Hello guys. This is not really a homework exercise, but I'm currently preparing for an exam and this is a question I got from a textbook. I'm currently sort of stuck, but here are the details. 1. The problem statement, all variables and given/known data Link to a scan of the problem: http://img443.imageshack.us/img443/4593/question1y.png [Broken] 2. Relevant equations The problem is supposed to be solved by using the Lagrangian Method. (Since it's from a "theoretical-mechanics"-book) 3. The attempt at a solution Okay. What I thought was that i have to choose a suitable accelerating coordinate system and relate this coordinate system with an intertial frame of reference. First one sees in the picture that: l * q(t) = s(t) * l_1(t) So if one finds an equation of motion for l_1(t) one is done. Now the problem is I'm not sure which moving coordinate system to choose. I could choose one with origin at B, which would mean that there's gravity but only the spring-force acting on the mass m. However I'm not quite sure how to tackle this problem in general and I'm a bit confused. I'd be more than happy if you could help me!! Thanks in advance!