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Gravity and Centrifical Force

  1. Oct 27, 2004 #1
    I'm not sure where to put this post, so I will put this post in this. Let's imagine for just a bit that anti-gravity was possible, and we built a machine do it. That is, the machine -somehow- made a piece of mass totally weighless, there is nothing teathering it to Earth. But, since it has a great bit of centrifical force, it should sling out of the Earth at an alarming rate. It's like the ball on the string, you swing the ball, then you cut the string, the ball flies off. So, how would someone who's wanting to make ant-gravity get passed this?
  2. jcsd
  3. Oct 28, 2004 #2


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    Clamp the object to a table or something. Of course, bouyancy would also be a problem, otherwise you could just make things weightless at the North or South poles, where cetrifugal force is not an issue.
  4. Oct 29, 2004 #3
    Long time no-see!

    Hi again Lurch,
    Isn't it possible that you are speaking of the Coriolis cross-product phenom?
    I can't imagine standing at the North Pole and firing a bullet horizontally or above and not having it fall back to the earth. Cheers, Jim
  5. Oct 31, 2004 #4
    Whoa! never thought of that. Amybe if you limited how strong this Anti-Grav field was made.
    Of course there is still that issue of creating the machiene first.
  6. Oct 31, 2004 #5

    Doc Al

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    Keep it in your pocket! :smile:
  7. Nov 3, 2004 #6


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    Right, but that's gravity bringing the bullet down. If the bullet were exempt from the influence of gravity, it would not come back down. But if it were set on a table-top near the equator, it would be flung into space by centrifugal force. Or you coul;d say that the bullet (or other object) keeps going in a strait line while you and the room you're in follow the curvature of the Earth and drop out from under the oject. At the pole, the ground would not "drop out" from under the object, but it would still be displacing its own volume of atmosphere. Of course, that much air doesn't weigh much, so the object won't require great effort to restrain.
  8. Nov 19, 2004 #7


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    Was just curious as to how fast it would rise once freed from gravity.

    My calcs show that it would rise about 400 feet in the first minute. That's roughly 5 miles per hour. It would accelerate gently as it rose, and a gentle Westward veering to its otherwise vertical rise would become apparent.

    My mental trig fails me here, so I'm ballparking. It would finally disappear over the Western horizon after around 8 hours, at an altitude of around 500 miles.
  9. Nov 19, 2004 #8


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    If it has no inertial mass, how would you impart any centripetal force to it? And not being subject to gravity, the earth would simply run off and leave it as if it were empty space.
  10. Nov 19, 2004 #9


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    Ignoring, for the time being, the problems with the violations of the equivalence principal....

    You should calculate the magnitude of the centripetal acceleration due to the earth's rotation sometime. The formula is r*w^2. Using google calculator, one can approximately calculate the result

    like this

    (Note that 24 hours isn't quite the correct rotational period, one really wants a sideral day rather than a solar day)

    to find that the body accelerates upwards at .03 M/s^2, which is 3/1000 of the earth gravity.

    If left to itself

    In 1 second, the body would move 1.5 mm
    in 10 seconds, the body would move .15 meter
    in 100 seconds, the body would move 15 meters

    However, it would be easy to hold the body down as the acceleration is very small.

    Note that this sort of "device" occured fairly often in early SF. One particular place it occured was in "The Skylark of Space" - a VERY old and VERY dated story by EE Smith. The author more or less correctly described the results in this particular case - not a problem if the device is on for a short period of time, it could be a major problem if the device won't turn off and you're in nside it for an hour before you're able to shut it off. (Note that said author didn't have a great track record with regards to physical accuracy, except compared to other SF authors of the same time period).
  11. Nov 29, 2004 #10
    wait hold on, isn't that like saying if you could float, you would be flown out into space? I don't think you would encounter those kinds of problems because in the ball and stick example you showed, the ball has mass. Even if one is weightless, I wouldn't think the acceleration of the body would be great, but barely moderate. Now my head's spinning
  12. Nov 29, 2004 #11
    Well, I'm not sure what speed the Earth rotates, so...I'll estimate 1,000 MPH. So, if you were standing there, the ball would at the vertex of the ends of Earth in four hours.

    OOO |
    OO OO |
    OO OO |
    OO OO |
    OOO |
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