Past exam of the mathematical methods II course of the University of Oxford / 2013 / 8 / a / Solution

With angles measured from the downward vertical, the upper mass rises by $2l(1-\cos {\theta }_1)$ and the lower by $2l(1-\cos {\theta }_1)+l(1-\cos {\theta }_2)$. Taking zero gravitational potential energy at the hanging equilibrium therefore gives the exact resultBoth masses contribute to the elevation change caused by the upper rod.