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Silly question about gravity

  1. Apr 9, 2004 #1
    why do we get so many orbits in nature? how narrow is the speed between escape velocity and impact velocity so that the moon for example will 'fall into' the earth whilst moving horizontally across it like 5mps or whatever? what are the odds that all these planets and moons have taken up this magic ratio of speed around the sun and around their host planets, etc.? why do stellar objects seem inclined towards orbit?
  2. jcsd
  3. Apr 10, 2004 #2


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    I believe that the near circularity of the planetary orbits is taken as a sign that the solar system was formed as a unit from a single primitive mass of spinning material. The planets were created in their current orbits, orbital velocity is determined by distance from the center of the orbit, so they have the correct velocity for that orbit because that is where they formed.
  4. Apr 10, 2004 #3
    but that doesn't explain why! why were they formed in orbit? why wouldnt' gravitational attraction simply have sucked everything together at the time of planetary formation? and it doesn't explain all the other orbits of moons and satellites, why is it so easy for an artificial satellite to maintain an orbit if it is such a tight speed ratio?

    i'm sorry if this stuff is too simple but i really would like to understand this better...
  5. Apr 10, 2004 #4


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    Consider doing the following experiment: flip a coin 10 times. If it comes up heads every time, write "Good" on a slip of paper, if it doesn't, write "Bad" on the paper. Repeat that 1000000 times. You should have about 1023 "Bad"s for each "Good" because the probability of 10 consecutive heads is 1/1024.

    NOW throw away all the slips of paper on which you have written "Bad" and keep only the ones on which you have written "Good"!!! Tell a friend that these slips of paper represent the number of times you got 10 consecutive heads but don't tell him about the slips of paper you threw away!

    The point is that that the huge majority of "objects" DID fall into the sun- that's one reason why it's so big! Take a look at the moon- all those craters represent debris that fell into the moon (and the "back side" is much worse- the earth tends to shield the side we can see). What is in orbit now is precisely the very, very few pieces that happened to have precisely the correct velocity and distance to orbit. The overwhelming majority that did not are long gone.
  6. Apr 10, 2004 #5
    you're right, thanks ivy (and integral, sorry)
    Last edited: Apr 11, 2004
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