1. The problem statement, all variables and given/known data A homogeneous wooden bar of length 10 cm, thickness 4 cm and weight 1 Kg is balanced on the top of a semicircular cylinder of radius R as shown below. Calculate the minimum radius of the semicircular cylinder if the wooden bar is at stable equilibrium. 2. Relevant equations Potential energy E=mgh and its derivatives. 3. The attempt at a solution Stable equilibrium means the first derivative of potential energy is zero and its second derivative must be greater than zero(local minima). So, I have to express the PE of the wooden bar in terms of R and find minimum R to satisfy above conditions. But here the CM of the bar is at R+(4/2)=R+2 cm above the ground. So, second derivative of PE is always 0? Where am I going wrong? Also, how to approach problems like these in general? I had read about equilibrium a long time back and the concepts are a bit muddled up.