Crank OA oscillates between the dashed positions shown and causes small angular motion of crank $BC$ through the connecting link $AB.$ When $OA$ crosses the horizontal position with $AB$ horizontal and $BC$ vertical, it has an angular velocity $=1\mathrm{.}20ra\frac{d}{s}$ and zero angular acceleration. Determine the angular acceleration ${}_{BC}($positive if counterclockwise, negative if clockwise$)$ of $BC$ for this position. The distances $\frac{?}{?}=400mm$ and $r=300mm.$