A car windshield wiper blade sweeps the wet windshield rotating at a constant angular speed of `omega`. R is the radius of innermost arc swept by the blade. Length and width of the blade are l and b respectively. Coefficient of viscosity of water is `eta`. Calculate the torque delivered by the motor to rotate the blade assuming that there is a uniform layer of water of thickness t on the glass surface.

Answer

`(etarhoomegaR^(3))/(3t)[(1=(L)/(R))^(3)-1]`