 Solution

First convert the dynamic viscosity to SI units:Using the stipulated effective-size version of Stokes's law with $a=20\times 10^{-6}\mathrm{m}$, the drag coefficient isThe speed is $v=10^{-7}\mathrm{m}{\mathrm{s}}^{-1}$, henceThe drag acts opposite the velocity. Its magnitude is about $1.06\times 10^5$ times smaller than the substrate force in (a); balancing surrounding-fluid drag alone therefore cannot account for the much larger substrate traction. The calculation uses the size parameter exactly as supplied. An exact drag calculation for a cube moving close to a wall would need geometry and boundary corrections beyond this stipulated approximation.