[tex]0.5[/tex] - kg block B slides without friction inside a slot cut in arm OA that rotates in a vertical plane. The rod has a constant ngular acceleration [tex]\dot{\theta}=10 \mathrm{rad} / \mathrm{s}^2[/tex]. Knowing that when [tex]\theta=45^{\prime}[/tex] and [tex]\mathrm{r}=0.8 \mathrm{~m}[/tex] the velocity of the block is zero, determine at his instant, (a) the force exerted on the block by the arm, (b) the relative acceleration of the block with respect to the arm.