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

["Fakhruddin, Hasan" <hfakhrud@bsu.edu>]

Okay, I was wrong.  My wrong choice of C is what now I stick to.  Yes, there is no g field in the cavity.  I did not extend my ideas of Gauss Law for EM filed to gravity.  Thanks, John, I stand corrected.

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


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Monday, October 04, 2010 2:21 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Clarification: A ball at the center of a planet

Hasan Fakhruddin wrote:

> I believe the radius will decrease due to the planets pull on the  
> particles of the ball, directed toward the center of the planet.
> So Choice C.

I assume you mean "B," but the planet doesn't exert any force  
whatsoever on the ball (at least under Newtonian gravity) so the  
reasoning here is clearly wrong.

John Mallinckrodt
Cal Poly Pomona

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