An oil film of viscosity μ and thickness h<< R lies between a solid wall and a circular disk as in below figure. The disk ratites steadily at an angular speed velocity, 2. Noting that both velocity and shear stress vary with radius r, derive a formula for the torque M required to rotate the disk. Air drag is neglected. Oil film thickness (a) r = R Fixed wall dr r=R dM=(t dA)r dA = 2πr dr