E$5\mathrm{.}3\mathrm{.}5$

$5\mathrm{.}3\mathrm{.}6.[$Level $2]$ As depicted, a simple pendulum of length $L$ and mass $M$ is free to pivot about the point $O$ on a collar of mass $m$ that is constrained to move along a smooth horizontal rail. Let $x$ denote the collar's horizontal position along the rail, and take $$ to be the angle that the pendulum makes with the vertical. Write expressions for the system's total kinetic and potential energies.