$6/3$ The force P is applied to the $50-$kg block when it is at rest. Determine the magnitude and direction of the friction force exerted by the surface on the block if $($a$)$ P $=0,($b$)$ P $=200$ N$,$ and $($c$)$ P $=250$ N$.($d$)$ What value of P is required to initiate motion up the incline? The coeffi cients of static and kinetic friction between the block and the incline are $$s $=0\mathrm{.}25$ and $$k $=0\mathrm{.}20,$ respectively. $20$