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What happens when one's gravity attracts the other?

  1. Feb 5, 2008 #1
    "the Moon is falling towards Earth. However, it also has an orbital speed from the momentum gained during its formation that allows it to fall around Earth with a trajectory that follows the same curve as Earth's surface. Because these paths are parallel, the Moon perpetually falls around Earth without ever touching it."
    I know this.

    But when did moon gain this orbital speed in the first place?

    Hypothetically, if we disintegrate the moon and put an object with a same mass out of the blue where the moon was how that object would behave? Would it fall into world gradually leading a catastrophic collision due to the earth's gravity or would it orbit around the earth just like the moon?
  2. jcsd
  3. Feb 5, 2008 #2


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    No one is sure where the moon originated. It might have just been aggragate matter that coelesced when the planets were formng, but I've seen recent suggestions that it might have been a piece of Mars that was knocked loose during a collision. Space Tiger will be able to provide better information.
    As for the orbital insertion of a foreign body, it is entirely dependent upon what speed and trajectory are in play.
    Last edited: Feb 5, 2008
  4. Feb 5, 2008 #3
    No speed, no trajectory.
    Out of the blue.

    I'm trying to comprehend the reason heavenly bodies form circular/elliptical orbits. Is it because they're already-owned velocity turn into a circular motion due to the gravity or what?
  5. Feb 5, 2008 #4


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    Straight in, out of the blue, would be one humongus Kaboom. Bye-bye dinosaurs.
    Again, this is not my area, and someone like ST can give you the straight ****. From what I've studied, though, solar system dynamics were established when the original dust cloud started liking itself enough to get married to itself and have babies.
    As the cloud shrank, it started into a 'wirlpool' movement due to conservation of angular momentum. Keep in mind that our system is composed of materials that were formed in first-generation supernovae, so there were heavy elements on board right from the beginning. Because of that, nuclei of planetary bodies came together due to mutual gravitational attraction among those heavy bits. The bigger a body became, the more crap it swept up along its path and incorporated into its structure. The angular momentum scenario applies to the planetary rotation as well as to the movement around the sun; each captured particle contributed.
    That is really all that I have to offer, and some of it might be at odds with current knowledge. Seriously, man... wait for an expert for more info.
  6. Feb 5, 2008 #5


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    This intro article on celestial mechanics should help:

  7. Feb 5, 2008 #6


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    i saw an episode of Nova Origins, and according to that episode, they said it was pretty widely accepted that something big banged into the Earth when it was around a billion years old, spewed a sh1tload of matter out that coelesced into the Moon. and the Moon used to be much closer. too bad that wasn't the case in 1993 or 1994 when i lived in Lebanon NH, i got to see a perfect, but annular solar ecilpse, i might give my left testicle to see a decent total solar eclipse (easy to say at my age).
  8. Feb 5, 2008 #7


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    Yeah, I guess that you don't have much use for them any more... :biggrin: (Unfortunately, I'm not too far behind you.)

    I've never even heard of that show, let alone seen it, but it sounds great. Thanks for the update.
  9. Feb 6, 2008 #8
    From the book called "Light&Matter: Newtonian Physics" I found this to my question:

    "It would just stay where it was. Plugging v = 0 into eq. F = ma = mv2/r
    would give
    F = 0, so it would not accelerate from rest, and would never fall into the sun. No astronomer
    had ever observed an object that did that!"

    Why wouldn't it accelerate? now there's a Force of gravity of sun and yet it wouldn't accelerate from rest towards to sun linear????
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