Kuldeep Singh Rathore
The matter is broadly categorized into two classes namely the fluids and the solids. Classically, fluids and solids have been defined on the basis of thermodynamic conditions and influence of external forces. This classification based on the geometry of the matter is simple to understand and useful but it has its own limitations.To overcome the above limitations, another criterion is based on the deformation of the matter. Whenever external force is applied, it causes deformation in the matter. In solids, only deformation matters while in fluids, rate of deformation matters not the only deformation. Hence fluid undergoes deformation continuously when subjected to an external force, however small it may be. If the deformation in the material increases continually without limit under the action of shearing force however small, the material is called a 'fluid'. This continuous deformation under the action of forces is manifested in the tendency of fluids to flow. Fluids are usually classified as liquids or gases. A liquid has intermolecular forces which hold it together so that it possesses volume but no definite shape. Liquids have but slight compressibility i.e. the ability for changes in volume of a mass of fluid. For most purposes, it is however sufficient to regard liquids as 'incompressible fluids'. A gas on the other hand, consists of molecules in motion which collides with each other tending to disperse it so that a gas has no set volume or shape. The intermolecular forces are extremely small in gases. A gas will fill any container into which it is placed and is therefore known as a compressible fluid. It is necessary to remark here that for speeds which are not comparable with that of sound i.e. if the Mach number, which is the ratio of the velocity of flow to the velocity of sound, is small compared with unity the effect of compressibility on atmospheric air can be neglected and it may be considered that to be a liquid, and in this sense it is called incompressible air. Viscosity of a fluid is that characteristic of real fluids which exhibits a certain resistance to alteration of form. Viscosity is also known as internal friction. All known fluids (gases or liquids) possess the property of viscosity in varying degrees.
