Consider an airfoil in a wind tunnel (i. e., a wing that spans the entire test section). Prove that the lift per unit span can be obtained from the pressure distributions on the top and bottom walls of the wind tunnel (i. e., from the pressure distributions on the walls above and below the airfoil). For this problem, assume that the pressure distributions on the upper and lower walls of the wind tunnel are pu(x) and pl(x), respectively. They are functions of the x-coordinates. You will need to apply the momentum equation in integral form in the y-direction.