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Where does the energy for gravity come from?

  1. May 11, 2015 #1
    Basically my question is this( and sorry if my formulations are at times a bit sloppy, English is not my first language) :

    1. we know energy can neither be created nor destroyed, it can only neconverted from one form into another

    2.We know that an object in space, that is moving, will move along a straight line unless acted upon by some kind of force.

    3. if I want to change the direction of an object in space, I have to invest some energy to do that

    4. The moon moves around the earth, because there is a constant pull from gravity. Very roughly we could say that gravity pulls the moon towards earth , but by the time the moon would have hit the earth , the moon has already moved forward enough enough, so that it is now a quarter turn further (yea, i know it the orbit is not really round, but it is not too bad an approximation to understand the basics. I w ill ignore for now the influence of the sun, th e other planets and whatnot else)

    5. If the pull of gravity is constant , where does this pull get all the energy from ? I mean the moon is a big heap of rock. If we would have to move it, we would need some serious thrust engine or something, needing one hell of a lot of energy. So where does this energy come from, which the earth constantly "invests" to force the moon into it's orbit.

    6. Another example would be a comet or a meteorite. When it flies towards earth it constantly accelerates, getting more and more kinetic energy while flying towards the planet, till it hits it often with impressive speed. Again where does this energy come from ??? my first point says energy cannot be created, so it earth loosing energy over time?

    7. In the end there can be only 2 solutions , either gravity gets weaker over time, loosing it's energy slowly but surely (which does not really make sense, since afaik that is clearly not what happens) some other kind of energy is constantly transformed into gravitational energy, which begs the questions what that energy that is , if and how it is replenished (and if it not, what happens when it runs out), and how come this transfer is so exact (since gravity does not seem to get weaker or stronger, so the transfer seems to exactly make up for the energy lost all the time, which would it make it quite a curious energy source)


    I hope my english and my science sucked not too bad to get my point across
     
    Last edited by a moderator: May 16, 2015
  2. jcsd
  3. May 11, 2015 #2

    rcgldr

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    The energy comes from the objects initial position, going all the way back to just after the universe was created (big bang theory). So at some point in time, the moon ended up orbiting the earth. In an ideal case, the total energy consisting of gravitational potential energy and kinetic energy of the earth and moon is constant. In reality, only angular momentum is conserved and some energy is lost to heat due to tidal interaction. Wiki article about this effect:

    http://en.wikipedia.org/wiki/Tidal_acceleration#Angular_momentum_and_energy
     
  4. May 11, 2015 #3

    Dale

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    This point is not correct. It takes energy to increase speed, but not to change direction. Of course, you can come up with examples where energy is used to turn, but those are examples of inefficiencies where energy is being wasted somewhere.
     
    Last edited: May 11, 2015
  5. May 11, 2015 #4
    What goes up, must come down. Well, not really, but basically, it takes energy to launch something up from the ground, because it must move against the direction that gravity is pulling. The object slows down as it increases height because the energy is converted into gravitational potential energy. Eventually (if it isn't launched with greater than the escape velocity), the object will fall back down and the energy is returned to the object. The gravitational potential energy is converted back into kinetic energy, and you end up with the object coming down with the precisely the same speed as it was launched up with, if nothing got in the way.

    Objects in space have a large gravitational potential energy to start with. Where did this initial energy come from? The big bang.
     
  6. May 12, 2015 #5
    Let us get back to my meteorite. If it accelerates towards earth, it gains momentum hence energy, where does that energy come from ? In my naive understandingn it must somehow come from earth , since earth is accelerating it
     
  7. May 12, 2015 #6

    A.T.

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    It gains kinetic energy, but loses potential energy. The sum of all energies in the system doesn't change.
     
  8. May 12, 2015 #7

    mathman

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    Not so. Changing direction requires force, so energy must be used.
     
