Hi, I am trying to understand a theoretical problem involving the contact between two surfaces. I have uploaded a screen shot of the mathematical formulations of the solution. I understand most of the solution, except the principal curvatures. I have tried to look up principal curvature, but still not sure how is applied to this problem. I would really appreciate any help for the mathematicians out there. https://dl.dropbox.com/u/47274064/Prin_curve_1.JPG [Broken] Thanks.