11th Class Physics Chapter 6 Fluid Dynamics Short Questions Answer

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11th Class Physics Chapter 6 Fluid Dynamics Short Questions Answer

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1. Explain what do you understand by the term viscosity?
An opposing force comes into play when one layer of fluid moves relative to another layer. The internal friction between two layer of a fluid in relative motion is called Viscosity(or fluid friction ).
2.What is meant by drag force? What are the factors upon which drag force acting upon a small sphere of radius r ,moving down through a liquid ,depend?
The retarding force experienced by an object moving through a fluid is called a drag force. It increases with a increase of speed of the object .
As a drag force ‘F’ on a sphere of radius ‘r’ moving slowly with speed ‘v’Through a fluid of viscosity ‘π’ is given by stoke ,s law as under,
According to the formula ,the drag force depends upon the following factors:-
1.Speed the sphere
2.Radius of sphere
3.Co. efficient of viscosity
3. Why fog droplets appear to be suspended in air?
A fog dropt is a tiny drop having a very small weight. As the droplet falls, the drag force acting on it becomes equal to the weight of droplet then net force acting on it is zero. Thus , droplet will fall with terminal velocity (constant speed) whose value is very  small. Therefore, the droplet drops so slowly that it appears to be suspended in air .
Note:- Terminal velocity is given by the formula
Vt=mg/6πɳr       i.e            vt    α mg
It shows, smaller the weight of sphere,smaller will be the terminal velocity
4. Explain the difference between laminar flow and turbulent flow?
Laminar flow:- The flow is said to be laminar, it every particle that passes a particular point, moves along exactly the same path ,as followed by particles which passed that points earlier. In this case each particle of the fluid moves along a smooth path called streamlined or laminar. Turbulent Flow The irregular or unsteady flow of the fluid is called turbulent flow.In this case , the exact path of the particles of the path cannot be predicted as the velocity of the fluid may change abruptly .In turbulent flow ,there is a great disorder and a constantly changing flow pattern.
5 A person is standing near a fast moving train. Is there any danger that he will fall towards it?
Yes ,there is a danger that the man may fall towards a fast moving train.When a train is moving fast, the velocity of air between the persons and the train also increases as the train drags it. according to a result of Bernoulli,s equation, when velocity of a fluid increases, its pressure decreases.Thus,the pressure of the air brtwween the person and train decreases.The greater pressure behind the man may push him towards low-pressure side i.e. train. So there is a danger that bi may fall towards it.
6.Identify the correct answer. What do you infer from Bernoulli’s theorem? (i) Where the speed of the fluid is high the pressure will be low. (ii)Where the speed of the fluid is high the pressure is also high (iii)This theorem is valid only for turbulent flow of the liquid.
The correct answer is (i) where the speed of the fluid is high the pressure will bi low.
According to the relation of Bernoulli’s theorem
P1 ⁺1/2pv12=P2+1/2 pv22
7.Two row boats moving parallel in the same direction are pulled towards each other?
When two boats are moving parallel in the same direction, the velocity of the water flowing through them will be larger as compared to rest of the water. According to the result of Bernoulli’s equation, the pressure of water between the boats must decrease as its velocity is larger. As a result the pressure of water on the outer sides is greater than inner side between the boats due to which two boats are pulled towards each other.
8. Explain, how to swing is produced in a fast moving cricket ball?
When a fast moving cricket ball moves in such a way that it spins as well as moves forward,the velocity of on oneside above the ball increases due to spin and air speed in the same direction below the ball and hence, the pressure decreases. according to Bernoulli’s theorem, the pressure of the air bellow the ball is greater than that above the ball. The greater pressure on the other side below the ball deflects the path of ball and give an extra curvature to the ball known as swing which deceives the batsman.
