Maybe you sci.astro.amateur and sci.astro readers can explain this

Common Man wrote:
For Pearl: Evidence that a rapidly spinning object would flatten out,
not become hollow.

It depends on the object, obviously.

If the Earth's crust were infinitely rigid, and its insides were made
of some compressible substance, then you could spin the Earth fast enough
to make it hollow out on the inside. It would not, however, be hollow
like a spherical shell. It would be more like a donut, encased within
the crust.

However, the Earth's crust is not infinitely rigid, so if one is going to
contend that centrifugal force pushes the insides out toward the periphery,
one is also going to have to explain why the Earth's crust isn't also
pushed out to infinity.

In particular, if the insides are being forced out against the crust,
it's clear that what's holding the insides in place is not the force of
gravity, but the integrity of the crust. That means that the linear
speed of rotation must exceed escape velocity. In that case, why doesn't
everything in and above the crust fly out into space? The fact that it
doesn't means that the insides aren't being flung against the crust.

But I'm afraid it's all academic. Maybe you don't have enough experience
with cranks, Common Man, but over here, it's de rigeur for them to ignore
any argument or observational evidence, no matter how sound, that goes
against what they desperately wish to believe.

Brian Tung
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