$($a$)$

Two blocks A and $B($Figure $3)$ have a weight of $W_A=45\mathrm{.}8kN$ and $W_B=30kN$ respectively. They are connected together with a cable and placed on the inclined plane. If the angle $$ is gradually increased, determine $$ when both the blocks begin to slide. The coefficients of static friction are ${}_A=0\mathrm{.}15$ and ${}_B=0\mathrm{.}25.$

$($b$)$

If the cable $CD$ is replaced by a spring with a stiffness $k=40\frac{N}{m},$ determine the angle $$ for which both blocks begin to slide. Also find the required stretch or compression in the connecting spring for this to occur.

$($c$)$

With the same spring as in Question $3($b$),$ determine the angle which will cause motion of one of the blocks. What is the friction force under each of the blocks when this occurs?