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About Gravity

  1. Jun 28, 2010 #1
    I have been pondering on this for a while.

    Gravity is a force, not energy. But where is the energy that produce gravity?

    For an example.

    If there only Earth and Moon existed and nothing else exists. (People and water are located underground inside the Earth or whatever the case that may be.) If the moon kept in perfect orbit where it does not move away or closer and creates the tides for water underground. People could produce energy from the tide, so where all the energy coming from? If gravity is not a form of energy, then what is the energy that produce gravity?

    Does this break the "Law of Nature" philosophy in term of energy being not created?

    Please and thank you for your time.
  2. jcsd
  3. Jun 28, 2010 #2
    In this context think of energy as a force over a distance. The force of gravity only involves energy if it acts on an object over a distance. Book on a table -> its not traveling a distance so there is no energy involved. Book falling through the air -> The force of gravity is acting over a distance so the book is gaining energy.

    As for your example, you shouldn't expect a 'perfect' orbit to produce energy from tides! The energy from tides comes from the energy of the earth and moon's orbits. The distance between the earth/moon as well as the speeds of the earth and moon are changing (slightly) as a result of us harnessing tides.
  4. Jun 28, 2010 #3
    Can you explains more on that?

    For gravity, when there is force, there have to be energy. Am I wrong?
  5. Jun 28, 2010 #4
    A force does not require energy. Consider a spring that you can compress. The act of compressing it does require energy. Keeping it compressed with a force does not require energy. You can latch it in a compressed state and toss it into space, it will stay latched and compressed and no energy need be consumed.
  6. Jun 29, 2010 #5
    what you're describing is impossible. it's impossible for the moon to maintain its current distance from the earth, especially with all the energy the moon is using up by creating the tidal forces!
    so where does all this energy from the moon exerting its forces on the earth go? according to the conservation of angular momentum law, the moon is constantly moving closer to the earth as it revolves around the earth.
    again, it's not possible for the moon to maintain its current distance from the earth.
  7. Jun 29, 2010 #6


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    It moves further. Check your work.
    There is usually an associated potential, so an object in gravitational field has potential energy.

    Field itself also has energy. Gravitational field is not an exception, but this is getting into something a bit more complex.

    Don't think of force as manifestation of energy, but rather as means of transfer of energy. If force is applied over distance, work is done, and energy is transfered from one body to another.
  8. Jun 29, 2010 #7
    the energy that produces gravity is from the potential energy the planet earth has, this energy is by nature come with all heavy mass objects, one evidence for that, that the gravity and the mass of the planet are direct proportional to each other, and otherwise you may think, this coincides with newton's theories.
    i recommend you to see chapter 13 of the serway/jewett physics book, google it, the chapter's title is "universal gravitation", you may find what you are looking for in it.
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