Fluid Mechanics: Flow Between Parallel Plates--Force Balance Equation 1. The problem statement, all variables and given/known data 2. Relevant equations I know the equation for shear, tau=viscosity*velocity gradient. and pressure is obviously force/area. 3. The attempt at a solution I solved this problem using navier-stokes equations, and using the assumptions given in the problem. however, the homework graders wrote to use force-balance equations instead, as stated in the problem. ><;; The professor went over a similar example of a cylindrical fluid element, where the area factor was simply pi*r^2. however, i'm confused by the plates being infinite, so i can't seem to figure what the area is. how do i calculate the area of the fluid element? is it simply dy*dx? how is the pressure/pressure gradient affect being that the plates are infinite? I just need to know what the force balance equation looks like. Here's a picture of my professor's lecture slide about the cylindrical fluid element and its force balance equation: link Thanks!