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This post was last edited by huaiqiang on 2010-2-11 17:30. Fluids are a general term for ( ) and ( ). 　　 A fluid is composed of a large number of molecules that are in constant thermal motion and have no fixed equilibrium position; its basic characteristics are ( ), ( ) and ( ). All fluids have a certain degree of compressibility; liquids have very low compressibility, while gases have higher compressibility. When the shape of a fluid changes, there is also a certain amount of resistance to motion between its different layers (i.e., viscosity). When the viscosity and compressibility of a fluid are very low, it can be approximated as an ideal fluid, which is an ideal model introduced to study the motion and state of fluids. 　　Solids and fluids have the following difference: at rest, a solid’s surface can withstand both shear stress and normal stress simultaneously. A fluid can experience both normal and tangential stresses only when it is in motion; in a stationary state, its surface can only withstand normal stress, which is a compressive stress, namely static pressure. A solid deforms under the action of force; within the elastic limit, there is a Hooke’s law relationship between the deformation of the solid and the magnitude of the applied force, that is, the degree of deformation is proportional to the size of the force applied. The fluid, on the other hand, is related to the angular deformation rate and shear stress; the relationship between these two quantities differs in laminar and turbulent flow states. In laminar flow, they are governed by Newton’s law of internal friction. When the force is removed, the solid can return to its original shape, while the fluid can only cease deforming but cannot return to its original position. Solids have a definite shape, while fluids require very little shear force to deform, allowing them to easily adapt their shape to that of the container and maintain it under certain conditions.