9.Explain the working of carburetor of a motorcar using Bernoulli’s equation?
The carburetor of a car uses a ventury duct to feed the correct mixture of air and petrol to the cylinders.Air is drawn through duct along a pipe to the cylinders. A tiny inlet at the side of the duct is fed with petrol. The air through the duct moves very fast ,creating a low pressure in the duct which draws petrol vapours into the air stream.
10. For which position will be maximum blood pressure in the body have the smallest value?(a) standing up right (b)sitting (c)lying horizontally(d)standing on one’s head ?
Correct answer is (c) Maximum blood pressure in the body has the smallest value when lying horizontally, because in this position ,all the parts of the body will be in level with the heart. Thus, heart will not have to work heard as pumping against gravity.
11.In an orbiting space station would the blood pressure in major arteries in the leg ever be greater than the blood pressure in major arteries in the neck
Blood pressure will remain the same due to stat of weightlessness in an orbiting space station.
12.A meteor burns into ashes when enters into outer atmosphere. Why?12.A meteor burns into ashes when enters into outer atmosphere. Why?
when a meteor enters into earth’s atmosphere, it will burn due to the friction between the particles of the atmosphere and meteorite. Hence, the energy possessed by a meteor is converted into heat energy and its momentum is transferred to the particles of atmosphere.
13.A spherical body is dopped in two different fluids and its terminal velocity is found to be different. Give the reasons?
14.A liquid was passed through a pipe and it was found that the rate of influx is equal to the rate of efflux .what information do you get about liquid?
From the above statement e get information about the liquid that it is incompressible, its density is constant and has a steady flow, such a statement is represented by the equation of continuity. Mathematically it is stated as
15.An incompressible liquid is passed through a horizontal pipe and it is observed that the speed of a mass of liquid when emerges is greater than its speed when it enters the pipe. How can it be possible?
As the liquid emerges from the pipe,the diameter of emerging stream is decreased and becomes less than inner diameter of pipe. This is because there is surface tension that tends to decrease the diameter of stream. This inside and outside the pipe increases the velocity of liquid in order to maintain the same rate of flow.
16 Why does the pipe of paper squeeze when air is blown through it?
When we blow under the paper ,the pressure on the underside gets decreased ,and the atmospheric, pressure at the topside will bend the paper and thus paper will not remain horizontal but will be squeezed.
17. Why does a car has oblong shape design?
Cars are streamlined for giving a special shape to them .This is done to ensure that flow of air around the car (oblong shape design) as streamlined even when the car moves at a very high speed .This helps in smooth running of the car and therefore less energy is wasted against drag force.
18.How does the pulsation in plus show the heart beat?
When heart pumps into the blood ,it squeeze and pressure in arteries rise momentarily upto120(Hg). After that the heart relaxes and so the pressure decreases to about 80 mm(Hg).This periodic rise and fall of the pressure is called plus.
19.When water falls from a tap, its cross-sectional area decreases as it comes down. Why?
As the water falls out of tap, its velocity increase due to g (acceleration due to gravity).As the velocity of water increase ,the diameter and hence area of cross section of stream decreases, so that rate of flow of water is same at all points.
20.It is often seemed that leaves laying on the road start following the fast moving car when it passes through the road. Why?
When the fast moving car passes through the road, the leaves laying on the road are pushed towards the car. This is because as the car moves at high speed, the pressure between leaves laying on the road and the car decreases .Therefore, the air behind the leaves which is at atmosphere pressure pushes the leaves towards the car, Thus, the leaves follow the fast moving car when it passes through the road.
21.What is co-efficient of viscosity. Write its unite?

