FRICTION

JKBOSE Solutions for Class 8 Science

JKBOSE  Solutions for Class 8 Science :

 JKBOSE solutions have been provided below to aid the students with answering the questions correctly, using a logical approach and methodology. The solutions provide best and smartest material to enable students to form a good base and improve the fundamentals of the subject. 

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JKBOSE Solutions for Class 8 Science

The links below provide the detailed solutions for JKBOSE science class 8 textbook.

JKBOSE Solutions for Class 8 Science

JKBOSE textbooks are prescribed by JK BOARD OF SCHOOL EDUCATION as the best books for preparation of the school as well as board examinations. The textbooks are considered as more than enough, without any aid from other refreshers. The solutions are made and designed keeping in mind the lucid language and the simplicity of the explanations that are given in the NCERT textbooks. These are not just the board and school examinations, NCERT textbooks are known to play a very important role in JEE and NEET.

JKBOSE science book class 8 acts as a stepping stone for further higher classes where u might feel that the topic is out of your league and hard to comprehend. The concepts of science social science math syllabus form the backbone of higher classes. JKBOSE class 8 science solutions help to clear our basic concepts and make them very easy and interesting for the students.

We prepared JKBOSE Solutions as per CBSE remodeled assessment structure.  We have discussed and elaborated extensively how are different food crops produced. Which living organisms do we see under a microscope in a drop of water etc ? Are some of our clothes synthetic? How do we conserve biodiversity? What is the internal structure of a plant? and many more.

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CHAPTER       14    FRICTION

FRICTION

When different surfaces comes into contact and walk or slide over one another friction comes into play Which is the force that opposes the motion of a solid object over another. static friction, sliding friction, rolling friction, and fluid friction are the different types of friction.

 Friction and force are always directly proportional to the contacting surfaces, and it doesn’t depend on the hardness of the contacting surface but With the increase in relative speeds, the sliding friction reduces, whereas fluid friction increases with the increase in the relative speed and fluid friction is dependent on the fluid’s viscosity.

Friction is defined as the resistance offered by the contacting surfaces  when they move past each other and Friction provides traction that is needed to walk without slipping. 

The factors affecting the frictional force are mainly affected by the surface texture and the amount of force impelling them together.

We are not completely sure what causes friction but it is thought to be caused by the interactions between the tiny ups and downs on surfaces as they rub against each other, These ups and downs or bumps on each surface bend and exert a force on each other making it hard for the surfaces to slip over each other.

The Following are the laws of friction:

Frictional force always comes into play against the motion. i.e it opposes the motion.

It acts tangentially along the surface of contact.

Force of friction does not dependent of area of contact of two bodies.

The nature of the surfaces in contact determine the friction.

Let's describe Types Of Friction

Following are the friction types which depend on the types of motion:

 

Static Friction

It is defined as the frictional force that acts between the surfaces when they are at rest with respect to each other and does not move.

The magnitude of the static force is equal in the opposite direction when a small amount of force is applied. When the force increases, at some point maximum static friction is reached.

Static Friction Examples

Following are the examples of static friction:

Skiing against the snow

Creating heat by rubbing both the hands together

Table lamp resting on the table

 

Sliding Friction

What Is Sliding Friction?

Sliding friction is defined as the resistance that is created between any two objects when they are sliding against each other.

Examples Of Sliding Friction

Following are the examples of sliding friction:

Sliding of the block across the floor

Two cards sliding against each other in a deck

 

Rolling Friction

 

What Is Rolling Friction?

Rolling friction is defined as the force which resists the motion of a ball or wheel and is the weakest types of friction.

Examples Of Rolling Friction

Following are the examples of rolling friction:

Rolling of the log on the ground

Wheels of the moving vehicles

 

Fluid Friction

 

What Is Fluid Friction

Fluid friction is defined as the friction that exists between the layers of the fluid when they are moving relative to each other

Examples Of Fluid Friction

Following are the examples of fluid friction

The flow of ink in pen.

