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NCERT Solutions Class 11 Physics Chapter 9 Mechanical Properties of Fluids - PDF Download

JEE Mains & Advanced

NCERT Ch 9 Physics Class 11 is all about fluids and their mechanical properties. It calls for an extensive study and understanding of this section, something that you often struggle in this chapter 9. NCERT solutions for Class 11 Physics mechanical properties of fluids mainly aim to solve this common problem among students by simplifying learning and help you to score good in your exam.

The topic given in fluids class 11 is a broad chapter. To get mastered in this subject and score good marks, it is very important to understand each concept of mechanical properties of fluids class 11. NCERT Solution for Class 11 Physics Chapter 9 is your guide and tool towards your better learning experience.

Liquids and gases can flow and that is why we call them fluids. This fluid property that distinguishes liquids and gases from solids in a basic way. NCERT Solutions for chp 9 are one of the best tools to prepare for Class 11. Mechanical Properties of Fluids is an important chapter in class 11th. You must prepare this chapter in a better way to score high marks in your examination and to understand the topic in depth.

Subtopics of Chapter 10 Mechanical Properties of Fluids

  1. Introduction

  2. Pressure

  3. Streamline flow

  4. Bernoulli’s principle

  5. Viscosity

  6. Reynolds’ number

  7. Surface tension


In this chapter 9, you will study some common physical properties of liquids and gases. Liquids and gases can flow and that's why we call them fluids.This property that differentiate liquids and gases from solids in a basic way. Fluids are everywhere around our surroundings. Earth consists of air and 2/3rd of its surface is covered with water. Water is not only necessary for our existence but for everyone, every mammalian body consists mostly of water. All the processes happening in living beings including plants are mediated by fluids. Thus understanding the behaviour and properties of fluids is important for this chapter 9.


A sharp needle when pressed against human skin pierces it. Our skin, however, remains intact when a blunt object with a wider contact area is pressed against it with the same force. If an elephant were to step on a man’s chest, the man's chest ribs would crack. Let's take an example if a circus performer across whose chest a large, light but strong wooden plank is placed first, is saved from this accident. Such day to day experiences convince us that both the force and its coverage area are important. Smaller the area on which the force acts, greater is the impact of pressure. When an object is submerged in a fluid or in a water at rest, the fluid exerts a force on its surface. This force is always normal to the object’s surface. This is so because if there were a component of force parallel to the surface, the object would exert a force too on the fluid parallel to it; as a result of Newton’s third law. So, this force will cause the fluid to flow parallel to the surface. Since the fluid is not moving, this cannot happen. So, the force exerted by the fluid at rest has to be perpendicular to the surface in contact with it.

Pascal’s Law 

The scientist Blaise Pascal observed that the pressure in a fluid at rest is the same at all points if they are at the same height.

Hydraulic Machines

Whenever external pressure is applied on any part of a fluid contained in a vessel, it is transmitted full and equally distributed in all directions. This is another form of Pascal's law and it has many applications in our daily life.

Streamline Flow

In this section we will learn about fluids in rest position. The study of the fluids in motion is called fluid dynamics. When a water tap is turned on slowly, the water flow is smooth in starting, but it will lose its smoothness when the speed of the outflow is increased. In studying the motion of fluids, we have to focus our attention on what is happening to several fluid particles at a particular given point in space at a particular time. The flow of the fluid is said to be steady if at any given point, the velocity of each passing fluid particle will remain constant in time. This does not mean that the velocity at different points in space will remain the same. The velocity of any given particle may change as it moves from one point to another. That is, at some other point the particle may have a different velocity, but every other particle which passes the second point behaves exactly as the last particle that has just passed that point. Each particle follows a smooth path, and these paths of the particles do not cross each other. 

Bernoulli’s Principle

Bernoulli’s principle says that the total mechanical energy of the moving fluid comprising the gravitational potential energy of elevation, the energy associated with the fluid pressure and the kinetic energy of the fluid motion, remains constant.

Speed of Efflux: Torricelli’s Law

Efflux means fluid outflow. The deviation can be partly explained on the basis of Bernoulli’s principle. 

  • Ball moving without shows the streamlines around a non-spinning ball moving relative to a fluid. From the symmetry of streamlines it is clear that the velocity of fluid (air) above and below the ball at corresponding points is the same resulting in zero pressure. Therefore, the air, no upward or downward force on the ball.

