Example 1.10 (from reference book by Frank White) An oil film of viscosity 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 r=R Fixed wall (b) =R dr dM= (t dA)r dA= 28r dr