Fluid Mechanics, science concerned with the response of fluids to forces exerted upon them. It is a branch of classical physics with applications of great importance in hydraulic and aeronautical engineering, chemical engineering, meteorology, and zoology. A fluid is any substance which flows because its particles are not rigidly attached to one another. This includes liquids, gases and even some materials which are normally considered solids, such as glass. Essentially, fluids are materials which have no repeating crystalline structure. Several properties of fluids were discussed in the Thermodynamics section of this text. The field of fluid mechanics examines the behavior of fluid systems and objects moving through fluids, such as liquids, gases and plasmas. Fluid dynamics is the study of the movement of fluids, including their interactions as two fluids come into contact with each other. In this context, the term “fluid” refers to either liquid or gases. It is a macroscopic, statistical approach to analyzing these interactions at a large scale, viewing the fluids as a continuum of matter and generally ignoring the fact that the liquid or gas is composed of individual atoms. Fluid mechanics has a wide range of applications in mechanical and chemical engineering, in biological systems, and in astrophysics.