1. The problem statement, all variables and given/known data I am studying a question in Marion's classical mechanics: I am successful in obtain the equation of motion, which is where theta is the theta shown in . However, in the second part of the solution, , it puts derivative of theta to be zero and I can't understand this. Also, the solution concludes theta=pi/2 is the solution. It is very confusing. 2. Relevant equations 3. attempt In my mind, I can image that derivative of theta is zero at equilibrium as the mass stops moving but i don't know why such criteria is still valid when expanding around a certain angle. pi/2 is the equilibrium angle which I can derive by finding minimum of the potential energy but the result does't seem to be related to the "half-angle" asked.