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Last updated date: 17th Apr 2024
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What is Fluid?

Do you ever wonder why some substances flow? And why do some substances flow differently from others? Do only liquids flow? What are these substances? Answers to all of these questions will be answered today. All these phenomena are associated with fluids.

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.

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 in 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 understand the types of fluid flow:

Fluid flow has 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 vicious or non-viscous.

The resistance offered by a fluid to flow is known as viscosity. A fluid that offers high resistance to flowing is known as a viscous fluid and its flow is said to be vicious.

On the contrary, a fluid that flows relatively freely is said to have low viscosity or is known as a non-viscous fluid and its flow is said to be 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

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. In the real world, ideal fluid does not exist.

  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. E.g., Water suspension of clay and fly ash 

  3. Real Fluid: Fluids having viscosity are real fluids. E.g., petrol

  4. Newtonian Fluid: A fluid obeying Newton’s law of viscosity is known as a Newtonian fluid. E.g., glycerol thin motor oil.

  5. Non-Newtonian Fluid: A fluid not obeying Newton’s law of viscosity is said to be a Non-Newtonian fluid. E.g., cornflour and water

  6. Incompressible Fluid: When the density of the fluid remains invariant with the application of external force, it is said to be an incompressible fluid. E.g., The stream of water flowing at high speed from a garden hose pipe

  7. Compressible Fluid: When the density of the fluid varies with the application of external force, it is a compressible fluid. E.g., gas, steam, etc.

Interesting Facts About Fluids

A very common fact about fluids is that they have a constant volume, meaning that they are confined 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. Water has three forms - solid, liquid, and gas(vapor)

  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.

Why study Fluid from Vedantu?

Fluid is an extremely important topic in Physics. An entire branch is dedicated to its study. Here Vedantu has tried to cover the topic for you as holistically as possible. After going through this, you will gain enough theoretical understanding of the topic to tackle questions based on basics. It will also facilitate you to understand the advanced level concepts. Going through it thoroughly will not only help you fetch marks in the upcoming exams but also in other future exams which may include this topic. 


This section has discussed the topic of Fluid for you in brief. The aim was to cover the fundamentals of the topic. Here we have covered standard definitions, characteristics, types, examples, and fun facts about the fluid and related phenomenon. All the content made available to you is of the utmost quality. To fuel your curiosity further, Vedantu has also added a few fun facts in the last part of the topic. You should go ahead and make the best use of this content and many other materials made available to you by Vedantu.

FAQs on Fluid

1. How will Fluid help me in scoring marks in physics?

Fluids are extremely important in Physics. It is a part of Class 12 physics. It is also a part of the Engineering syllabus. An entire subject called Fluid Dynamics is introduced to students in engineering. Therefore it is important to have your basics clear from the beginning. This is why Vedantu has brought Fluid. By studying this, you will get your basics clear and you will be ready to learn advice concepts with ease. This will help you in scoring excellent marks in all the standards.

2. From where can I get reliable information about Fluid?

The Internet is flooded with material on Fluid, but their relevance is questionable. To help you with this problem, Vedantu has brought this topic in detail for you. The content is made by subject matter experts who have dealt with fluid dynamics in the past. All the concepts covered are factually correct and verified. It has been made sure that the topic is covered holistically and cohesion is maintained throughout. Thus Vedantu’s Fluid is the reliable material you have been looking for.

3. What topics are covered under Fluids by Vedantu?

The importance of Fluid is well known to students. It is a recurring topic in many standards, right from school to master's degree education. Following topics have been covered by Vedantu in Fluid:

  • Various definitions of a fluid

  • Daily life examples of fluids

  • Definition of fluid in Physics

  • Types of flow- stead, unsteady

  • The viscosity of a fluid- viscous, non-viscous

  • Types of fluids- ideal, ideal plastic, real, Newtonian, non-newtonian, incompressible, compressible

  • Fun facts about fluids

Though Vedantu has covered these many topics, the list isn’t exhaustive. You should go ahead and learn more about this topic.

4. What are some real-life examples of a fluid?

A fluid can be defined as a continuous, amorphous substance molecule that moves freely one after another and tends to acquire the shape of the container in which it is placed. The fluid shows an interesting property through which it resists any permanent deformation. They show relative rates of deformation by virtue of friction. Some real-life examples of fluid are which you may be aware of are as follows:

  • Honey and emulsions are fluid.

  • Fruit jams are fluids on heating. This fluid can be spread over the cake to enhance its taste.

  • Topping creams are used to decorate the pieces of bread and cake.

5. What are the principal types of fluid?

Fluid mechanics is a very vast topic that covers a variety of fluids and all of these hold significance in exams as well as real life. These are broadly classified into 7 categories which are as follows:

  • Ideal Fluid: It is an imaginary fluid that has zero compressibility and no viscosity 

  • Ideal Plastic Fluid: A fluid becomes this type of fluid when the shear stress becomes proportional to the velocity gradient and more than the yield value

  • Real Fluid: These are your regular fluids that show some viscosity

  • Newtonian Fluid: These are fluids that adhere to Newton’s laws of fluid motion

  • Non-newtonian Fluid: These are fluids that do not follow Newton’s laws of fluid motion 

  • Incompressible Fluid: The density of this type of fluid is invariant with respect to an external force applied

  • Compressible Fluid: The density of this type of fluid varies with respect to an external force applied

6. What does the flow of a fluid depend upon?

Before applying any fluid for any application, it is very important to study the factors on which its flow depends upon. The factors are taken into consideration depending on which type of fluid is being studied.  But these factors can be put under a broad umbrella of three factors. Thus, the flow of any fluid (gas or liquid) depends upon three factors: 

  1. The characteristics of the fluid 

  2. The speed of flow, 

  3. And the shape of the solid surface

Characteristics of fluids consist of their viscosity, density, and compressibility.

7.  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 has 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 aging.

  • 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.

8. State Real-Life Examples of Fluids.

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. 

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