Two identical parallel discs have a common axis and are located at a distance `h` from each other. The radius of each disc is equal to `q`, with `a gt gt h`. One disc is rotated with a low angular velocity `omega` relative to the other, stationary, disc. Find the moment of friction forces acting on the stationary disc if the viscosity coefficient of the gas between the disc is equal to `eta`.

Explanation

We consider two adjoining layers. The angular velocity gradient is `(omega)/(h)`. So the moment of the frictional force is
`N = int_0^a r.2 pi r dr.eta r (omega)/(h) = (pi eta a^4 omega)/(2 h)`.