I am told that parabolic coordinates in a plane are defined by [tex]\xi[/tex] = r + x and [tex]\eta[/tex] = r - x. after this i am then asked to show that this leads to a given expresion for the kinetic energy (if i knew x and y i could find this without a problem). From this I am then told to find the equations of motion, an expression which i find very vague. I am in fact familiar with the Lagrangian and Hamiltonian formalisms (loosely) but I do not know what my expression for potential energy will be in this system (if i am on the right track here). Anyway, the main problem I am having at the moment is that I am very unsure as to what "r" is. Any help would be greatly appreciated.