Consider the pendulum of Figure $7.4,$ suspended inside a railroad car, but suppose that the car is oscillating back and forth, so that the point of suspension has position $x_{\mathrm{s}}=A \cos \omega t, y_{\mathrm{s}}=0 .$ Use the angle $\phi$ as the generalized coordinate and write down the equations that give the Cartesian coordinates of the bob in terms of $\phi$ and vice versa.