1. The problem statement, all variables and given/known data Find vz(r) Boundary Conditions: 1. vz(Ro) = 0 2. vz(Ri) = W 2. Relevant equations (1/r) d/dr [r dvz/dr] = 0 (from previous problem) Let v = dvz / dr d/dr [r v] = 0 r v = c1 v = c1 / r vz = c1 ln r + c2 BC 1: 0 = c1 ln Ro + c2 c2 = - c1 ln Ro BC 2: W = c1 ln Ri - c1 ln Ro W = c1 (ln Ri - ln Ro) W = c1 ln(Ri/Ro) c1 = W / ln(Ri/Ro) My Soultion: vz = (W ln r) / (ln(Ri/Ro) - (W ln Ro)/ln(Ri/Ro) vz = W[ln (r / Ro) / ln(Ri/Ro)] Unfortunately, the answer key says it should be: vz = W [ln (Ro / r) / ln(Ro/Ri)] So I am close but no cigar. I have recompleted this problem several times and keep arriving at the same solution. Can anyone point out my mistake?