Consider a bead that is threaded on a rigid circular hoop of radius $R$ lying in the $x y$ plane with its center at $O,$ and use the angle $\phi$ of two-dimensional polar coordinates as the one generalized coordinate to describe the bead's position. Write down the equations that give the Cartesian coordinates $(x, y)$ in terms of $\phi$ and the equation that gives the generalized coordinate $\phi$ in terms of $(x, y)$.