$92$ A large plate is pulled at a constant speed of $U=4\frac{m}{s}$ over a fixed plate on $5-$mm$-$thick engine oil film at $20C.$ Assuming a half$-$parabolic velocity profile in the oil film, as sketched, determine the shear stress developed on the upper plate and its direction. What would happen if a linear velocity profile were assumed? $=0\mathrm{.}8374N*\frac{s}{m^2}$ should be given $**$