A jet plane flying at a constant speed $v$ at an altitude $h=10km$ is being tracked by radar located at $O$ directly below the line of flight. If the angle $$ is decreasing at the rate of $0\mathrm{.}020ra\frac{d}{s}$ when $=60,$ determine the value of $r^{}$ at this instant and the magnitude of the velocity $v$ of the plane.

Ans. $r^{}=4\mathrm{.}62\frac{m}{s^2},v=960k\frac{m}{h}$