$6\mathrm{.}4\mathrm{.}12.[$Level $2]$ A rigid bar $\frac{?}{\text{b}\text{a}\text{r}}(AB),$ hinged at $A,$ is being drawn upright by a rope attached to the bar at $B$ and being pulled at the other end by someone standing at $O,$ directly below a pulley at $C\frac{*}{b}ar(AB)\frac{?}{b}=ar(OC)\frac{?}{b}=ar(OA)=2m.$ Rope is being reeled in at $0\mathrm{.}5\frac{m}{s}.$ Find ${}^{}$ when $=30.$

E$6\mathrm{.}4.$