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

["Dr. Richard Tarara" <rtarara@saintmarys.edu>]

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.

Rick

***************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
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----- Original Message ----- 
From: "brian whatcott" <betwys1@sbcglobal.net>
To: <phys-l@carnot.physics.buffalo.edu>
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?
> >
> > Thanks
> >
> > ~ 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
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