Fluid Mechanics - Find the required power

Hi I don't know where to start with this problem. Any help would be greatly appreciated.

A cone is rotating in a slot at constant angular speed w. The gap h separating the cone and the lsot's wall is much smaller than the cone's height b. A fluid with viscosity mu fills the gap. Obtain an expression for the power required to just overcome the viscous resistance. Calculate this power for a rotating speed of 500rpm, h = .2 mm, b= 10cm mu= .025 Pa.s cone's half angle alpha = 25 degrees

Because the example given uses the formulas below but it was for a cylinder and velocity was in m/s. As a result I have no idea what to do.
Shear stress= mu * (change in velocity/gap between cones)
Shear force = shear stress * area (surface?)
Power = Force * Velocity

I know I have to convert rpm to radians and that as the radius changes the velocity changes as well.

As height b changes so does the radius (which is the lever arm?) but I am not sure how to express that. Any pointers on how to get started would be great.

Thanks.