supose a spherical distribution of charge like this:
ρ=α for r ≤ R/2
ρ=2α(1-r/R) for R/2 ≤ r ≤ R
ρ=0 for r ≥ R
where ρ=volumetric density of charge. I need to get α, and i have to demostrate that the movement of an electron oscilating around the sphere center is an simple armonic movement.
just assume that it is at a position x from the centre now calculate the force by integration and you would be able to get the acceleration is directly proportional to the displacement and you can prove it ( i will try to solve and mail it to you)