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Where does gravity get its energy from?

  1. Dec 23, 2009 #1
    A large, very large body like earth exerts a gravitational force on moon or a smaller object and there is displacement (neglect displacement of earth). Thus work is done. So where does the energy come from to do this work?
  2. jcsd
  3. Dec 24, 2009 #2
    Well classically, the idea is that the moon would need a certain amount of work done on it if it were to escape to infinity, or equivalently it would need sufficient kinetic energy to completely escape the gravitational pull of the earth. Therefore we create this notion of 'gravitational potential energy'. When the moon is attracted to the earth it loses some of its GPE which is converted to its kinetic energy. In other words the moon becomes more constrained as it moves inwards. It has less 'potential' or freedom than if it were a free body.

    However, its a bit of a fudge really to make the energy equation balance and to make calculations easier. I suspect the real insight is general relativity, where the presence of mass is curving spacetime so that particles travelling on geodesics appear to be travelling in curved paths around a massive body. There is a notion of mass/energy equivalence but im not sure how it applies in this case. I tried to follow the Susskind lectures on youtube and its pretty tricky stuff, so hopefully someone else can illuminate us here?

    The other thing to consider is whether there is a fundamental particle(the graviton) that mediates the force. If there is such particle then it is this that is actually transferring energy between the earth and the moon.
  4. Dec 24, 2009 #3
    As a result of the fact that an object is under the influence of the earth's gravity, it has a form of energy called gravitational potential energy. So when you drop an apple and it falls to the ground, the energy needed to do the work comes from this gravitational potential energy.

    Now you may be wondering how exactly the apple got the gravitational potential energy to begin with, since energy isn't free! Well, the apple acquired that energy because work was done on it to lift it to that height.

    I hope this helps!
  5. Dec 24, 2009 #4
    Say you had a massive lump of stuff and pulled it apart into two equal pieces. You would apply a force to separate them under their mutual gravitational pull.

    If they were originally together this is where the energy came from. It came from the energy applied to get them separted to some distance. Does this make sense?

    So you might ask, if the Universe began as some small point where did the energy come from the separate all the pieces?
  6. Dec 25, 2009 #5


    Staff: Mentor

    Be a little careful here. For a circular orbit there is no work done since the force is always perpendicular to the displacement. Work is only done when the force has a component parallel to the displacement. A better example is a comet where the orbit is highly elliptical.

    The comet starts far away and falls closer to the sun, work is done, the gravitational potential energy is reduced and the kinetic energy is increased. The reverse happens as the comet goes out away from the sun.
  7. Dec 25, 2009 #6


    User Avatar

    Staff: Mentor

    Ultimately, the GPE was first acquired when the universe was created.
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