1. The problem statement, all variables and given/known data http://img13.imageshack.us/img13/5793/84188411.jpg [Broken] 2. Relevant equations Find a condition on b such that x = 0 is a local minimum of the potential function. 3. The attempt at a solution To find local minimum, potential function (V) of the system should be written. V must be positive definite and derivative of V must be negative semi definite. I tried to write hundreds of potential functions that provide local minimum constraints but i can't get rid of sin and cos terms from derivative of V so i couldn't find a condition for b to show x = 0 is a local minimum of the potential function.