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PHYSICS-IC As bulk moduli of liquids are large, it is difficult to compress liquids Generally it is very difficult to compress liquids i.e. bulk moduli of liquids are sufficiently large so that liquids can be treated as nearly incompressible for usually applied pressure but gases can easily be compressed by applying pressures in the order of atmospheric pressure. Also, a liquid has a free surface of its own while a - gas does not. 2.2 MECHANICAL TREATMENT OF A FLUID I In 'Fluid Mechanics', we study fluid at a macroscopic level. It is concerned with characteristics that can be observed and measured on the laboratory scale. We A fluid can exert three types of contact forces : Pressure force, Viscous force and force due to surface tension cannot resolve the motions of individual molecules by measurements in our laboratories. But we recognize that it is the aggregation of propeties of individual molecules that determine the behavior of a macroscopic element of a fluid. Our measuring instruments average the behavior of a very large number of molecules in the vicinity of a point in a fluid. Hence, all the results obtained in 'Fluid Mechanics' are applicable at macroscopic level only. It means in 'Fluid Mechanics' we treat the fluid as a continuous matter which is true at a macroscopic level. A fluid can exert three types of contact forces: Pressure force, Viscous force and force due to surface tension. Contact force exerted by a fluid on its surrounding In reality, a fluid may exert all these three forces simultaneously and considering the effects of all these forces at once can be very complicated. So, it will be very convenient to deal with these forces one at a time and then combine all these effects. For this reason, we are going to define a concept of ideal fluid in which intermolecular forces are absent. Infact, an ideal fluid (also known as a dry fluid), cannot be realized in reality. An ideal fluid cannot exert viscous force or surface tension force because the cause of these two forces is the existence of intermolecular forces. Contact force exerted by a fluid on its surrounding Pressure Viscous force Force due to force surface tension It is the tangential It is the force exerted by free surface of the i) It is the normal component of fluid contact force component of fluid contact force fluid It is a dragging ii) It is a pushing or pressing force iii) This force is possible This force is possible It is a pulling force force This force is possible only when the free surface of a real fluid for state of a fluid only when the real fluid is flowing [Fluid Dynamics] exists. iv) Both ideal and real Only real fluids can Only real fluids can exert fluids can exert a exert viscous force a surface tension force. pressure force v) It is a non-dissipative It is dissipative force It is a non-dissipative force force FLUID MECHANICS 41