Question.
My confusion comes from a problem and its solution (both partially).
Here is the problem.
Here is the solution.
Now, what baffles me is the highlighted part in the solution; I do not understand it.
My Attempt. I tried to understand the derivative, by definition, as the rate of change in potential energy following some change in the angle $\theta$. But why its being zero represents equilibrium? Does equilibrium occur at some turning point of potential energy? I am puzzled.
Comment. Any kind of help would be appreciated. Personally I prefer some short but helpful hint. Thank you in advance!