viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion. for instance, when a fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. in such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. this is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity.