Solving Pipe Flow: Understanding Why $\tau =\mu(-dV/dR)$

[foo9008]

Homework Statement

why the shaer stress ( tau) formula is given by μ(dv/ dy ) = μ(-dV / dR) , why there is a negative sign there ?

Homework Equations

The Attempt at a Solution

 

[mfig]

The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.

 

[foo9008]

The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.

why when we are considering the velocity as a function of the distance from the the wall , it's positive ? or we can also consider the distance from the wall as positive ?

 

[foo9008]

The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.

can we consider the distance from the centerline of pipe to wall as positive ?