Dynamics of fluids: Pressure and Pascal's principle.

Pressure
In general, pressure exerted on one object is defined as the ratio between the perpendicular force applied to the object's surface and the area of the involved surface. The unit of measurement in the SI system is called Pascal (Pa). One Pascal equals one Newton per meter squared.

Pascal's principle
Blaise Pascal (1623-1662) discovered that in a fluid, any pressure applied insists uniformly in all the direction of the fluid , and any change in pressure is transmitted identically throughout it. For instance, all the hydraulic devices take advantage from the principle in order to obtain larger forces starting from a small force applied. (See the description of how hydraulic jacks work in the textbook on page 161).

Atmospheric pressure
Atmospheric pressure at a given height from the sea level is the pressure due to the weight of atmosphere insisting at that height. Evangelista Torricelli (1608-1647) demonstrated that atmospheric pressure at sea level can support a column of mercury 76 cm in height; in other words, air pressure at sea level balances exactly the weight of such a column. Hence, by knowing the density of mercury, the evaluation of the atmospheric pressure at sea level is a simple matter. If the column of mercury is contained in a tube with a cross sectional area A atmospheric pressure is

Since the density of a substance is defined as the ratio between its mass and its volume:

the pressure will be

where “h” is the height of the column, “g” is the acceleration of gravity, “” the density of mercury whose value can be read in following table:

hence the evaluation yields to:

that means atmospheric pressure at sea level is approximately 100 kilopascal.
As we go up from sea level, there is less atmosphere above us, and the pressure decrease. However atmospheric pressure depends not only from the height from the ground, but also from the changes in the weather, Pascal observed that columns height were lower on stormy days and higher on clear days. This pressure variation is actually used to indicate and to forecast weather conditions.