Magnetic ball falling inside conducting tube

[masterjoda]

Homework Statement

I have a magnetic ball with magnetization M, mass m and radius R that starts to fall from rest through conducting tube of radius a little big larger than R, thickness $\Delta$ and conductivity $\sigma$. The ball is not rotating, it said that magnetization is oriented vertically down.

Homework Equations

Professor send me this problem, usually I know how to solve the problems that he sends me, but his one I don't know even how to start.

 

[CWatters]

I know it falls slowly and why but I've never looked at the maths. Google found a paper on the subject ...

http://arxiv.org/pdf/physics/0603270.pdf

 

[masterjoda]

They here use approximation with two monopoles but in my case I have a sphere and I think I sound me approximation with a one dipole but I don't know how to write flux then?