The two constants can be found from boundary conditions. Referring to Fig. 4, since the plates are fixed, so at (no-slip conditions for viscous fluids). In order to satisfy this boundary condition, and . Thus, the velocity distribution becomes,
                                  (23)
The Eq. (23) indicates that the velocity profile between two fixed plates is parabolic and is shown in Fig. 4(c). The volume flow rate per unit width, passing through the plates is given by,
                                  (24)
Since the pressure decreases along the direction of the flow, so the pressure gradient is negative. Thus, if represents the pressure drop between two points at a distance , then .

So, the Eq. (24) can be written as,
                                                   (25)