newton's law of viscosity is not a fundamental law of nature, but rather a constitutive equation (like hooke's law, fick's law, and ohm's law) which serves to define the viscosity formula_10. its form is motivated by experiments which show that for a wide range of fluids, formula_10 is independent of strain rate. such fluids are called newtonian. gases, water, and many common liquids can be considered newtonian in ordinary conditions and contexts. however, there are many non-newtonian fluids that significantly deviate from this behavior. for example: