Gravitation: Which formula exactly?

[StingerManB]

I am not looking so much for answers to problems, but I was sick the day of this lecture and need some clarification. I think I understand what to do, but some support would be great.

Homework Statement

What is the gravitational force acting on a sphere on particle 'm' located at distance 'r' from center of sphere, assuming it is a uniform sphere of matter that has mass M, radius a, and a concentric cavity of radius a/3.
Solve for r= a/6, r= 2a/3, r= 3a/2


I believe all I need is:

F= G(m1m2/r^2)

I have not attempted to solve, I would like to know it if this is the correct equation to use.
Thanks.

 

[StingerManB]

Anyone? Anything at all?

 

[ruggler]

yes, that's right. I don't understand why they gave you a/3 and radius of the sphere though, so maybe I'm missing something.

 

[fluidistic]

Maybe you have to calculate the center of mass of the "sphere with a cavity" which shouldn't be the center of the sphere. Once you've done that, you will have that the distance from the particle and the center of mass is not the same than the distance from the particle to the center of the sphere. Unless I'm misunderstanding what a cavity is.
EDIT : And about "F= G(m1m2/r^2)", yes it is the good equation to use here. Note that r is the distance from the particle to the center of mass of the sphere.