First I was confused about H(\theta,p_{\theta})\neq E, but after reading some books this seems to be a classic example of this type of problem. If I understand this correct r(t) is a holonomic constraint.
Anyway the solution seems to be correct. Thanks for your responses.
[SOLVED] Variable Length Pendulum
Hi, everybody! I'm not sure about my solution to the problem.
Homework Statement
Consider a simple pendulum with variable length r(t). The suspension point remains fixed.
(i)Write the Lagrangian for the pendulum and the equations of motion.
(ii)Write the...