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So i have this question that is bothering me for a while. Suppose you are inside a hollow planet with no contact with the exterior. How do one show that the planet is rotating? I've been thinking along the line of Coriolis force.

Any suggestions?

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  • $\begingroup$ You obviously mean you rotate along with the planet, otherwise it’s obvious! Then, yes, Coriolis. Or Sagnac effect. The problem is usually stated in terms of a rotating box. $\endgroup$ – user154997 Oct 8 '17 at 13:34
  • $\begingroup$ @LucJ.Bourhis Yes. I rotate with the planet. Are you referring to the foucault pendulum when talking about coriolis effect? $\endgroup$ – ross999 Oct 8 '17 at 13:38
  • $\begingroup$ Foucault, yes, that’s the easiest, but you could also fire big guns! $\endgroup$ – user154997 Oct 8 '17 at 13:55
  • $\begingroup$ Or drop things down mineshafts, though this is a harder experiment than it sounds. (Actually so are Foucault pendulums and the gunnery approach, the Earth simply rotates slowly enough that it's hard to detect at human scales.) $\endgroup$ – dmckee Oct 8 '17 at 15:42
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Assuming you live on the internal surface of the planet, you would feel "gravity" due to the centrifugal force of rotation pushing you against the surface. This "gravity" would be the strongest at the equator and zero at the poles. There also would be secondary effects as mentioned in the comments. Without rotation you would be suspended weightless regarding of the mass of the planet, because the gravity inside a sphere is canceled due to the inverse square law.

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  • $\begingroup$ +1 for sure and it's the OP's question, but.. Suppose you are inside a hollow planet with no contact with the exterior OK, I'm a thin Earther.....in other words, we believe that because we have no knowledge of the outside world, your rotation claim is wrong. Instead God simply made our world thinner in some places than others, (easier for sinners to get to hell). Now what? $\endgroup$ – user167453 Oct 8 '17 at 18:04
  • $\begingroup$ Careful mentioning God and sinners going to hell on this forum ;) Could you clarify what you mean by "thin" and "thinner"? I can't grasp what you're asking. If you mean the planet is thicker at the equator and thinner at the poles and for this reason attracts you stronger at the equator, then no, it still wouldn't work, because inside a sphere or a circle you are equally attracted to the side you are staying on and to the opposite side no matter how far it is (even light years). Like no field inside a charged sphere, no field inside a hollow planet, because of the same inverse square law. $\endgroup$ – safesphere Oct 8 '17 at 18:30
  • $\begingroup$ I kinda meant if it was unsymmetrical and random in mass distribution, lumpy on the outside, large mountains etc, then I might argue that I was not being pulled against the thinner parts as much as the thicker ones. That's not really clear, apologies, and I need to think about it before making silly comment's. Best I can do is ask my own question, as I wonder also how a pendulum would work on the inside, with totally symmetrical mass distribution assumed. Regards $\endgroup$ – user167453 Oct 8 '17 at 19:10

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