  9. May 12, 2015 #8

    phyzguy

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    This is incorrect. The rate of doing work on an object (which is the time rate of change of the energy) is F⋅V. In a circular orbit, F and V are perpendicular, so F⋅V = 0. There is a force, and there is an acceleration, but no work is done, and the energy of the orbiting body does not change. Note that DaleSpam said the same thing.
     
  10. May 12, 2015 #9

    Dale

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    No. Not all forces use energy:

    http://hyperphysics.phy-astr.gsu.edu/hbase/work2.html
    http://en.m.wikipedia.org/wiki/Work_(physics)#Constraint_forces

    As phyzguy said if f.v is 0 then the power is zero. You can think in terms of the potential energy. If you stay at the same potential then no energy is used. The force is the gradient of the potential. So if you move perpendicular to the force then you stay at the same potential and no work is done.
     
  11. May 12, 2015 #10
    Let's break this down into even more ultra basics, for the newbies. What is the first formula you learn for energy transferred by work: force times distance. I can apply any force I want without doing work, so long as I don't move the object in the direction of the force. In the orbits mentioned the distances between, for example earth and moon, are fixed. No change in distance, no energy.
     
  12. May 13, 2015 #11
    I don't think gravity uses energy cause according to Newton anything made of mass has its own gravity the more mass the more gravity.as far as I can say it is related to the natural curvature of space. And I bet gravity does not use energy and gravity is not exactly a force
     
  13. May 13, 2015 #12
    A handy way of visualising this is to view gravity as like a spring or elastic band. The potential energy stored in the gravity 'band' between two objects is a result of separating the two objects. You can put this energy in yourself - putting a satellite in orbit using a load of chemical energy rocket fuel, for example. If the satellite falls back to Earth the kinetic energy it acquires in falling is the energy you put into putting it up there to begin with.

    In the case of a meteorite, the potential energy in the gravity 'band' is also the result of the original separation of the meteorite and Earth, and that separation work was done by the Big Bang and the expansion of the universe. The "natural" position for everything is as a single point, separating things out from that point required energy, that energy is recovered when the two things fall together under gravity.
     
  14. May 13, 2015 #13

    phinds

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    I think it would be more correct to say that it takes energy to CHANGE speed, not just increase it. Either one (increase or decrease of speed) will change an orbit [Dale, I know you know this, I'm just pointing it out for the OP]
     
  15. May 13, 2015 #14

    jbriggs444

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    Increasing speed takes energy. Decreasing speed releases energy. Changing direction does neither.

    Of course if you use an inefficient process (like squirting a hose at the front of a super-tanker to slow it down), you may wind up using energy even though none is required.
     
  16. May 13, 2015 #15

    phinds

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    How do you decrease the speed of a spacecraft without expending energy?
     
  17. May 13, 2015 #16

    jbriggs444

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    Reverse slingshot, for example.

    Agreed that there are there are no efficient ways if you are deep space depending on using reaction mass and fuel that are carried on board.
     
  18. May 15, 2015 #17
    Strictly speaking, a reverse slingshot only doesn't use the spacecraft's energy - the work is done by the planet.
     
  19. May 15, 2015 #18

    jbriggs444

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    In a slingshot, energy is gained by the spacecraft at the expense of the planet. In a reverse slingshot, energy is gained by the planet at the expense of the spacecraft. So yes it does use the spacecraft's energy.
     
  20. May 15, 2015 #19

    CWatters

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    The moon stays at a constant distance from the earth (at least for the purposes of this question) so gravity, which only works in that direction, doesn't move the moon. Fridge magnets need no energy source to stay up for the same reason.
     
  21. May 15, 2015 #20
    Probably the only time I can recall Richard Feynman getting testy, was when an interviewer implied he should surely be able to say how the magnetic force works. Professor Feynman's reaction was, more or less, that one would have to go back into decades of learning and build up, step by step, on the most fundamental foundations of that facet of science before such a question could be answered.

    That was magnetism, and given enough time, he could have answered that question. I feel fairly sure he wouldn't have used gravity, even in the most simplistic of analogies, to describe that answer.
     
    Last edited: May 15, 2015
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