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Question about gravity

  1. Feb 11, 2010 #1
    Hello, I'm new here and am very glad I found these forums, I've been seeking answers for some questions that have been bothering me for some time.

    Ok, first of all I want to ask about orbits. I know what an orbit is ( a very fast object virtually falling endlessly , never touching the ground because of the Earth's curvature , like Newton's canon demonstrates). My question is though, what forces keep the certain object from not slowing down? Like the Moon, what keeps its velocity so accurate that allows the moon to complete an orbit in exactly the same amount of time as the previous one?

    That will be all for now, because I'd like to keep you guys concentrated on one question, to get the best answers possible for them. I'll keep asking as we move on :D
  2. jcsd
  3. Feb 11, 2010 #2
    Because there is no wind resistance in space. The moon does however slowly lose energy (velocity/distance from the earth) from the gravitational interaction between it and the earth (but this is very small).
  4. Feb 11, 2010 #3


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    You don't need a force to stop it slowing down - you just need there to be no force to slow it down.

    In practice drag from tiny bits of the outer atmosphere does slow down satelites and cause them to change orbit and eventually re-enter.
    At the distance of the moon there is nothign to cause drag. There are other effects to do with tides but these actually cause the moon to speed up - the moon is moving away from us at a measurable rate
  5. Feb 11, 2010 #4
    My 2nd question is..

    Does orbit depend on distance from the planet? Say, if i would have had superman powers and could throw a rock from just 6 feet above the earth with enough speed so that the rock can "fall" into orbit, considering there's no atmosphere to create drag, could the rock orbit the planet at just 6 feet of height, given the speed of the rock is high enough?
  6. Feb 11, 2010 #5
    Six feet above the surface of the Earth there definitely IS wind resistance.

    Also mgb_phys, I didn't know it was speeding up! Snap... I guess it's stealing the energy from own own rotation?
  7. Feb 11, 2010 #6


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    Yes the days are getting longer
  8. Feb 11, 2010 #7
    noo, i said imagine there's no atmosphere, like on the moon.
  9. Feb 11, 2010 #8
    Oh, then yeah... that's fine.
  10. Feb 11, 2010 #9
    so it can orbit?
  11. Feb 11, 2010 #10


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    So long as you don't stand in its path! :biggrin:

    (or there aren't any hills, etc.)
  12. Feb 12, 2010 #11
    My third question is.. if orbiting is just a continuous free fall without any resistance , wouldn't you feel the same weightlessness when skydiving out of the plane? if yes, the how are you able to distinguish up and down while skydiving and not in space where every way you turn you feel the same?
  13. Feb 12, 2010 #12


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    Yes. Suicide method :

    Go to the moon. Fire a bullet horizontally at 1679 m/s, and wait for 108 min in the same spot. :wink:

    Initially yes, but then you feel resistance. You are not in free fall in the atmosphere.
  14. Feb 13, 2010 #13


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    Yes. thats how you can train astronauts in a falling plane

    Only by wind resistance - if you skydived inside a sealed box you couldn't tell
  15. Feb 13, 2010 #14
    Hi pr1,
    excellent...this is a great forum..just take answers as "tentative" and "mostly correct" most of the time, especially mine!!!!

    does not have to be fast...for example, higher orbits require less speed..


    very complicated question..one I could not answer in a few sentences....but looking into gravitation and studying its various aspects will begin to give you insights for further understanding...Wikipedia is a free place to start...

    In another discussion posted within the last year I posted an interesting fact that I had come across: the moon moves closer to the earth by a small amount every year... it might have been a cm every ten years, but don't quote me on that..in any case orbits do vary for complex reasons....

    it's velocity is not really "accurate" but varies from earth perspective as it passes in a non perfect, non circular orbit...I guess mostly elliptical....anyway the moon does not really orbit the earth, they both orbit their combined center of mass...hence the roughly elliptical orbit and the moon does NOT orbit "in exactly the same amount of time..." all we can say is that it's orbit IS fixed enough that it has been in place for many millions of years and likely will remain up there for many more millions of years....

    If you want to get some insights here is a good place:

    Last edited: Feb 14, 2010
  16. Feb 13, 2010 #15


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    When skydiving on earth, you reach terminal velocity.
  17. Feb 13, 2010 #16
    Yes. period squared is proportional to radius cubed. As for the rock, yes, that is possible.
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