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Centrifugal gravity

  1. Dec 7, 2006 #1
    Centrifugal "gravity"

    Imagine a giant circular ring, which spins to generate a type of "gravity" (think of the video game "halo" even)

    My problem with this is:

    1. A man is walking along this spinning ring. He feels gravitational effects no doubt.

    2. The man takes a ball, and tosses it up high into the air.

    3. The wind blows on the ball, forcing it backwards. So the ball no longer has the forward motion.

    The problem with this "gravity" is I don't see how the ball is going to "come down" anymore. It is no longer in contact with anything that is spinning. The gravity only works when you are actually in contact with the spinning object thru some means.

    Am I wrong?

  2. jcsd
  3. Dec 7, 2006 #2
    yes. you need no contact. the "gravity" is just an artifact of a rotating frame of reference, there is no force acting. the ball will move in a straight line as the ground moves in a circular arc toward the ball so it will appear that the ball is falling

    as the ball's distance from the centre of the rotation changes though, other effects come into play
    Last edited: Dec 7, 2006
  4. Dec 7, 2006 #3
    But if the ground is a circular arc, it will never "come to the ball" , because it is always still underneath it...
  5. Dec 7, 2006 #4
    Here is a drawing of what I'm talking about..

    Attached Files:

    Last edited: Dec 7, 2006
  6. Dec 7, 2006 #5


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    I think what you're trying to say is that it's possible to put a ball into a trajectory parallel to the motion of the spacecraft's center of mass. The ball will move freely through space, while the spaceship moves "around it," never touching it.

    You are correct that the ball, in such a trajectory, would never come "down" and hit the floor.

    - Warren
  7. Dec 7, 2006 #6
    the second pic is right if there's no air in the spaceship. you've effectively put the ball into orbit. the same as if you threw up a ball on the moon and applied a strong enough sideways force
    Last edited: Dec 8, 2006
  8. Dec 7, 2006 #7
    I have thought about how cool it would be to be in a ring shaped space ship in which you normally stand on the walls normally, and take a fast vehicle and drive along it until you match the linear speed of the ship's rotation. Then you can get out of the car and feel like you're actually flying and you could be watching the ground move by at incredible speed, and so forth O.O! Of course, if this weren't in a vacuum, you would face some...problems.

    If the radius of the ship was 100 meters, then you would only have to travel at 70 miles per hour (31 meters per second). This is if you wished to have the "artificial gravity" be similar to earth's.
  9. Dec 8, 2006 #8
    However in my analysis, objects would only fall back "down" if you either threw them upwards, or forwards (in the same direction as the ring's spin). If any object moves backwards it would never come down. Well, it might eventually, if it's backward velocity was not enough to counteract the foward spin velocity. However, it would still give rise to a very imbalanced gravitation. Jump forward and you come down quick, jump backward and you stay up a long time.

    Hm So I guess the size and speed of the ring have to be carefully chosen, because if it's too slow, it will be too easy to overcome the spin velocity, and get into orbit...

    *ugh I'm so confused :P*

    Last edited: Dec 8, 2006
  10. Dec 8, 2006 #9
    it is an imperfect simulation of gravity, definitely. you should look into the coriolis "force" for why
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