Why does this system have zero potential energy and only one degree of freedom?

[sayf alawneh]

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 !

Homework Equations

here we can find T and V and use the lagrangian easily

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 anybody 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 ![/B]

 

[sayf alawneh]

oops sorry i miss readed the part which is saying that its moving on the horizontal plane XD
but i havnt figured out why theta is the only DOF

 

[TSny]

There are definitely two degrees of freedom, $\theta$ and $\phi$, unless there is some sort of constraint that was left out in the statement of the problem.

It's possible to find a differential equation of motion that involves only one of the degrees of freedom. But I'm not sure what your professor was saying about treating the system as effectively having only one degree of freedom.