The Polar Coordinate

I am trying to model the behaviur of a telescopic actuator today. I found out that I can do it with a 2D model and only 3 degrees of freedom. It will be a 2D mesh with rectangular lagrangian elemtns. But, the problem right now is the weak form that comes from equilibrium equation. I had trouble working with polar coordinate. I am going to learn how to work with radial coordinate that comes into almost every equilibrium equation.
The Constitutive model wont be hard and I am sure that I can use the same thing as Cartesian for this. The main problem will be the equilibrium equation.