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

I suspect that the intent of the original question is much more simple than we are making it, but the original condition should have been that the ball is a perfect sphere in the vacuum of deep outer space. Then taking it to the center of a planet should increase the radius--at least in theory (sort of like the earth moves up to meet the dropped ball somewhere in between. ;-) As a question for students, this is not so easy as many will assume the zero net gravitational force at the center of the planet is equivalent to no force in outer space. Others (at least my students) get totally confused between pushes and pulls and might well assume the planet is pushing from all sides.

If I am mistaken, and the intent is really to include the deformation forces at the surface of the planet, then previous posts have covered the fact that there is insufficient information here.


Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
Free Physics Software
A new series of updated software will be appearing as completed--providing higher resolutions. A new airtrack simulation is available now.
----- Original Message ----- From: "brian whatcott" <>
To: <>
Sent: Monday, October 04, 2010 12:48 PM
Subject: Re: [Phys-l] A ball at the center of a planet

On 10/4/2010 9:43 AM, Fakhruddin, Hasan wrote:
Greetings folks!

Here is a question for your intellectual entertainment:

A solid rubber ball has a radius of r in vacuum at the surface of a planet that is a solid uniform sphere. The ball is now placed in vacuum at the center of the planet. Will the radius of the rubber ball
(a) Increase
(B) Decrease
(C) Stay the same?


~ Hasan Fakhruddin
Instructor of Physics
I believe I can respond to a question that is the physics counterpart of
angels dancing on pinheads

If a highly elastic ball forms a perfect sphere at the surface of some
planet with appreciable gravity,it would be asymmetrical when
transported to some location where the gravity gradient is reduced,
taking a slightly oblate form.

Brian W
Forum for Physics Educators