Fluids will exert a normal force on the surface area of an object.  In turn, this will result in a pressure exerted by the fluid on that object. Generally, when we are talking about pressure will are only talking about the force per unit area caused by a gas or a liquid.  However, a solid is capable of exerting a force per unit area.  When this is the case we call it a normal stress.

The units of Pressure

First let’s discuss the units of pressure.  In the standard international system, (SI system) this is a pascal ($Pa$).  The units that make up a Pascal are newtons per square meter.


One pascal however is a very small amount of pressure.  Instead, to truly describe most pressures seen in practice you would either need to use kilopascal ($1~kPa=10^3~Pa$) or megapascal ($1~MPA=10^6~Pa$).  Some other units that are commonly used are bar and standard atmosphere.

$1~bar = 10^5~Pa$

$1~atm = 101325~Pa$

Next let’s take a look at the English system.  In this case pressure is described as pound-force per squared inch.

$1~atm = 14.696~psi$

Normal Stress

As stated above a pressure within a solid is called a normal stress.  A normal stress is caused by a force that is acting perpendicular to the surface per unit area. For example if a $200~lb$ object had a $45^in^2$ foot print it would exert and $4.4~psi$ pressure on the floor.   Hence, the larger the foot print, the smaller the pressure, while the larger the foot print, the greater the pressure.  This is the reason why a sharp knife can cut through food so easily.  This is because there is such little surface area contacting the surface of the knife that a small amount of force can create a large amount of pressure.

Absolute vs Gage Pressure

Finally, the last thing that you should understand about pressure is the difference between absolute pressure and gage pressure.  First, absolute pressure is the actual pressure that a fluid is exerting.  On the other hand, gage pressure is the difference between absolute pressure and the local atmospheric pressure.  It is called gage pressure because it is the pressure that is measured using a common pressure gage.  Finally, any pressure that measures bellow atmospheric pressure is called a vacuum pressure.

(Eq 1) $p_{gage}=p_{abs}-p_{atm}$

(Eq 2) $p_{vac}=p_{atm}-p_{abs}$

In order to clarify if a pressure is absolute or gage normally the letters “a” (absolute pressure) or “g” (gage pressure) are added after the pressure unit.  For example psia is absolute while psig is gage.

Finally, pressure is a compressive force per unit area.  However, looking at a point within a fluid the pressure acting on that point will be the same in all directions.  Hence, pressure has a magnitude but no specific direction.  This makes it a scalar quantity.



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