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## Homework Statement

Find the potential and force on a mass m outside and inside the earth in terms of g, the acceleration due to gravity, assuming earth has uniform density and radius R.

## Homework Equations

For a mass m, the potential energy of it in the gravitational field of a spherical shell of radius r' and uniform mass distribution m' is given by

γ=const. if m is inside the shell

γ=-Cm'/r if m is outside the shell, where r is the distance from the center of the sphere to the mass and C is a constant

It has the answers provided,

Outside:

F=mgR^2/r^2, γ=-mgR^2/r

Inside:

F=mgr/R, γ=mg/2R*(r^2-3R^2).

As a hint, to find the constants, when r=R, F=mg.

## The Attempt at a Solution

I managed to get the first part outside the earth, though whether or not the method I used is legitimate I don't know.

I integrated from 0 to R [itex]\int-Cm'/rdr'[/itex] to find the total potential β=-CMR/r, where M is the mass of the earth (integrating from 0 to R, the sum of the masses of the shells will total M). Taking the negative gradient to find the force, I find

**F**=-CMR/r^2

**e**. At r=R, F=mg, so C=mgR/M. β=-mgR^2/r and

_{r}**F**=-mgR^2/r^2

**e**

_{r}Getting the inside potential is a little more difficult. I know that it involves integrating from 0 to r [itex]\int-Cm'/rdr'[/itex] and maybe r to R [itex]\int Adr'[/itex]. I've tried two separate methods, where the second term was the integral as above and the second as just a constant in itself. I do know that at r=R, both inside/outside forces and inside/outside potentials should be equal, but neither attempt yielded anything remotely similar to the answer provided in the problem.

Since the method I used in part 1 didn't work for this, I believe it is at least partly a fluke.