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

I think the reason we all got caught up in picky minutiae is that the question seems like it can't possibly hold any interest absent the effects of minutiae.

Now, however, I think we have a fairly well-defined question. I'm not sure what a "massive run away planet" is, but assuming the intent is to distinguish between a position at the center of a cavity in a gravitating body in free space and that same position minus the gravitating body, I'll choose C; no difference. I suspect that isn't literally true general relativistically, but I'll trust Hasan's implication that he isn't interested in such quibbles.

John Mallinckrodt
Cal Poly Pomona

On Oct 4, 2010, at 10:31 AM, Fakhruddin, Hasan wrote:

Alright folks, Here is a little modification:

A solid elastic rubber ball is a perfect sphere of radius r in free space. A solid homogeneous spherical massive run away planet also in free space has a spherical cavity formed at its center. The cavity is much larger than the ball and has vacuum in it. The ball is now transported to the center of the planet. Its radius now is
(A) > r
(B) < r
(C) = r

(now come on....don't ask me to define free space)

How did you guys ever take any exam without giving your instructor ulcers?!

'Burp'....excuse me; I just took Alka-Seltzer!

~ Hasan Fakhruddin
Instructor of Physics
The Indiana Academy for Science, Mathematics, and Humanities
Ball State University
Muncie, IN 47306
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