the question is : particles m1 and m2 each of mass m are connected by a massless rod with length l , these particles move on a frictionless horizontal plane as shown in the screen shot , the movement of m1 is fixed on a frictionless circular track of radius R , find the E.O.M ! 2. Relevant equations here we can find T and V and use the lagrangian easily 3. The attempt at a solution i found the kinetic energy and found the potential energy but for some reason my doctor said that the potential energy is zero for this system can any body tell me why , please i need help :( also i solved for theta and phy but the doctor said that this system can be solved for theta only considering it the only degree of freedom ! why is that we have 2 degrees of freedom theta and phy so why we consider theta the only degree of freedom !