Ah, I see thank you. I should have recognized the definition of a derivative.
My only question is that in method 1 it doesn't look like you are making any approximations because you're just applying the definition of a derivative, while in method 2 it looks like you use an approximation because...
Yea, my problem was realizing that I was supposed to approximate the function around r and evaluate the function at r + dr. Should have seen that. Thank you for your help.
I'm reading a derivation and it says that the following approximation can be used:
I do not under stand how Taylor's theorem allows for this approximation. Can anyone explain this a little?
Homework Statement
I am trying to derive Poiseuille's equation using an infinitesimal volume element of the pipe in cylindrical coordinates as seen below. I am using x for the flow direction, u for the velocity in the flow direction, and μ for the viscosity constant...