  •  Ball moving with spin: A ball which is spinning drags air along with the ball. If the surface is not smooth more air will be dragged. 


Most of the fluids are not ideal and offer some resistance to motion. This resistance to fluid motion is like an internal friction to friction when a solid moves on a surface and it's called viscosity. This viscosity force exists when there is relative motion between layers of the liquid. Let's suppose the bottom plate is fixed while the top plate is moved with a constant velocity v relative to the fixed plate. If  oil is replaced by honey then a greater force is required to move the plate with the same velocity. Hence we can say that honey is more viscous than oil. The fluid in contact with a surface has the same velocity as that of the surfaces. 

Stokes’ Law

When an object falls through a fluid it drags the layer of the fluid in contact with it. A relative motion between the different layers of the fluid is set and, as a result, the object experiences a retarding force. Falling of a raindrop and swinging of a pendulum bob are some most common examples of these motions. It is seen that the viscous force is directly proportional to the velocity of the object and is opposite to the direction of motion. The other quantities on which the force F depends are viscosity η of the fluid and radius a of the sphere. 

Surface Tension

The property of the surface of any liquid or fluid that allows it to resist an external force, due to the cohesive nature of its molecules.

Surface Energy

A liquid stays together due to attraction between molecules. Let us consider a molecule well inside a liquid and intermolecular distances are such that it is attracted to all the surrounding molecules. This attraction results in a negative potential energy for the molecule, which depends on the number and distribution of molecules around the particular particle. But the average potential energy of each molecules is the same. This is supported by the fact that to take a collection of such molecules (mostly liquids) and to disperse them far away from each other in order to evaporate or vaporise, the heat of evaporation required is quite large.

Surface Energy and Surface Tension

Surface tension is a force per unit length or area acting in the plane of the interface between the plane of the liquid and any other substance; it also is the extra energy that the molecules at the interface have as compared to molecules in the interior.

At any point on the interface besides the boundary, we can draw a line and imagine equal and opposite surface tension forces S per unit length of the line acting perpendicular to the line, in the plane of the interfac and line is in equilibrium. To be more particular, imagine a line of atoms or molecules at the surface. The atoms on the left pull the line towards them; those to the right pull it towards them. This line of atoms is in equilibrium under tension. 

Angle of Contact

The surface of liquid near the plane of contact with another medium is in general curved so the angle between tangent to the liquid surface at the point of contact and solid surface inside the liquid is termed as angle of contact. 

Benefits of NCERT Solution for Class 11 Physics Chapter 9 PDF

All of the students study reference books alongside their NCERT books as an effort to gain further insight into the subjects. Referring to our Class 11 Physics NCERT solutions chapter 10 will help students in numerous ways, some of which are:

Simplify Learning : NCERT Solution for Class 11 Physics Chapter 9 was curated with the prime objective of helping you in your studies and exams.

Your Go-to Guide: NCERT solutions for Class 11 physics chapter 9 PDF is created by our subject teachers expert. These expert subject teachers are the masters in their field - teachers, lecturers or PhD holders in the subject.

Problem-Solving Approach: A problem-solving approach to studying by NCERT solutions for Class 11 Physics Chapter 9 PDF is designed to maximise your understanding of this chapter.

Answers all Questions: Class 11 Chapter 9 Physics NCERT solutions answer every question and clarify all your doubts for a better learning experience. Getting your doubts cleared by these solutions makes learning a fun activity.

Free Resource: Fluids NCERT solutions Class 11 is available in a PDF format for your convenience on this website.

Frequently Asked Questions

Q1. What is Bernoulli’s Principle?

Ans: Bernoulli’s Principle says that in an ideal state, the density and pressure of a fluid are inversely proportional to each other. Class 11 chapter 9 explain this property of fluids.

A point to be noted here is that fluid does not mean liquid, fluids include gas. This principle says that a fluid with high speed exerts more pressure than one that is moving slower. Bernoulli’s principle on the nature of fluids has laid the foundation for many practical applications. The design of airplane wings stands out as a great practical examples of this principle.

Q2. What is Streamline Flow?

Ans: Fluids have a natural tendency to flow separately.

These layers flow independently, without collision or disruption at any particular point. This kind of flow in fluids is known as a streamline flow. Streamline flow is a way to study fluids in motion. Our team suggest you consider referring to our NCERT Class 11 Physics Chapter 10 solutions to solidify your understanding of the concept.


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