Suppose a fluid whose coefficient of viscosity is η and density ρ, flows through a cylindrical tube of radius r and length L. Let P be the pressure difference in the liquid at both ends of the tube. If the volume of the liquid flowiing per unit time through the cylindrical tube depends on the pressure gradient P/L, the coefficient of viscosity η and radius r, obtain an expression for the volume of liquid flowing per unit time through the tube. ​