PROBLEM $12\mathrm{.}69$

A $0\mathrm{.}5-kg$ block $B$ slides without friction inside a slot cut in arm $OA$ which rotates in

a vertical plane. The rod has a constant angular acceleration ${}^{}=10ra\frac{d}{s^2}.$

Knowing that when $=45$ and $r=0\mathrm{.}8m$ the velocity of the block is zero,

determine at this instant, $($a$)$ the force exerted on the block by the arm, $($b$)$ the

relative acceleration of the block with respect to the arm.