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.