1. piston having a diameter of D = 5.48 in. and a length of L = 9.50 in. slides downward with a velocity V through a vertical pipe. The downward motion is resisted by an oil film between the piston and the pipe wall. The film thickness is δ = 0.002 in., and the cylinder weighs W = 0.5 lb. Estimate the velocity V if the oil viscosity is μ = 0.016 lb*s/ft^2. Assume the velocity distribution in the gap is linear. 2. Relevant equations I think I need Tau=μ(du/dy) and tau=P/A. I don't know what du/dy is. 3. The attempt at a solution A=pi*(2.74)^2=23.586 in^2 tau=0.5lb/23.586in^2=0.0212. I don't know what to substitute for du or dy.