萬有引力 r=0

Simply speaking, gavitational force arises due to presence of mass, r = 0 means there is no mass at all. Hence there is no gravitational force.

2013-02-06 20:10:03 補充：
In your teacher's example, the mass m is placed inside the earth and is enclosed by a spherical shell on the outside, and a sphere of radius r inside.
The forces due to the outside spherical shell balance away. There is only gravitational force from the sphere where the mass m is on its surface.

2013-02-06 20:12:34 補充：
But when r = 0, that means the sphere shrinks to zero. Obviously, there is no gravitational force.

2013-02-06 20:18:52 補充：
You could refer to the following web-page for the mathematical derviation of the relevant formula:

http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/earthole.html