$6\mathrm{.}15$ If in Problem $6\mathrm{.}14$ the plane has an inclination with the horizontal of $20,$ calculate

$($a$)$ the frictional resistance when the block is at rest

$($b$)$ the force parallel to the plane necessary for motion to impend up the plane

$($c$)$ the force parallel to the plane necessary to prevent motion down the plane