What is the pressure of gas, liquid and solid

Already in the ancient world people suspected that such pressure of air and liquids actually. Some of the ideas about the atomic structure of matter have come down to us in the poem of Lucretia Kara "On the Nature of Things", and this is a period of antiquity, while the properties of pressure were effectively used already in ancient Egypt. Priests with the help of heated and expanded gas "by magic" opened the doors of the temples, and builders used a hydraulic lift for heavy stone blocks.

Today, when asked what pressure is like a physical quantity, it answers: it is equal to the ratio of force to unit area. Therefore, the pressure of the air, the pressure of the liquid in the vessel and the pressure of the solid on the support are similar phenomena. Because they use force, pressure can be made to do the work (which was used by enterprising ancient Egyptian priests).

With the pressure of the solid on the support, in principle, everything is clear. Body weight is strength, and it is divided into the area of contact of the body with the support. But in a liquid and gas, the particles are not at rest. They move all the time, either chaotic Brownian or directional transfer due to external forces or internal conditions of the system. Pressure is created by striking the particles against the wall of the vessel.

The force that participates in creating pressure in this case is the momentum that each particle gives in a unit of time. Whence there is an impulse and force, we will understand, if we recall the kinematics formulas describing the elastic collision of bodies. A molecule or atom of a liquid and a gas is regarded as an elastic sphere. Inside the liquid and gaseous matter, particles constantly collide with each other, exchange energy and momentum. Therefore, pressure exists also not only with respect to the wall of the vessel, but also inside any substance.
Even inside the vacuum there always remains a certain amount of particles, which create a small pressure in it. True, that such pressure exists in a vacuum, it turned out not immediately. Initially it was believed that the vacuum is an absolute void, and it creates a zero pressure. The physics of the school course uses this assumption even now.

Let us return to the motion of the particles. It will help us understand what kinetic and static pressure is. When the particles are in chaotic thermal motion, which is constant, a static pressure arises. When an external action is applied to the system, and predominant directions arise in the particle motion, these same particles begin to exert kinetic pressure.

Static pressure can be observed, for example, at the bottom of a bath filled with water. If you open the tap, a falling jet of water will create additional kinetic pressure. Simplified, it can be calculated on the basis of the same considerations as described above with respect to elastic collisions of particles. The jet has a measurable velocity and exchanges momentum with the bottom of the bath during a collision. The total pressure of the system (bath with water) will be equal to the sum of the static and kinetic pressures.