When we press on the table with our hand, we exert a certain pressure with the palm of our hand.
The pressure is the physical scale which characterizes the amount of applied force on a surface area of contact.
The letter symbol for pressure is “p”.
![Pressure mathematical equation equals force devided to surface area](https://hvac-learning.com/wp-content/uploads/en_112-02.gif)
- The greater the applied force, the greater the pressure.
- The larger the area, the less the pressure.
Conversely the pressure will be greater the smaller the area of applied force. Hence, we can easily stab the point of a needle into the surface of a table.
In our profession the most common pressures are due to weight (weight is a force)
![](https://hvac-learning.com/wp-content/uploads/en_112-03.gif)
To reduce the pressure due to the weight of machines, we sometimes install them on large blocks, which due to their large surface area reduce the pressure on the supporting floor.
![](https://hvac-learning.com/wp-content/uploads/80-02.jpg)
In everyday language we confuse weight with mass. It is normal to say “the weight of the object is, for example, 50 [kg]”. In reality this is its mass (weight being expressed in Newtons and not in [kg]).
- Mass is in proportion to the amount of molecules which make up a given body. This never varies whatever the position of this body in the universe.
- Weight corresponds to the force with which a mass is attracted to the earth. It depends on terrestrial attraction.
![](https://hvac-learning.com/wp-content/uploads/80-01-1.jpg)
The international unit to calculate force is the Newton.
The Newton is approx. the weight of a mass of around 100 [g] (US: 3.527 oz).
The international unit of pressure is the Pascal.
It corresponds to the applied force of 1 Newton on an area of 1 [m²] (US:
10.763 sq.ft).
The Pascal corresponds to the pressure due to a mass of 100 [g] (US: 3.527 oz)
distributed over 1 [m²] (US: 10.763 sq.ft).
This is a very low pressure. The Pascal is not suitable for our profession.
Therefore we use:
[kPa]= 1000 [Pa] (US: 0.15 psi) and the [bar] = 100 000 [Pa] (US: 14.5 psi).