Physics class 11 notes Fluid Dynamics Chapter 6 for kpk

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Physics class 11 notes Fluid Dynamics 2021

Fluid Dynamics Chapter 6 Physics class 11 notes 2021 pdf download, short question, long question, and numerical problems.

Q.1 Smoke rises in a chimney faster when a breeze is blowing. Use Bernoulli’s printer ciple to explain these phenomena.

As the wind blows across the top of the chimney, the pressure over there will be less than that of the inside. And Bernoulli’s principle for fluids tells us that it is the nature of the fluid that it always flows from an area of high pressure to an area of lower pressure.

When a breeze is blowing, it will create a pressure difference between inside and outside of the chimney, which in turn causes a faster rise of smoke in the chimney.

Q.2 Why do many trailer trucks use wind deflectors on the top of their cabs? How do such devices reduce fuel consumption?

Fluid Dynamics

Wind deflectors are designed in such a manner that they reduce the air resistance offered by a vehicle or trailer truck. Thus lesser the air resistance experienced by the vehicle, the lesser the amount of work it has to be done to move forward.
Owing to this reason many trailer trucks use wind deflectors on the top of their cabs because in such a way the amount of work done against the air resistance is reduced significantly. Therefore, engines have to do less work which will, in turn, lower the consumption of fuel in these trucks, especially when they have to travel in windy areas.

Q.3 Consider the cross-section of the wing on an airplane. The wing is designed such that the air travels faster over the top than under the bottom. Explain why there is a net upward (lift) force on the wing due to the Bernoulli effect.


The wing of the airplane is designed such that the air will blow faster over the top of the wing than under the bottom. Bernoulli’s equation tells us that the pressure of a fluid is less in areas where the speed of the fluid is greater. Therefore due to faster blow, the pressure over the wing of the surface will be low as compared to below the wing.

And as air always blows from the area of higher pressure to the area of lower pressure so there will be thrust from the lower side of the wing. This thrust makes it possible for an airplane to keep up in the air and not to fall under gravity because gravity is balanced by that upward thrust.

Q.4 When a fast-moving train passes a man standing on the platform at rest. He is likely to be drawn towards the train. How does the Bernoulli effect explain this phenomenon?


The speed of air near the fast-moving train is very high as compared to its speed away from the train.

According to Bernoulli’s principle, the pressure is low where the speed of air is high. Thus the pressure near the fast-moving train is less as compared to the pressure behind the man standing on the platform. Since the air flows from the region of high pressure to the region of low pressure. As a result of this, the man will feel a push towards the train. Thus he is likely to be drawn towards the train.

Q.5 If you suddenly turn on your shower water at full speed, why is the shower curtain pushed inward?


When we turned on our shower at full speed, water came out of it at high speed. And as Bernoulli’s principle tells us that pressure of the fluid is low in areas of faster flow so this will create an area of low pressure near the water flow.

Therefore the shower curtain pushed inward due to the flow of air from an area of higher pressure i.e. from behind the curtain to the area of lower pressure i.e. towards water flow.

Read more: Physics class 11 notes Rotational and Circular Motion

Q.6 A baseball moves past an observer from left to right spinning counterclockwise. In which direction will the ball tend to deflect?


As the ball is spinning as well as moving in the forward direction so the spinning will make the motion of the ball faster, due to which pressure in that region decreases. Thus the pressure to the other side of the ball will be maximum.

When the ball will hit the ground, then due to lower pressure to its side of rotation, it will get an extra curvature to that side and will swing to its direction of spinning.

Q.7 Aspherical body is dropped in two different fluids and its terminal velocity is found to be different. Give reasons.


As terminal velocity of a body of mass ‘m’ and of radius ‘r’, moving through a fluid of viscosity ‘η’ is given by,

A spherical body is dropped in two different fluids

This shows that terminal velocity and the viscosity of fluid has an inverse relation. As the viscosity of different fluids is different so if we drop the spherical body in the fluid having a higher value of viscosity, it will have a lower value of terminal velocity.

Similarly, if we drop the body in the fluid of lower viscosity, it will have a higher value of terminal velocity.

This is the reason that when a spherical body was dropped in two different fluids then its terminal velocity was found to be different.

Q.8 A liquid passed through a pipe and it was found that the rate of the influx was equal to the rate of efflux. What information do you get about the liquid?


If a liquid passes through a pipe and its rate of influx is equal to the rate of efflux, it means that liquid has a steady-state flow. The liquid having such flow is in-compressible and will have a uniform density as well.

