Viscosity We can think of viscosity as the internal friction of a fluid in motion It has the effect of smoothing out differences in velocity by increasing the shear stresses in proportion with the velocity gradient This is called Newton s law of viscosity and fluids that obey it are said to be Newtonian Newton s law of viscosity looks a lot like Hooke s law for shear that we see in solids the shear stress is a function of the rate at which strain changes over time at room temperature The viscosity of water is equal to one centipoise typical engine oil at room temperature is 500 times more viscous than water pitch is a thick tarlike fluid that has extremely high viscosities it is the world s longest running continuous lab experiment in liquids viscosity is caused by cohesive forces which bond layers of the fluid together in in gases it s caused by intermolecular collisions which create interaction between adjacent layers of fluid viscosity is highly dependent on temperature in liquids It decreases as temperature increases This is because at the molecular level an increased temperature allows molecules to more easily escape the attractive forces of the adjacent molecules the assumption that viscous forces are negligible is typically only applied to certain regions within larger flow system Viscosity really is a fundamental parameter in the study of fluids inviscid flow is also a key assumption in the derivation of Bernoulli s equation we said earlier that fluids that obey Newton s law of viscosity are said to be Newtonian but for some fluids the relationship between shear stress and strain rate is nonlinear these are called non newtonian fluids