Swimming

FRICTION AS NECESSARY EVIL

Friction is said to be a necessary evil because it is useful as well as harmful. Friction helps us to walk, write, hold things, lift objects. So it is necessary for our lives. Without friction, many essential processes can not be done. On the other hand, it is also unnecessary at times. It sometimes results in loss of energy, wear and tear of parts of machinery, due to which it also damages things. So it is an essential evil.

FRICTION CAN BE REDUCED BY:

lubricating the surfaces.

use of ball bearings (i.e. replacing sliding friction with rolling friction)

streamlining the body.

Friction is not always an advantage. It is a necessary evil. There are situations when we want to reduce friction. Let find out the methods of reducing friction with some examples.

Methods of reducing friction

1. Make surfaces smooth by grinding

2. Lubrication

3. Streamlined body

4. Reduce pressure or weight on the object

5. Reduce contact between surfaces by electrostatic magnetic levitation etc

6. Use rolling friction instead of sliding friction

7. Use fluid friction instead of the dry friction

 

JKBOSE TEXTBOOK QUESTIONS

Question 1. Fill in the blanks.
(a) Friction opposes the ______ between the surfaces in contact with each other.
(b) Friction depends on the ______ of surfaces.
(c) Friction produces ______
(d) The sprinkling of powder on the carrom board _______ friction.
(e) Sliding friction is _______ than the static friction.
Solution: (a) relative motion (b) smoothness (or irregularities or nature) (c) heat (d) reduces (e) less

Question 2. Four children were asked to arrange forces due to rolling, static and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding             (b) rolling, sliding, static
(c) static, sliding, rolling             (d) sliding, static, rolling
Solution: (c) static, sliding, rolling.

Question 3. Alida runs her toy car on a dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper.
Answer:  (a) wet marble floor, dry marble floor, newspaper and towel.

Question 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Answer: Frictional force will act upward, i.e., the direction opposite to that of sliding book.

Question 5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer: The layer of soap makes the floor smooth due to which the friction is reduced. This makes the floor slippery and the foot cannot make a proper grip on the floor. Therefore it is difficult to walk on a soapy floor. We may slip on the floor.

Question 6. Explain why sportsmen use shoes with spikes.
Answer: Sportsmen use shoes with spikes to increase the friction between shoes and the surface. So the shoes with spikes do not slip while the sportsmen run and play.

Question 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer: A heavy object produces more friction as it is pressed hard against the opposite surface. So Seema will have to apply a larger force.

Question 8. Explain why sliding friction is less than static friction.
Answer: The sliding friction is less than static friction because the sliding object get less time to interlock into the contact points on the floor. So it is somewhat easier to move an object already in motion than to get it started.

Question 9. Give examples to show that friction is both a friend and a foe.
Answer: Some points are given below which show that friction is both a friend and a foe:

Friction as a friend:

It allows us to grip and catch any object.

It helps us to walk comfortably on the floor.

It helps to minimise the speed or to stop any moving object.

It helps us to write.

Friction as a foe:

It causes wear and tears in objects.

It causes damage to the parts of machines and tools which further require money to get them repaired.

It reduces the speed of moving objects, so more force is required.

It produces hurdles in moving any object freely.

Question 10. Explain why objects moving in fluids must have special shapes.
Answer: The objects moving in fluids must have a special shape to overcome the fluid friction acting on them. Efforts are therefore made to minimise the friction, so objects are given special shape having pointed fronts with little broader middle portion which gets tapered at the back called streamlined.

 

Activity 1
Gently push a book on a table [Fig. 12.2(a)]. You observe that it stops after moving for some distance. Repeat this activity pushing the book from the opposite direction [Fig. 12.2 (b)]. Does the book stop this time, too? Can you think of an explanation? Can we say that a force must be acting on the book opposing its motion? This force is called the force of friction.

Friction opposes relative motion between the surfaces of the book and the table
Solution:
Yes, the book stops this time too.
After sliding for some time, the book stops. As the surface of the table provides the frictional force to oppose the sliding • of the book.
Yes, we can say that a force must be acting on the book opposing its motion.

Activity 2
Tie a string around a brick. Pull the brick by a spring balance (Fig. 12.3). You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.

