# Introduction to Viscosity

Generally, fluid is the substance which has the ability to flow. Fluid can also be defined as the substance which is unable to resist shearing stress permanently. When the fluid flows, it comes across a frictional force which is called Viscosity or internal friction (As, this frictional force plays within the substance (fluid)). Viscosity is the property of fluid by virtue of which it opposes the relative motions between its different layers. Viscosity plays a great role in the flow of fluid when the velocity of the fluid is well below the limiting velocity called critical velocity of the fluid$$(V_c)$$. When the velocity of a liquid (liquid is a fluid) is below its critical value, the flow follows an orderly pattern. At that time, we call it a streamline flow, or a viscous flow. Here, we consider that fluid flows in the form of layers or laminae, and thus. We call the streamline flow as