2-87 A thin plate moves between two parallel, horizontal, stationary flat surfaces at a constant velocity of 5 m/s. The two stationary surfaces are spaced 4 cm apart, and the medium between them is filled with oil whose viscosity is 0.9 N.s/m². The part of the plate immersed in oil at any given time is 2-m long and 0.5-m wide. If the plate moves through the mid-plane between the surfaces, determine the force required to maintain this motion. What would your response be if the plate was 1 cm from the bottom surface (h) and 3 cm from the top surface (h₁)? Stationary surface h₁ V = 5 m/s F h₂ Stationary surface FIGURE P2-87 2-88 Reconsider Prob. 2-87. If the viscosity of the oil above the moving plate is 4 times that of the oil below the plate, determine the distance of the plate from the bottom sur- face (h) that will minimize the force needed to pull the plate between the two oils at constant velocity.