That branch of mechanics which deals with fluids is called “Fluid Mechanics”. Fluids are subdivided into two classes: liquids and gases. Both, liquids and gases take the shape of the vessel which contains them. The difference between liquids and gases as that a liquid only occupies a definite volume independent of the dimensions of the container and forms a free surface if the volume of the liquid is smaller than the volume of the container, whereas a gas expands and fills the container in which it is placed. Gases are compressible, liquids are normally considered incompressible since the effect of compressibility is so small that it in general may be neglected, although in some cases the compressibility may be significant.
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