CFD Online Discussion Forums - Weak Contribution

I am looking to implement a surface stress via a weak contribution in COMSOL using the "Two-Phase Flow Laminar Mesh" physics module. My model is a 2-D axis symmetric model.

The equation governing the stress on the surface is:

$P_{mag} = \frac{1}{2} \mu_{0}\left(\vec{M} \cdot \hat{n} \right )^2$

Where M is the magnetization of the liquid and mu is a universal constant.

The equivalent form using COMSOL variable notation is:
0.5*mu0_const*(down(mfnc.Mr)*nr + down(mfnc.Mz)*nz)^2

However, I realize that this is not sufficient. I believe the solution is to implement this using the weak form of the equation.

Can anyone guide me in the direction to implement this equation. What I have now is:

0.5*mu0_const*(down(mfnc.Mr)*nr + down(mfnc.Mz)*nz)^2*sign(down(mfnc.Mr)*nr + down(mfnc.Mz)*nz)*(test(u)*(tpfmm.nr-u) + test(w)*(tpfmm.nz - w))

The bold term there is an addition that I don't completely understand and ended up grabbing from another form. I am pretty sure it is wrong.

Thanks in advance for any help.