Fluids :Liquids and Gases

Introduction to fluids

What is fluid?

  • Fluids play a vital role in many aspects of our life.
  • We breathe them, drink them and it circulates through our bodies in our cardiovascular system.
  • If we look around we find airplanes fly through them and ships float through them. So there are fluids in the ocean and the fluids in the atmosphere.
  • A fluid is any substance that can flow. We can use term fluids for both liquids and gases.
  • We usually think of gases as easily compressible and liquids as nearly incompressible (there can be exceptional cases)

Fluid definition :-

A Fluid is a substance that can flow. It ultimately assumes the shape of the containing vessel because it cannot withstand sharing stress. Both liquids and gases are fluids.

In fluids molecules are not held together in a position like solid molecules are. In solids, the strain set up under sharing stress lasts throughout the period of application of the stress. Liquids cannot stand up under share stress because they do not have set positions and they generally move randomly. When you are applying a force to the system you get a preferred motion of the atoms, molecules or particles.

Important Properties of fluids

  1. Unlike solids, the atoms or molecules in a fluid are arranged in a random manner.
  2. Fluids continuously yield to tangential or shearing stress. It begins to flow when a shearing stress is applied.
  3. A liquid has no definite shape of its own. It ultimately assumes the shape of the containing vessel. So fluids have no molecules of rigidity.
  4. A fluid can exert/withstand a force in a direction 1 to its surface. So fluids do have bulk molecules of rigidity.

Examples of fluids

Some examples of fluids consists of :-
air, blood, water, milk, honey, etc.
All these fluids can be divided under following two catagories
  1. Liquids
  2. Gases


  • A liquid is a fluid which has no shape of its own.
  • It occupies a definite volume which cannot be altered.
  • In other words, a liquid is a fluid which is quite incompressible and has a free surface of its own for example water, alcohol, honey etc.


  • A gas is a fluid which can easily be compressed when subjected to pressure.
  • On reduction of pressure on gases. They can also be made to expend indefinitely, occupying all the space made available to it.
  • Thus all of the gas will escape out of a vessel if there is the tiniest aperture in it.
  • So, gas is a fluid which has neither shape nor a free surface of its own. For example oxygen, hydrogen, air, \(CO_2\) etc.
Note:- All liquids do get compressed a little when subjected to very high pressures. The compression is almost negligible.

What is fluid mechanics

Fluid mechanics is a branch of physics. It deals with the mechanics of fluids. It has applications in a wide range of disciplines, including mechanical, civil, chemical and biomedical engineering, geophysics, astrophysics, and biology.
Fluid Mechanics can also defined as the science which deals with the study of behaviour of fluids either at rest or in motion.
In fluid mechanics we study about (i) fluid statics and (ii) fluid dynamics

Fluid statics:-

  • Fluid statics is the study of fluids at rest in equilibrium situations.
  • Like other equilibrium situations, it is based on Newton’s first and third laws.
  • Here we will study the concepts of density, pressure and buoyancy and Archimedes principle.

Fluid dynamics:-

  • Fluid dynamics deals with the fluids in motion.
  • It is one of the most complex branches of mechanics.
  • Here we deal with the concepts like stream line flow, Bernoulli’s theorem, viscosity etc.

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