When a viscous liquid flows , adjacent layers oppose their relative motion by applying a viscous force given by ` F = - eta A (dv)/(dz)` where , eta = coefficient of viscosity , A = surface area of adjacent layers in contact ,` (dv)/(dz)` = velocity gradient Now , a viscous liquid having coefficient of viscosity `eta ` is flowing through a fixed tube of length l and radius R under a pressure difference P between the two ends of the tube . Now consider a cylindrical vloume of liquid of radius r . Due to steady flow , net force on the liquid in cylindrical volume should be zero . ` - eta 2pirl (dv)/(dr) = Ppir^(2)` ` - int _(v)^(0),dv = P/(2 eta l) int_(tau)^(R) rdr ( :' ` layer in contact with the tube is stationary ) ` v = v_(0) (1- (r^(2))/(R^(2)))` , where `v_(0) = (PR^(2))/(4nl)` ` :. " " Q = (piPR^(4))/(8sta l)` This is called Poisecuille's equation . The velocity of flow of liquid at r = R/2 is