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What is Fluid?

The scientific fluid definition is very simple to understand. Fluid meaning refers to a substance that does not have a definite shape and yields easily to external pressure. It can be a liquid or a gas. We say that a fluid can flow easily and we find its applications more in clutch or coupling, where fluid helps transmit the power supply.

In the above text, we got a brief of what is fluid and now, we will dig into the scientific definition of fluid. Now, let’s see what is fluid with some fluid information and facts about fluid science.

Fluid Examples in Everyday Life

You might have heard of the word ‘Fluid’, now, we will look at some examples of everyday use of this term to understand it better.

  • The patient is severely dehydrated, and he needs fluids urgently.

  • The tanks on the rooftop are placed for storing fluids like oil, kerosene, and gas.

  • We generally keep our household cleaning fluids out of the reach of the children.

  • Our stomach collects gastric fluids around its edge to digest food.

  • We saw some kind of fluid oozing (a viscous liquid) coming out from the trunk of the tree.

Fluid Definition Physics

In Physics, the fluid definition is:

A fluid is a substance that continually flows or deforms when we subject it to shear stress or external force. 

The fluid is one of the phases of matter and we consider fluids like liquids, gases, and plasmas. One of the scientific definitions of fluid is that these are substances with zero shear modulus, or, in simpler words, these substances cannot resist the shear force applied to them.

Fluid Information Definition

To understand fluid information definition, let’s consider the below one-line statements:

The term ‘fluid’ is more closely related to being a solid plasma; however, we consider it as liquid and gas. A fluid carries no shear modulus. Now, let’s types of fluid flow:

Fluid flow has is of the following types:

  1. Steady or unsteady

  2. Compressible or incompressible

  3. Viscous or non-viscous, and 

  4. Rotational or irrotational.

Steady or Unsteady Flow

The flow of fluid can either be steady or unsteady, depending on the velocity of the fluid:

  • Steady: In steady fluid flow, the fluid’s density remains constant at every point.

  • Unsteady: During the unsteady flow, the fluid’s velocity differs between any two points.

Viscous or Non-Viscous Flow

Liquid flow can be viscous or non-viscous.

Viscosity is a measure of the thickness of a fluid, and very sticky fluids viz: motor oil or shampoo are called viscous fluids.

Fluid Characteristics

We see ‘fluid definition’ a lot many times; however, ‘fluid’ is used in everyday life and is understood with some of its characteristics.

Now, let’s look at some characteristics of a fluid:

  1. Capable of flowing and easily changing its shape.

  2. Constantly changing or apt to variations.

  3. Smooth in movement, i.e., easy flow.

Types of Fluids

The below diagram shows the types of fluids.

[Image will be uploaded soon]

Now, let’s look at types of fluid.

The Types of Fluids are:

1. Ideal Fluid

An ideal fluid is incompressible and it is an imaginary fluid that doesn’t exist in reality. Also, it doesn’t have viscosity.

2. Ideal plastic Fluid

When the shear stress becomes proportional to the velocity gradient and more than the yield value, the fluid is said to be the ideal plastic fluid.

3. Real Fluid

Fluids having viscosity are real fluids.

4. Newtonian Fluid

A fluid obeying Newton’s law of viscosity is known as a Newtonian fluid.

5. Non-Newtonian Fluid

A fluid not obeying Newton’s law of viscosity is said to be a Non-Newtonian fluid.

6. Incompressible Fluid

When the density of the fluid remains invariant with the application of external force, it is said to be incompressible fluid.

7. Compressible Fluid

When the density of the fluid varies with the application of external force, it is a compressible fluid.

Interesting Facts About Fluids

A very common fact about fluids is that they have a constant volume, meaning that they confine within the vessel they are kept in.

Now, let’s look at a few more interesting facts about fluids.

  • A fluid that is a liquid has the following properties.

  1. After cooling liquids, we can compress them, except for ice, which expands.

  2. We cannot compress water.

  3. Liquids have three forms.

  4. The liquid form of fluid is a prerequisite of carbon-based life

  5. Most of the water present on the earth is from outer space.

  • Cats release a lot of fluid in the saliva.

  • Fluid mechanics is the study of the forces on the fluid and how fluids move. Fluid mechanics can be divided into parts: fluid statics and fluid dynamics. Fluid statics is the study of fluids at rest, and fluid dynamics is the study of fluids in motion. 

  • Fluid mechanics is a branch of continuum mechanics, a subject that models matter without using the information of what atoms they are made of.

FAQ (Frequently Asked Questions)

1.  Describe Some Fun Facts About Fluids.


  • During our average lifetime, our heart pumps about 1.5 million barrels of blood ( a fluid) which is enough to fill 200 tanks of cars and trains

  • One inch of rain (a liquid or simply a fluid) is equal to 10 inches of snow

  • Do you have any idea that amazing Helium superfluid climbs on the wall and does not follow gravity law?

  • Have you ever observed that water, fluid have a high pressure; however, when you add a handful of salt to it, its level goes down?

  • One of the interesting scientific facts is that 90 minutes of sweating can temporarily  (for the time being) shrink the brain as much as one year of ageing.

  • Do you know that when a baby is born he/she has only one cup of blood in his/her body? That’s amazing to know!

  • One of the interesting facts is, our saliva is six times more powerful than morphine.

2.  State Real-Life Examples of Fluids.

Answer:  A fluid is a continuous, amorphous substance whose molecules move freely one after another and it has the tendency to take the shape of its container; a liquid or gas in which it is placed. 

Fluids lack resistance to permanent deformation, they can resist only relative rates of deformation in a dissipative or a frictional manner. However, they bear the following applications:

  • A jam is solid, on warming it, it melts and becomes a fluid. You can spread this fluid over the cake to make it delicious.

  • A topping cream melts into a fluid that can be used to decorate pieces of bread and cakes.

  • Honey, emulsions are fluids.