# Fluid statics

The static fluid studies the balance of gases and liquids. From the concepts of density and pressure , the fundamental equation of hydrostatics is obtained , from which Pascal’s and Archimedes ‘ principles can be considered consequences. The fact that gases, unlike liquids, can be compressed makes the study of both types of fluids have some different characteristics. In atm ósfera phenomena pressure and thrust that they can be studied in accordance with the given principles of gas statics.

Fluid is understood as a state of matter in which the shape of the bodies is not constant, but adapts to that of the container that contains them. Fluid matter can be transferred from one container to another, that is, it has the ability to flow. Liquids and gases correspond to two different types of fluids. The former have a constant volume that cannot be appreciably mortified by compression. This is why they are said to be incompressible fluids. The latter do not have their own volume, but occupy that of the container that contains them; They are compressible fluids because, unlike liquids, they can be compressed.

The study of fluids in equilibrium constitutes the object of fluid statics, a part of physics that includes hydrostatics or the study of liquids in equilibrium, and aerostatics or the study of gases in equilibrium and in particular of air .

1. The density of the bodies

Density

Bodies generally differ in mass and volume. These two physical attributes vary from one body to another, so if we consider bodies of the same nature , the greater the volume, the greater the mass of the body considered. However, there is something characteristic of the type of matter that makes up the body in question and that explains why two bodies of different substances that occupy the same volume do not have the same mass or vice versa.

Even though mass and volume are directly proportional for any substance, the proportional relationship is different for each substance. It is precisely the constant of proportionality of that relation that is known by density and is represented by the Greek letter

m = cte · V

that is to say:

m = · V

Solving for the previous equation results:

equation that facilitates the definition of and also its physical meaning.

The density of a substance is the mass that corresponds to a unit volume of that substance. Its SI unit is the ratio of the unit of mass to that of volume, that is, kg / m3.

Unlike the mass or volume, which depend on each object, its quotient depends only on the type of material it is made of and not on its shape or size. Therefore, density is said to be a characteristic property or attribute of each substance. In solids the density is approximately constant, but in liquids, and particularly in gases, it varies with the measurement conditions. Thus, in the case of liquids, the temperature to which the given value for density refers is usually specified, and in the case of gases, the pressure must be indicated, along with this value.

1. Density and specific weight

Density is related to the degree of accumulation of matter (a compact body is generally denser than a more dispersed one), but it is also related to weight. Thus, a small body that is much heavier than a larger one is also much denser. This is due to the relation P = m · g existing between mass and weight. However, to refer to the weight per unit volume, physics has introduced the concept of specific weight, pe, which is defined as the quotient between the weight P of a body and its volume.

The specific weight represents the force with which the Earth attracts a unit volume of the same substance considered.

The relationship between specific weight and density is the same as that between weight and mass. Indeed:

where g is the acceleration due to gravity.

The unit of the specific gravity in the SI is N / m3.

1. Relative density

The relative density of a substance is the quotient between its density and that of a different substance that is taken as a reference or standard:

For liquid substances, water whose density at 4 ºC is equal to 1000 kg / m3 is usually taken as the standard substance . For gases, the reference substance is frequently made up of air, which at a temperature of 0 ºC and a pressure of 1 atm has a density of 1,293 kg / m3. Like any relative magnitude, which is obtained as a quotient between two equal magnitudes, the relative density lacks physical units