Momentum flux

We have seen in Chapter 1 that a fluid (either a gas or a liquid) is a substance that takes the shape of the vessel containing it. We have also seen that all real fluids have a property called viscosity associated with them. Let us consider two parallel flat plates with a fluid (say water) in between them. Now let us consider the situation when the bottom plate is carefully moved in the x direction with a reasonably small velocity, vx. If the velocity is small enough, we can assume that the bottom-most liquid layer adhering to the plate will also move with the same velocity as that of the plate. The shear stress due to the shear force exerted by the bottommost layer of fluid influences the velocity of the fluid layer above it. The shear stress exerted by the layer above the bottom-most layer influences the velocity of the layer above it, and so on.