PROBLEM $8\mathrm{.}7$

The $10-kg$ block is attached to link $AB$ and rests on a moving belt. Knowing that ${}_s=0\mathrm{.}30$ and ${}_k=0\mathrm{.}25$ and neglecting the weight of the link, determine the magnitude of the horizontal force $P$ that should be applied to the belt to maintain its motion $($a$)$ to the left as shown, $($b$)$ to the right.