Helping find moments in mass-spring-ball system

[syndrome]

I have this mass-spring-ball system:

https://dl.dropbox.com/u/1723401/massa-molla/sistema.png

There is friction, but the ball only rolls.
I have to find the model of the system.

I started to divide the 2 systems and for mass1 I found the follow equation:

m_1 \cdot \ddot{x_1} = F_k - F_1 - F_{a1} = k \cdot (x_2 - x_1) - F_1 - F_{a1}

Then I find the equation for the ball, but I have some problem! I don't know how to consider that F2 in the sum of the moments!
The sum of the moments will be r * F2, or F2 must be perpendicular to R?

What I've done:

m_2 \cdot \ddot{x_2} = F_2 - F_k - F_{A2} = F_2 +k \cdot (x_2 - x_1) - F_{a2}
J \cdot \ddot{ \alpha } = F_{a2} \cdot r + M_{FP}

But now I don't know how to find Mfp..

any help?

 

[syndrome]

Up.. please