Where's the Science in the Science Forum?

OK, person gave me a calculous proof that I didn't understand showing me wrong but I still think that a planet could have a hollow core.

First, the forces of gravity of every particle comprising the planet balance out at the center so there is zero force of gravity at the center.

Second, the presumed high presure at the center is caused by the force of gravity of the overlying layers. But #1 indicates that maximum pressure must be found in a sphere that is somewhere between the surface and the center. I suggest this sphere might be defined as (3/8)r.

Third. Thought experiment. Consider a planet sized hollow iron sphere with a wall thickness of 1% of the diameter. a force of Gravity would be detected at the outer surface and a very slightly less force of gravity would be detectable at the inner surface. Less because the effect of the opposite wall must be subtracted. What happens to these gravity readings if the wall thickness is increased?

 

Your conclusions about the thought experiment aren't valid. On the outer surface, the force of gravity would be exactly equal to all the mass of the iron sphere as if concentrated to a point in the center of the sphere but felt at that distance. The force of gravity on the inner surface of the sphere would be zero. This remarkable result was proved by Newton in that ANY hollow sphere will have zero gravity everywhere within the sphere. This tremendous range of variation would be reached, across the boundary from the surface to the inner surface.

The outer walls, experiencing an inward compression force due to gravity, will compress inwardly unless oppossed by a force sufficient to hold them in place. When the force is greater than the structural strength of the material, the inner hollow space will be collapsed. Solid rock is known to self-compress into spherical bodies at about half the mass of our moon.

 

OK, Then consider a "hollow" hemisphere. A hemispherical shell. Would not all surfaces exhibit gravitational attraction with respect to another object?

 

Hmm. It would experience gravitational forces that would draw it into a sphere. It might be strong enough to keep its shape if it is small enough so that the gravitation is weak.