Q.9 An incompressible liquid is passed through a horizontal pipe and it is observed that the speed of a mass of liquid when it emerges is greater than its speed when it enters the pipe. How can it be possible?


As equation of continuity for in-compressible fluid tells us that,                                                  

  A1v1 = A2v2

k 2

Equation (1) shows that velocity of the emerging fluid is inversely proportional to the area of emerging end of the horizontal pipe.

So, if the fluid passes through the horizontal pipe and its speed when it emerges is greater than its speed when it enters then it means that the cross-sectional area of the emerging end is less than that of entering.

i.e.                                                A2 < A1

Q.10 Why does the pipe of a paper squeeze when air is blown through it?


When we blow air through the pipe of paper then it squeezes.

As Bernoulli’s principle tells us that when the velocity of the fluid in a certain area increases then pressure in that region decreases. So by blowing air in the pipe the velocity of air increases inside the pipe and so there will be a drop in pressure. To level this drop with external pressure the air from outside the pipe will try to get inside and that is the reason that the pipe squeezes when the air is blown through it.

Q.11 When waterfalls from a tap, its cross-sectional area decreases as it comes down. Explain why?


The equation of continuity tells us that the product of the area and velocity of the fluid is constant. Mathematically;

                                            A1v1 = A2v2

This means that area is inversely proportional to the velocity of the fluid so, when waterfalls from the tap its velocity increases due to gravity. An equation of continuity requires its area to squeeze with an increase in velocity to keep its product constant.

So that is the reason that the cross-section area becomes more and more narrow as the water falls down due to gravity.

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Q.12 Why does a car have an oblong shape design?


We know that all fluids offer resistance if we have to move through them. So when a car moves, it has to overcome the resistance offered by the air. That is the reason that a car is designed in oblong shape so it can reduce the air resistance during its motion. As a result of which it attains a high speed with less consumption of energy.

Q.13 How do the pulsations in pulse show the heartbeat?


Pulsation indicates the change in blood pressure inside veins. When the heart pumps the blood, the pressure inside the veins will vary according to the speed of blood. Increase in pressure will make vessels expand and decrease in pressure will squeeze the veins.

This is the reason that we feel the ups and down in pulsation when we touch a vein. This pulsation is equal to the heartbeat because each time the heart pumps it will create a pressure difference and in return each time we will feel a pulse.

Q.14 Describe the working of an engine carburettor and paint spray.

(a)     Working of an Engine Carburetor:

 Engine carburettor works on the same principle as that of the filter pump. It follows Bernoulli’s principle in order to supply a cylinder with a mixture of air and fuel. The engine draws air through its horizontal pipe which is connected to the fuel tank and sends them to the cylinder.

When the air flows through the pipe with high speed, the pressure inside the pipe will drop. As a result, fuel will move to the pipe and it will form a mixture of the air and fuel which will then be supplied to the engine in form of a spray.

(b)     Working of Paint Spray:

 Paint spray machines also work on Bernoulli’s principle. When we push the spray machine, this will increase the pressure above the liquid surface than inside the spray machine. As a result, the liquid will move up in a vertical pipe inside it because fluid has a nature to flow from an area of high pressure to an area of low pressure.

Then there will be a spray of fluid from the nozzle having a small cross-sectional area with a high speed.

Comprehensive Questions

Q.1 What is a fluid? Discuss briefly, the viscosity of a fluid and explain how the flow of a fluid is characterized by viscosity?



    “A substance which flows is called a fluid”

Some examples of fluids are water, oil, gas etc.


    “The resistive force between different layers of the fluid during its flow is known as viscosity”

For a flowing fluid, the viscosity is a characteristic of the fluid. We can say that the flow of a fluid is characterized by its viscosity as it determines the rate of flow of fluid.

Coefficient of Viscosity:

                         The numerical value of resistance to the flow of a fluid between its layers is known as the coefficient of viscosity. It is represented by ‘η’.

More will be the value of ‘η’, greater will be its viscosity and harder it will be to get through such fluid. Its value varies from fluid to fluid.

The viscosity of a fluid depends on the temperature of the fluid.

                    In liquids, viscosity exists due to intermolecular forces. When we heat up a fluid, the strength of these forces decreases with the rise of temperature and so as the viscosity.  

In the case of gases, it depends on the velocity of the molecules. With the increase in temperature, molecules of gas will have more energy and will flow with greater velocities. As a result, the viscosity of gas will decrease.

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