Already in the ancient world, people suspected what the pressure of air and liquids really is. Some of the ideas about the atomic structure of matter came down to us in the poem “On the Nature of Things” by Lucretius Cara, and this is the period of antiquity, while the properties of pressure were effectively used already in Ancient Egypt. The priests used the heated and expanded gas to "magically" open the doors of the temples, and the builders used the hydraulic lift for heavy stone blocks.
Today, to the question of what pressure is as a physical quantity, they answer: it is equal to the ratio of force to unit area. Therefore, air pressure, fluid pressure in a vessel, and solid body pressure on a support are similar phenomena. Since they involve force, pressure can be made to do work (which was what the enterprising ancient Egyptian priests used).
With the pressure of a solid body on a support, in principle, everything is clear. The weight of the body is a force, and it is divided by the area of contact of the body with the support. But in liquid and gas particles are not at rest. They are constantly moving, either chaotic Brownian or directed transfer due to the influence of external forces or internal conditions of the system. Pressure is created by the impact of particles on the wallsvessel.
The force that is involved in creating pressure in this case is the momentum that each particle gives off per unit time. Where the momentum and force come from, we will understand if we remember the formulas of kinematics that describe the elastic collision of bodies. A molecule or atom of liquid and gas is considered as an elastic sphere. Inside a liquid and gaseous substance, particles constantly collide with each other, exchange energy and momentum. Therefore, pressure also exists not only in relation to the wall of the vessel, but also inside any substance.
Even inside a vacuum, there is always a certain amount of particles that create a small pressure in it. True, it took some time to find out that such pressure exists in a vacuum. Initially, it was believed that vacuum is an absolute void, and it creates zero pressure. The physics of the school course uses this assumption even now.
Let's get back to particle motion. It will help us understand what pressure is kinetic and static. When particles are in chaotic thermal motion, which is constant, there is a static pressure. When any external influence is applied to the system, and prevailing directions appear in the movement of particles, these same particles begin to exert kinetic pressure.
Static pressure can be observed, for example, at the bottom of a tub filled with water. If you open the tap, the falling jet of water will create additional kinetic pressure. Simplified, it can be calculated based on the sameconsiderations that were described above regarding elastic collisions of particles. The jet has a measurable speed and exchanges momentum with the bottom of the bath upon impact. The total pressure of the system (water bath) will be equal to the sum of the static and kinetic pressures.
