Equation for the shear stress of a Newtonian fluid:
\[ \tau = \mu \frac{du}{dy} \]
where \( \tau \) is shear stress
\( \mu \) is the viscosity
\( \frac{du}{dy}\) is the rate of strain
\( \tau \) can also be found by dividing the tangential force on a surface by the area of that surface.