$15\mathrm{.}12$ The rectangular block shown rotates about the diagonal $OA$ with a constant angular velocity of $6\mathrm{.}76ra\frac{d}{s}.$ Knowing that the rotation is counterclockwise as viewed from $A,$ determine the velocity and acceleration of point $B$ at the instant shown.

Fig. P$15\mathrm{.}12$ and P$15\mathrm{.}13$