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

But what if the ball on the surface of the earth has neutral buoyancy and is
suspended in water? Then deformability should have no effect. But he did
say in a vacuum. But while it is falling in the vacuum, not touching
anything there should be negligible deformation, assuming of course that
there is negligible tidal force on the ball because it is small.

Actually this question is sort of like some of the questions which are asked
in the Minds on Physics text. It is designed to get students to think about
various conditions. So MOP asks students to compare the gravitational force
on identical size and mass blocks under various conditions such as hanging
from a string, falling, on a table which is obviously deformed, in a
vacuum... Many students will tell you that the gravitational force
increases as you get to the center of the Earth, without thinking about the
fact that it has to switch direction, so it has to go to zero some where.

John M. Clement
Houston, TX