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this post was submitted on 13 Aug 2023
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If you had a planet that was hollow in the center*, the entire hollow region would have zero gravity. You could have a thin-skinned planet with the entire interior an empty weightless void. I doubt any planets like this actually exist.
* Assuming radial symmetry. If you can represent the planet as concentric spherical shells then you're good.
I thought this was wrong, but it is true:
Brb fixing my other comments.
Yeah it's a pretty counter intuitive result. I'd expect a greater pull of gravity towards the nearer side, but it turns out to be exactly cancelled out by the greater mass on the further side.
E: oops, looking at your edited comment, I should stress this is only for hollow bodies. Your comment pre-edit was correct for non-hollow bodies. If you're part way to the middle of a planet, you can think of the planet as two sections, a small sphere for the part that's below you, and a larger hollow shell for the rest. You experience no gravity from the outer shell, so only feel gravity of the smaller mass below. 10m from the earth's center, you feel equivalent gravity to if you were on a 10m radius iron sphere.
A center of gravity is a single point and couldn't be expanded to fill a planet's interior, even if the space was only 1m^3
You're right but that was not the point. The comment just explained that at any point inside a hollow sphere gravity forces cancel out so that effectively there is no gravity.