What's on the other side?

[gismo]

Huh? You are correct that it doesn't matter what shape the planet is - but it doesn't make any difference if the planet is solid or hollow, either. Inside a hollow planet the object will still feel a net gravitational force pulling it to the planet's, ahem, center of gravity. The CG, being the point around which the mass is equally distributed, is also the point around which gravitational attraction is equally distributed. The sum of the accelleration vectors from all directions will be zero at the CG, but non-zero anywhere else.

Regards,
Joe

I think Kath meant anywhere inside a hollow planet, not just at the center of the planet's mass. I haven't figured that one out yet as my mind wants to relate this to the zero electrostatic force inside a conducting sphere but there are some big differences in that analogy.

 

[kath]

I think Kath meant anywhere inside a hollow planet, not just at the center of the planet's mass. I haven't figured that one out yet as my mind wants to relate this to the zero electrostatic force inside a conducting sphere but there are some big differences in that analogy.

Ding ding, Lance!

Suppose you're inside a hollow spherical planet. The CG is at the center, yes, but you feel no net force even if you're near one of the walls.

If you can figure this problem out for the electrostatic force (inside a conductor), then it's the same problem. Both kinds of forces (gravity = GMm/r^2 and electrostatic = kQq/r^2) depend on 1/r^2 which is the key. (The planet does have to have uniform mass density in order for this to work, BTW)

--Kath