Fluid means the state of matter, which can flow. The liquids and the gases are collectively called the fluids. Viscosity is the property of the fluid, which is in the motion. It is called the liquid friction also. The viscous fluid means, which can flow less as compared to the non-viscous fluid. For example, in case of honey and water, honey is more viscous as compared to the water. Let us discuss the fluid mechanics.
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Viscosity is the property of a fluid by virtue of which an internal frictional force comes into play when the fluid is in the motion and opposes the relative motion of its different layers. There are some more forces which acts on the fluids, which also resists the motion of the fluid. Let us discuss the viscous drag. Consider the two adjoining layers P and Q of the fluid. The layer Q moving faster than P, tries to accelerate the P. The layer P, moving at the slower rate tries to retard the Q. Thus two layers tend to destroy their relative motion as if there is a backward dragging force acting tangentially on layers so to maintain the flow of the fluid an external force equal and opposite to backward dragging force must be applied. This backward dragging force is also called the viscous force which is given by
`F = - h A (dv) / (dx)`
Where, h = coefficient of viscosity of the fluid, A be the area of the layers, `(dv) / (dx)` = velocity gradient. The negative sign shows that the force is in the opposite direction of the motion.
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Here we conclude that the viscosity is due to the intermolecular forces which are effective when the different layers of the liquid are moving with different velocities. These forces are Vander Walls forces and vary inversely as the seventh power of the intermolecular distances. Due to these forces, every fast moving liquid layer tends to accelerate the adjoining slow moving layer and every slow moving layer tends to retard the adjoining fast moving layer of the liquid. So that a backward dragging force comes into play.