Measure of fluid resistance to deformation due to internal friction $ \tau=\frac{F_S}{A}=\mu\frac{\partial U}{\partial y} $ $\mu$ is dynamic viscosity, units $\text{N-s}/\text{m}^2$ In differential form: $ \tau=\mu\frac{\partial U}{\partial y} $ When $\mu$ is constant, the fluid is considered *Newtonian* Viscosity is the proportionality constant between an applied shear stress, and the fluid’s resulting rate of shearing strain In liquids, is a strong function of temperature - viscosity drops significantly as temperature increases