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A have a question

  1. Apr 21, 2008 #1
    this is my first time here so try not to make feel like an idiot.

    Someone told me that you can "cheat" or "delay" gravity by spinning on your own gravity. Kind of like when your doing a flip(which would explain why people don't fall on their heads)? I wondered if this was true, because I heard that the earth doesn't hurtle towards the sun because it spins at a certain speed.
     
  2. jcsd
  3. Apr 21, 2008 #2

    Danger

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    Welcome to PF, Peaceful.
    (Nice username, by the way; good counterpoint to mine. :biggrin:)
    Nobody here will ever make you feel like an idiot. If they did, I would have been gone years ago. We're here to help. My input is more as a 'greeter', since I'm not really qualified to answer most scientific questions.
    I can tell you, though, that 'cheating' or 'delaying' gravity is not possible. In your example of a flip, that's just the person's muscular power working against a constant gravitational field. If the muscles are stronger than the gravity, then one can leave the ground. The higher the ratio, the higher one can leap and thus the more time s/he can remain in the air to accomplish some manoeuvre.
    As for the Earth/Sun relationship, it's just basic orbital mechanics. The centrifugal force of the planet's motion exactly balances the centripetal force of gravity trying to haul it down. I'm afraid that I can't give you a better answer, but others who can will be along shortly.
     
  4. Apr 21, 2008 #3
    Newton explained an orbiting body with an analogy to a cannonball fired horizontally at exactly the right speed so that it falls to the ground the same amount that the curvature of the world bends away from it --it's always falling and yet always staying at the same height.

    Newton's example, cannonball and earth, are analogous in the present question to earth and sun.

    An object that revolves around something else often rotates around its own axis as well, but rotating isn't part of the reason that it revolves.
     
    Last edited: Apr 21, 2008
  5. Apr 21, 2008 #4

    Doc Al

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    No, it's not true. And the reason why the Earth doesn't fall into the Sun is not because it's spinning but because it's moving sideways at just the right speed.
     
  6. Apr 21, 2008 #5
    I think we're affected by some optical illusions. The way Olympic pole vaulters wiggle around, it seems that they do something in midair to get themselves a little extra push over the bar. That's only an illusion. The center of gravity of the pole vaulter's body always moves along a parabola. It may not seem like it because of all that wiggling makes it difficult for us to see where that center of gravity is at any gven moment.
     
  7. Apr 21, 2008 #6

    Danger

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    While I'm not sure about this, it always seems to me that the 'wiggling' is to impart extra tension to the pole, and thus gain more 'thrust' when it unwinds. There is also the matter of arcing one's body so as to give maximum clearance of the bar.
     
  8. Apr 21, 2008 #7

    DaveC426913

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    That's the wiggling he's talking about.
     
  9. Apr 22, 2008 #8

    Danger

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    Ahhh... okay. I thought that the last-second downward push against the pole might be part of what he meant.
     
  10. Apr 22, 2008 #9
    I was ambiguous - I meant after the person is a projectile, no longer using the pole.

    The high jump competition also.

    There was a time when I also thought that you could "cheat" gravity by moving in a particular way.

    The martial arts movies add to the misconception ... the person jumps into the air and does a lot of somersaults to go higher... to jump all way up to the roof of a tall building, that require a lot of rotations.
     
    Last edited: Apr 22, 2008
  11. Apr 22, 2008 #10

    Danger

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    And a lot of wires.
     
  12. Apr 22, 2008 #11
    Steroid rotation power ftw?

    No, why, guys, if you have some kind of a propeller attached, you can cheat gravity by spinning. Try it off your roof, like I did 3 years ago. It works!
     
  13. Apr 22, 2008 #12
    And a lot of patience to erase those wires from all the frames of the actual movie.
     
  14. Apr 22, 2008 #13

    DaveC426913

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    Well, you sort of can, though technically it isn't cheating.

    It is theoretically possible to have your centre of gravity actually pass below the bar on its parabolic trajectory while ensuring that each part of your body actually passes above the bar.
     
  15. Apr 22, 2008 #14

    russ_watters

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    True, and I didn't believe it until I thought about it for a minute. Pole vaulters go over with their stomach facing down and their torso bent, which could put the CoG below their belly button, under the bar, while their hands and feet are also below the bar, on opposite sides of it.
     
  16. Apr 22, 2008 #15
    Gravity is a "fact" of the natural world, it is an integral phenomenon originating from the nature of matter and spacetime.

    You can not cheat gravity anymore than you could cheat electromagnetism or the strong or weak nuclear forces. I think people have a gross misconception of what it means that gravity exist, it is so integral to everything it is impossible to form any concept of existance that is anything remotely based on our own universe without it.

    Peaceful, try not to think of gravity as a thing, or process, or phenomenon. The act of gravitation simply IS, as a consequence of the nature of our universe.

    However ya there are many ways to in a matter of speaking 'cheat' gravity, but not in actuality.

    And welcome to forum bye the way! Good first post! :cool:
     
  17. Apr 22, 2008 #16

    DaveC426913

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    To be fair, no one's using the word rigorously.

    Gravity, all other things being equal, causes everything to follow a elliptical path (on Earth it appears parabolic). This path is inexorable. If the apex of your trajectory is 12 feet, and the bar is 12 foot 1 inch, you're going to go under the bar, no two ways about it.

    But there are ways of thwarting this universal and inexorable Law of Gravity. Fiddling with your centre of mass is one way. Of course, having wings and a propellor is another.
     
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