Solution
= Solution
A coordinate absent from the <Lagrangian> is <cyclic coordinate>[cyclic]. Its <Euler-Lagrange equation> gives
$$
\frac d{dt}\frac{\partial L}{\partial\dot q_1}=\frac{\partial L}{\partial q_1}=0.
$$
Thus the corresponding <generalized momentum> $\boxed{p_1=\partial L/\partial\dot q_1}$ is conserved.