# Homework Help: Quick gravitation problem?

1. Dec 16, 2012

### Ishida52134

1. The problem statement, all variables and given/known data
A spherical shell has inner radius R1, outer radius R2, and mass M, distributed uniformly
throughout the shell. The magnitude of the gravitational force exerted on the shell by a point
particle of mass m located a distance d from the center, outside the inner radius and inside the

2. Relevant equations
F = -GMm/r^2

3. The attempt at a solution
I know that from gauss's law that the force from a spherical shell would have the same gravitational force as one from a point particle. How would you find the gravitational force of a particle located between two shells though?

2. Dec 16, 2012

### Ishida52134

any ideas

3. Dec 16, 2012

### haruspex

That's only true for points outside the shell. What is the force on a particle entirely inside the shell?

4. Dec 16, 2012

### Ishida52134

0. Oh do you just find the force and use density = m/v to substitute m with pv.
Then v = 4/3 pi r^3. Then u just subtract the total volume of the sphere by the volume of the sphere with distance d < the particle.

5. Dec 16, 2012

### haruspex

I'm not completely sure what you are saying there, but it doesn't sound quite right.
Think of the given shell annulus as made up of two shell annuli: one with outer radius d and one with inner radius d.