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

We'd also need to know if the ball has homogeneous elastic properties and, most important of all, perhaps, is the ball subject to any forces other than gravity at either location?

John Mallinckrodt
Cal Poly Pomona

On Oct 4, 2010, at 9:26 AM, LaMontagne, Bob wrote:

I think we need the problem to be defined a little better. Does solid include "deformable". If so, how is the radius measured, i.e., along a radius from the center of the earth, tangent to it, an average, etc....?

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Monday, October 04, 2010 11:05 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] A ball at the center of a planet

Ummmm.... trick question? Because of tidal forces, the ball is not
truly a sphere at the surface of the planet. Or if the ball is a
sphere at the surface, then the lack of tidal forces at the center
mean it won't be a sphere there. New to this listserv, so I figured my
first post ought to be something that's probably wrong. ;o)

Bill

William C. Robertson
Bill Robertson Science, Inc.
Stop Faking It! Finally Understanding Science So You Can Teach It.
Woodland Park, CO 80863

On Oct 4, 2010, at 8:43 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?

Thanks

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

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