Co-efficient of viscosity of the liquid is a defined as the tangential force per unit area required to maintain a unit relative velocity between its two layers. At unit distance apart.

Mathematically, it is expressed by a formula as given below
Where F is opposing tangential force is distance between two layers, A is surface area and v is velocity .
Its SI Unit:-
Its unit is kg  m-1s-1

22.State Bernoulli’s equation?

Bernoulli’s equation states that the sun of pressure, the kinetic and potential energies per unit volume in a steady flow of sun incompressible and non-viscous (fristionless) fluid remains constant at every points of its path.

Mathematically, it is expressed as
P+1/2 pv2 +pgh=Constant
Where ‘p’ is the pressure and v is the velocity and ‘h’ is the height of fluid under consideration of its path p is the density and ‘g’ is the acceleration due to gravity

23. Write two application of Bernoulli’s theorem?
Bernoulli’s equation is expressed in venture’s relation for horizontal pipes which is used in venture-meter, a device used to measure speed of liquid flow.  Ventura relation can be expressed as
(2) Left on an Aero plane:-
Bernoulli’s effect is applicable to the left of an aero plane. The wings of an aero plane are designed to deflect the air so that the streamline are close together above the wing than below it. Thus, the  air is faster on the upper side of wing than on the lower. The
pressure will be lower at the top of wing and as the wing will be forced upward
24.State stokes’s law .Is it applicable at high speed?
Statement: It states that the drug force ‘F’ on a sphere of radius ‘r’ moving slowly with velocity ‘v’ in a fluid of viscosity ‘ɳ’ is by given
A high speeds the force is no longer proportional to speed. But FαV2,Therefore,the stock’s law is not applicable at high speed.
25. (A)Define fluid dynamics (B)Define fluid statics.
(a) Fluid Dynamics:- The branch of physics which deals with the study of fluids in motion is called as fluid dynamics. (b)Fluid statics:- It is that branch of physics which deals with the study of properties of fluid at rest. Like other equilibrium problems, it is based on Newton’s laws of motion.
26. (a) What is meant by drug force(or viscous drag) (B)What are factor’s upon which the drag force in a small sphere of radius ‘r’ moving through a liquid depend ?
(a) Drag force :- A object moving through a fluid experiences a retarding force called drag force. The drag force F is given by stokes’s law as F=6πrɳv (b)It is clear from the above relation that drag force depends upon the following factors (i)The drag force depends the speed of the object i.e.it increases as the speed of the object increases. (ii)The drag force increases as the co-efficient of viscosity ’ɳ’ increases (iii)The drag force increases with the increase of radius ’r’ of the sphere.
27. What do you mean by terminal velocity?
when weight of the falling body and drag acting on the body become equal in magnitude, the velocity of the body becomes maximum, It is denoted by ‘Vt’
Mathematically it is expressed as

28. What is venture ‘s’ effect?
The effect of the decrease in pressure with the increase in speed of the fluid in a horizontal pipe is known as venture’s effect.
A well –known venture ,s relation is expressed as
which is used in Venturi-meter.
29.What is venture-meter?
It is a device used to measure speed of liquid flow. It has been prepared by the use of venture‘s relation.
30. Define equation of continuity?
For a steady flow of incompressible fluid, it states that the product of cross sectional area of the pipe and the fluid speed at any point along the pipe is a constant. Thus (i.e. Av) is equal to the volume floor per second of the fluid or simply flow rate. Mathematically, equation of continuity is expressed as AlVl=A2V2=constant As we know that (velocity=Distance/time AlVl=Al. Sl/t=A2.S2/t But area*length=Al*Sl=Volume=Vl V1/t=V2/t=Constant Hence volume flow per second (flow rate)is constant
31.What is torr ? Write down the value of torr in SI system?
Torr is the SI unit used for the measurement of human blood pressure. The pressure varies from a high (systolic)pressure of 120 torr (mm of Hg) to a low (diastolic) pressure of about 75-80torr between beats in a normal, healthy person.
32.What is Torricelli’s Theorem?
33.How are the blood pressure and the speed of liquid flow are measure?
The blood pressure is measured by an instrument called sphygmomanometer. They speed of liquid flow is measured by Venturimeter.

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  1. What you are driving a small car on the freeway and truck passes you at a high speed, you feel pulled toward the truck, why??

    1. the fluid should be steady.
      the fluid should be non-viscous.
      the fluid should be incompressible.

    2. Conditions are the following:
      1. Fluid should be non viscous
      2. Fluid should be incompressible.
      3. Fluid motion should be laminar or steady.

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