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Where does the energy of gravity come from?

  1. Apr 5, 2007 #1
    Say you were to slingshot around the moon and gain momentum. In so doing you would have gained Energy from gravity. Basically because energy is conserved there has to be some way that the force of gravity "radiating" out from the moon would come in contact with a space shuttle and speed it up. How does this work?
     
  2. jcsd
  3. Apr 5, 2007 #2
    I think that the energy you gained ,come from the "kinetic energy" of the moon!
     
  4. Apr 5, 2007 #3

    russ_watters

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    Gravity is a static force. There is no energy expended in static forces. Consider a book sitting on a table. It has a certain potential energy defined by its height, but that energy level never changes, so gravity is not doing any work on it.
     
  5. Apr 5, 2007 #4
    I don't understand. I don't see how it could speed something up without transferring some kind of energy.
     
  6. Apr 5, 2007 #5

    russ_watters

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    Speeding something up is different (I should have read more carefully/been more descriptive), but in order to drop something you first have to lift it. It is conservative, just like a spring. The total energy is the potential energy plus the kinetic energy and it is always constant (if there is no loss from other factors).

    A gravitational slingshot, however, does steal orbital energy. It is a bad (confusing) example for examining why there is no energy generation in gravity.
     
  7. Apr 6, 2007 #6
  8. Mar 27, 2009 #7
    Gravity is a force, if everything is static, there's no work and no energy required. But take two celestial bodies, say the Moon and the Earth. Gravity keep them revolving around each other but the Moon would fly off tangentially if it were not for gravity. The Moon's course is constanlty being altered by the Earth's gravitational pull. This is not a static situation, the moon is being accelerated, work is being done and the question is... where does the energy come from?
     
  9. Mar 27, 2009 #8

    russ_watters

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    No.
    Work is force * distance. Where is the distance here?
     
  10. Mar 27, 2009 #9
    Looking at the Moon's trajectory. It is constantly being accelerated towards the Earth. Relative to the Earth there is no movement, but from the Moon's perspective its course is being bent away from its straight line trajectory and therefore work is being done.
    Relative to the center of Earth there is no movement, but in rectalinear space the Moon is being constantly accelerated from a constant speed and course, and that is the distance.
    Thanks, for your reply, and I'm not tryiing to be argumentative, this puzzles me.
     
  11. Mar 27, 2009 #10

    DaveC426913

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    When your spaceship uses the Moon to slingshot, the Moon slows down a very, very small fraction. That is where the energy comes from.

    It is analagous to a skater grabbing the bumper of a passing car and pulling himself forward. The car slows down as the skater speeds up.
     
  12. Mar 28, 2009 #11

    russ_watters

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    No. The acceleration is always perpendicular to the direction of motion. It causes no change in the kinetic energy of the moon (caveat: elliptical orbits, but they are conservative too). Consider that once a month the moon gets back to where it started, going the same speed as it was a month ago: exactly the same state/energy. Acceleration perpendicular to the direction of motion does not produce a change in speed.
     
  13. Jan 18, 2010 #12
    Hey pedridge,

    I think you're onto something. I stumbled upon this thread when I googled the same question you asked.

    I agree that gravity introduces energy into the "closed system" of the Universe. Indeed, Newton claimed he was not attempting to explain where gravity came from or what the driving force behind it was.

    Physicists just make up terms like "static force" and "potential energy" in order to get around the "matter/energy is neither created nor destroyed." I think it's clear using the thought experiments you propose that matter continuously creates energy in the form of gravitational force from seemingly nothing.
     
  14. Jan 18, 2010 #13

    Matterwave

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    The moon actually gains energy in its orbit from the rotation of the Earth and the tidal forces the moon exerts on the Earth.

    As such, the moon is very slowly moving away from us, and the Earth's rotation is very slowly slowing down.

    Over the course of the Earth's history (4.5 billion years), the Earth day has slowed down from ~6 hours to 24 hours, and the moon has drifted something like 4 times further away (don't quote me on these figures).

    The point is, energy is not lost, just transferred.
     
  15. Jan 18, 2010 #14
    I can imagine one day the travesty of 'wasting moon energy' as space travel becomes more common. The moons energy is a limited resource!
     
  16. Jan 18, 2010 #15

    HallsofIvy

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    Newton did NOT say that. He did say he was describing gravity, not explaining it ("non hypothesen fingo"- "I frame no hypotheses") but he certainly never talked about a "driving force" behind gravity. One force does not require another "driving force".

    Yes, in a sense the different kinds of energy are a "bookkeeping" device but very useful ones. Physicists do not "just make up" such terms any more than accountants "just make up" the numbers they put in the books. Matter does not "create" energy from "seemingly nothing". All that the thought experiments baseplayer142 proposes show is that he (and you) do not understand the physics involved. He, at least, asked for assistance. You just tell him to ignore physicists who "just make up" things!
     
  17. Jan 18, 2010 #16
    I'll restate the problem I'm having.
    Take any two obvious large objects as an example, the two easiest are the Earth and the Moon, locked in an orbit around each other, by gravity. Simple. The question becomes... as the Moon is not being allowed to take off on a tangential course, it is being constantly being accelerated towards the Earth. Simple, again. The energy required must be supplied from somewhere, and the question is where?
    Mass and energy are two sides of the same thing, as there is no other source for the energy required, matter has to be converted to energy, but where is the loss of mass?
     
  18. Jan 18, 2010 #17

    Matterwave

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    The answer is...there is no energy required for a circular orbit to remain stable. That's the whole point people have been trying to tell you, but you seem to ignore.

    Remember P=F dot V

    Where P is the power (energy per second) radiated, F is the force, and V is the velocity.

    For a circular orbit, F and V are always perpendicular (elliptical orbits also don't radiate power, but it's more complicated). Therefore, F dot V is zero and there is no power radiated!
     
  19. Jan 18, 2010 #18
    And that makes no sense which is what we've been trying to tell you. Your equations don't help explain how the Sun is able to keep the Earth moving in a circular manner as opposed to flying straight. Or the Earth and the Moon, etc. To say that no energy is needed to maintain a circular orbit is nonsensical. It obviously requires energy.

    Take a table. If I kick the bottom of the table, in outer space it will move in the direction it was kicked. But Earth's gravity is continuously pulling it downward. This is work being done. Work requires energy. There is no matter --> energy conversion that explains this as far as I know, and petridge pointed this out as well. So where does it come from?

    I've raised this issue up in a conversation with a Stanford PhD physics professor about this. She didn't know how to answer it and said things like 'we don't know much about gravity' and 'we have this concept of potential energy.' If you have better credentials than that, by all means give a real answer, don't just refer to some equation as proof that you don't need to account for it.
     
  20. Jan 18, 2010 #19
    I apologize for getting the quote wrong, or even the subtleties of what he said. But how is "describing it, not explaining it" NOT the same as saying he was saying how it works, not where it comes from? Where what comes from? The energy that gravity is clearing introducing into the system.
     
  21. Jan 18, 2010 #20

    DaveC426913

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    Objects fall at 9.8m/s^2. That is describing it without explaining it. It says nothing about how or why objects fall at that rate.
     
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