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Re: [Phys-l] A ball at the center of a planet

1st response:That's why I said "pointing to the center of the ball AND earth". The self gravitation of the ball is the same in both cases. But the gravitational field inside the earth grows as a linear function of r and points inward. That adds to the compression of the ball because not all parts of the ball are at r=0.

2nd response (after brain was fed coffee): The very center of this hypothetical planet is hollow - so no linear gravitational field.

So I now agree with you John.

Bob at PC

and earth
-----Original Message-----
From: [mailto:phys-l-] On Behalf Of John Mallinckrodt
Sent: Monday, October 04, 2010 2:10 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] A ball at the center of a planet

Bob LaMontagne wrote:

If the ball is a perfect sphere of finite size when at the center
of the earth, then the center of the ball has zero net force acting
on it - so it does nothing. A point on the surface of the ball has
a weak but finite force on it pointing to the center of the ball
and earth. Therefore, shouldn't a deformable ball therefore
decrease in radius relative to deep space?

That same weak force is there in deep space (it is, after all, due to
the mass of the ball itself), so why would there be any difference?

John Mallinckrodt
Cal Poly Pomona

Forum for Physics Educators