The shear force applied to a flowing fluid at the point of contact with a surface. Where it is assumed that the wall shear stress is proportional to the kinetic energy of the moving fluid per unit volume, the proportionality constant is known as the Fanning friction factor, f. That is ${\tau }_w=f\rho v^2/2$ where ρ is the density of the fluid and v is the velocity. The SI units are N m⁻².