What is pressure? | Difference between pressure and stress.

What is pressure?

Pressure: As the term explained itself the word pressure comes from the word press. Means pressing is always observed when force is applied only in perpendicular direction to a specified area.

Let's try to understand the root cause of pressure:

Pressure

Basically pressure is created due to collision of molecules of fluid particles. Due to molecular activity when the fluid particles strikes to a particular surface having some momentum, after collision the molecule change it's direction and magnitude of velocity that causes change in momentum w.r.t. time results in force. And normal component of these forces per unit area is known as presssure.

Mathematically, pressure is defined as normal force per unit area. It's SI unit is N/m2 or pascal(Pa).

Some of the very important unit conversions.

1 N/m2 (or) Pa = 1 MP

1 Atm. = 101325 Pa

1 Atm. = 10.3 meter of water column

1 Atm. = 760 mm of Hg column

1 torr = 1 mm of Hg

One more important conversion is 1 kgf/ cm2 is equal to how much Pa?

let's try to make it step by step:

1 kilogram force = 1 kg * 9.81 m/sec2 = 1 N.

In shorts 1 kgf = 1 N.

hence 1 kgf/cm2 = 98100 Pa.

Difference between pressure and stress:

In spite of similarity in units of pressure and stress they also exhibit some basic difference.

Pressure	Stress
Generally used in fluid mechanics	Generally used in mechanics of rigid and deformable bodies
Scalar	Second order tensor
Created due to external force	Created due to internal force
Always compressive	May be compressive (or) tensile