τ =μ (du/dy)
The Attempt at a Solution
I got far enough to write down
dM = μ (Ωr/tanθ) dA
from just substitutions, easy enough.
I get confused when I'm solving for the differential surface area. I somehow need the dA for a cone.
For a circle it's easy enough. A=πr^2 so then dA = 2πr dr.
But how do I go about getting this for a cone? the book lists it as (2πr/cosθ) dr, without any explanation, of course.