- #1

- 603

- 6

## Homework Statement

## Homework Equations

Fluid Mechanics

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.