Past exam of the mathematics course of the University of Cambridge / 2015 / ii / Paper 2 / 35E / iv / Solution

The fluid's tangential traction on the lower stationary surface is $\tau =\mu u_y{|}_{y=0}$ in the positive $x$ direction. Using the pressure gradient found above givesThe drag on a plane beneath a translating cylinder follows by integrating along the wall:per unit cylinder-axis length. This is the force on the lower surface, so its direction follows the cylinder's translation.