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Question about the equivalence principle

  1. May 29, 2005 #1
    - Albert Einstein, Relativity: The Special and General Theory, Section 23

    I'm confused about how gravity could account for all of the observations of the observer on the rotating disc. If the observer dropped a ball, he would see it fall away from the disc initially, as expected if there was a gravitational force pulling in all directions around a disc at rest, however, wouldn't the ball also accelerate in the direction opposite of the disc's spin (as judged from the observer who calls the force centrifugal) and begin circling the disc from the perspective of the observer who calls the disc "at rest"? If the gravitational field just "increases proportionally to the distance from the centre as we proceed outwards" why would the ball begin to circle around the disc?

    Also, is it possible to describe the motion of the sun, planets, and fixed stars as due to a gravitational field that "increases proportionally to the distance from the centre" of the Earth?
    Last edited: May 29, 2005
  2. jcsd
  3. May 29, 2005 #2


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    Sure, there would be coriolis and also tidal forces. But Einstein here is speaking only about local effects (he doesn't make that clear, but he is - his applications of th EP are all local). In the limiting case of small neighborhoods, both coriolis and tidal forces go to zero.
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