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Astronomy/Getting a grip on gravity!


Is it correct to think of gravity as a filter/separator of sort allowing the heavier stuff to collect at the center of the earth and the lighter stuff at the surface during the accretion event?

Gravity just pulls everything downward, regardless of its mass or density (the mass per unit volume). But when the Earth was relatively young, it was probably either entirely liquid ("molten"), or mostly liquid. And in liquids, denser materials do tend to sink to the bottom. So on the average, the densest materials should sink to the core, and the least dense should rise to the surface. And to a great extent that is what happened to the Earth, as the core is the densest part of the Earth, the surface layers are the least dense, and those layers in between have in-between densities.

Of course part of the reason for the increase in density as you go down is the compression of the materials at the bottom by the weight of the materials above them. As a result, the materials in the core are even denser than they would be if they were at the surface of the Earth; but on the average, they are the densest common materials, such as iron.

I don't know if that makes gravity a "filter", as the separation of different densities has more to do with a comparison of buoyancy to density than anything else. In a liquid, a given volume of material has a buoyant force equal to the weight of the surrounding materials of similar volume. If it is denser than those materials, its weight will exceed its buoyancy and it will sink. If it is less dense, its buoyancy will exceed its weight and it will rise. So things are a little more complicated than the effects of gravity alone.


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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