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Please consider the differences in Newtonian physics and general relativity.

Newtonian physics

In Newtonian physics it makes sense that objects placed on a planets surface facing away from the planets star weight less than objects placed on a planets surface facing to the star due to a "centrifugal force"/ the objects inertia.

General relativity

Considering (my understanding of) general relativity, a planet follows a straight line and the space itself is bend into an orbit. As there is no change in velocity of objects on the surface of the planet there shouldn't be a centrifugal force. Objects on both sides of the planets should have the same weight.

Is there a way to understand this difference and why is there a centrifugal force?

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    $\begingroup$ Only the center of mass of the planet moves on a geodesic line, objects on its surface do not and they do experience tidal forces. The same is true for the ISS. True microgravity experiments can only be performed at the center of mass of the station, which, of course, keeps shifting all the time as people are moving around, air circulates trough the modules, thermal expansion changes from orbit to orbit. The ISS is therefor actually a pretty poor microgravity lab. $\endgroup$ – CuriousOne Dec 22 '14 at 22:39
  • $\begingroup$ @CuriousOne : Why don't you post you comment as an answer? $\endgroup$ – Sofia Dec 23 '14 at 10:50
  • $\begingroup$ @Sofia: Five votes? Wow... and I thought that was too trivial for an answer. OK, will do. $\endgroup$ – CuriousOne Dec 23 '14 at 10:52
  • $\begingroup$ @CuriousOne : 6 votes. Post your answer, what you wait for? $\endgroup$ – Sofia Dec 23 '14 at 10:55
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Only the center of mass of the planet moves on a geodesic line, objects on its surface do not and they do experience tidal forces. The same is true for the ISS. True microgravity experiments can only be performed at the center of mass of the station, which, of course, keeps shifting all the time as people are moving around, air circulates trough the modules, thermal expansion changes from orbit to orbit. The ISS is therefor actually a pretty poor microgravity lab. The Wikipedia article http://en.wikipedia.org/wiki/Micro-g_environment claims that the tidal forces in low Earth orbit amount to $0.33\mu g/m$.

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  • $\begingroup$ Thank you for your answer. I don't really understand the connection between a centrifugal force caused by rotation and the tidal force you mention though. $\endgroup$ – Luz Dec 23 '14 at 19:57
  • $\begingroup$ I was mainly responding to the second part of your question. How do we talk about the forces acting on extended objects in general relativity? The terminology in GR is tidal forces and they result from objects not moving on geodesic lines (which are basically the lines that free falling test particles would tale), whereas in the Newtonian picture we were summing what we thought to be real forces from gravitating bodies and pseudo-forces from non-inertial movement. In GR one can look at all of these as deviations of motion from the preferred (free falling) motion. $\endgroup$ – CuriousOne Dec 23 '14 at 21:37
  • $\begingroup$ I get the idea but I still have to learn more to fully understand it :) thank you! $\endgroup$ – Luz Dec 24 '14 at 22:26
  • $\begingroup$ You are welcome. Happy Holidays! $\endgroup$ – CuriousOne Dec 24 '14 at 22:29

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