1. The problem statement, all variables and given/known data http://fotozrzut.pl/zdjecia/ad3bbdd9f6.jpg [Broken] Mases are as stated on the picture, I is the moment of inertia of the pulley, angle marked is ω and a is a radius. 2. Relevant equations L=T-V 3. The attempt at a solution I try to formulate a lagrangian for this system, but I don't know what coordinates shall I use. How to relate the angle and position of the mass m?