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A fluid is a collection of molecules that are randomly arranged and held together by weak cohesive forces and by forces exerted by the walls of the container. A fluid can flow continuously under the application of a force. However, the solid substance does not deform continuously under the application of force. If the solid is deformed then it may come back to its original shape when the applied deforming force is removed or it may not be able to regain its original shape and size. In either of the case, the deformation is not continuous rather it takes place in a discrete manner. All liquids and gases are fluids. It is worth noting that the fluids are subset of the phases of matter including liquids, gases, plasmas, and to some extent plastic solids.
- A solid has a definite volume and shape where as a fluid does not.
- The fluids have the ability to flow unlike solids.
- The fluids form a free surface unlike the solids that form rigid surfaces.
- The fluids satisfy the conservation laws including the conservation of mass, linear momentum, angular momentum, and conservation of energy. The solids may or may not satisfy these conservation laws.
Pressure and Density:
Pressure in general is the force exerted per unit area. When a solid material is merged in a fluid, the force (F) exerted perpendicularly on the surface per unit area of the solid substance is called pressure (P):
P ≡ F/A
where A is the surface area of the solid material placed in the fluid. The unit of pressure is Pascal.
Density in general is defined as mass per unit volume of the substance: ρ = M/V, where ρ is the density of the considered object, M is the mass of the substance and V is its volume. The unit of density is kg/m3.
- Density explains how tightly the molecules of a material are packed. However, it has nothing to do with the hardness of a substance.
- For an incompressible fluid the density is always constant. Water may be considered as an incompressible fluid at times.
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