as energy in the sense of "doing work" is concerned, the Earth is doing no work and thus needs no energy, to keep the mass at rest on its surface. So, keeping it there forever is not an infinite amount of energy.
    This brings us on to "for ever", and infinity. These are always terms which need to be treated carefully in physics. (There have been a couple of threads on this here recently).
    The Earth has mass, but not an infinite amount, and this mass distorts space-time, but not by an infinite amount. So I don't see where the idea of an infinite amount of energy comes from. If you ask whether this lasts "for ever", that is another matter, and you are asking if the universe lasts for ever. Even if it does, it does not imply that the Earth is doing, or going to do, an infinite amount of work; and therefore doesn't need (or posses) an infinite amount of energy
  6. Apr 2, 2010 #5


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    Staff: Mentor

    Consider a mass sitting on a spring. Is the spring constantly exerting energy to hold the mass up?
  7. Apr 2, 2010 #6
    right, you need to get clear in your mind what the differences are between 'force,' 'energy,' and 'work.'
  8. Apr 2, 2010 #7
    ok lets take a continuous light ray moving past earth as it comes by earth it bends and changes it course of direction , what allows earth to bend this light , lets say the light beam is infinitely long .
    I kind of get what you are saying about the mass and spring
  9. Apr 2, 2010 #8


    Staff: Mentor

  10. Apr 2, 2010 #9
    Answer: same as your post #1. (you can use rocks, cannon balls..... satellites all the same. Light or photons (electromagnetic radiation) is definitely more mysterious and stumped scientists until Einstein explained it.

    You can attempt to get insights into such very subtle and in some respects difficult aspects of gravity by repeating thousands of years of "intuitive" studies by thouands of people studying thousands of examples...OR...you can try working from the theory which results from all those efforts, see if it makes snese, then try applying that theory to problems which interest you. For most of of us mere mortals (not Einstein's) the second path is a LOT faster and more instructive.

    If you understand potential energy, energy of position, calssical (Newtonian) gravity is more understandable:

  11. Apr 2, 2010 #10
    ok thanks for the replies i think i understand better , thanks for the link on GR dalespam
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