A viscometer (an instrument used to study characteristics of a non-ideal fluid) consists of a flat plate and a rotating cone. The cone has a large apex angle and the angle `theta` shown in figure is very small (typically less than `0.5^(@)` ). The apex of the cone just touches the plate and a liquid fills the narrow gap between the plate and the cone. The cone has a base radius R and is rotated with constant angular speed `omega`. Consider the liquid to be ideal and take its coefficient of viscosity to be `eta`. Calculate the torque needed to drive the cone.

Answer

`(2pietaomegaR^(3))/(3sintheta)~=(2pietaomegaR^(3))/(3theta)`