In both http://en.wikipedia.org/wiki/Dynamical_billiards" [Broken] we have a potential who's value is infinity. Now if it were just a finite number (rather than infinity), I would be getting a dirac delta function in the equations of motion (I think..) but when it's infinity I can't really do anything with it. So my questions are, How do you derive the equations of motion using this potential? What is the meaning of this infinity written here? Is it just a 'metaphor' or is there some formalism which gives it meaning? Thank you!