Now wrap a piece of polythene around the brick and repeat the activity. Do you observe any difference in the readings of the spring balance in the above two cases? What might be the reason for this difference? Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?
Solution:
It is observed that the reading of the spring balance in the case when brick is wrapped with a piece of poly thene is less than the other cases.
The reason for this difference could be that in the first case, since the brick surface in contact with the ground is rough, the surface gets greater force of friction. But in second case, when the brick is wrapped with polythene, the surface gets smoother. Hence, less frictional force is required to move it.
By wrapping a piece of jute bag around the brick, it is observed that the reading of spring balance is more than the second
case, when the brick was wrapped with polythene.

Activity 3
Make an inclined plane on a smooth floor, or on a table. You may use a wooden board supported by bricks, or books. [Fig. 12.4 (a)]. Put a mark with a pen at any point A on the inclined plane. Now let a pencil cell move down from this point. How far does it move on the table before coming to rest? Note down the distance. Now spread a piece of cloth over the table. Make sure that there are no wrinkles in the cloth. Try the activity again [Fig. 12.4 (b)].

The pencil cell covers different distances on different surfaces Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.
Solution:
The distance covered by the pencil cell is different every time because of the different amounts of friction provided by different surfaces. The distance covered with a thin layer of sand is least, as this surface provides the maximum friction.

Questions and Answers

Question 1. Define friction.
Answer: Friction is the force which resists the motion of a body, while moving on a surface.

Question 2. What is meant by sliding friction ?
Answer: When one surface is sliding over the other, sliding friction comes into play.

Question 3. When does static friction come into play ?
Answer: Static friction comes into play to counter balance the applied force on the body.

Question 4. What are lubricants ?
Answer: The substances which reduce friction are called lubricants.

Question 5. What is rolling friction ?
Answer: When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

Question 6. What are fluids ?
Answer: Fluids is the common name given to gases and liquids.

Question 7. Do fluids exert friction ?
Answer: Yes, fluids also exert friction.

Question 8. What is meant by air resistance ?
Answer: There is friction between a moving object and the air through which it moves, known as air resistance.

Question 9. What is meant by drag ?
Answer: The frictional force exerted by fluids is known as drag.

Question 10. Is friction a contact force or non-contact force ?
Answer: Contact force.

Question 11. Why is it difficult to walk on ice ?
Answer: It is difficult to walk on ice because the friction between feet and ice is less,

Question 12. When a boat is moving in the river, does the water offer force of friction to its movement ?
Answer: Yes, the water opposes the movement of the boat.

 Questions and Answers

Question 1. When your writing desk is tilted a little, a book kept on it starts sliding down. Mention the direction of the frictional force acting on it.
Answer: The frictional force acts in the upward direction.

Question 2. You spill a bucket of soapy water on a marble floor accidently. Would it make easier or more difficult for you to walk on the floor ? Why ?
Answer: It would be more difficult to walk on the floor with soapy water because when we walk on a soapy floor, there is little friction between the floor and the feet, so it becomes difficult to walk.

Question 3. Put a tick mark on the correct choice :
When one object moves over the surface of other object.

surface of only lower object exerts frictional force on the upper object.

surface of only upper object exerts frictional force on the lower object.

surfaces of both the objects exert force on each other.

surfaces of both the objects exert force in a direction opposite to each other.

Answer: Surfaces of both the objects exert force in a direction opposite to each other.

Question 4. Four children have got similar toy cars. They run the car on the dry marble floor, wet marble floor, newspaper and towel simultaneously. The force of friction between the car and different surfaces in increasing order will be :

Wet marble floor, dry marble floor, newspaper and towel.

Newspaper, towel, dry marble floor, wet marble floor.

Towel, newspaper, dry marble floor, wet marble floor.

Dry marble floor, wet marble floor, towel, newspaper.

Answer: Wet marble floor, dry marble floor, newspaper and towel.

Question 5. Explain why sportsmen use shoes with spikes.
Answer: Sportsman use shoes with spikes to increase the friction. In this way, the sportsman can run faster.

Question 6. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the Who will have to apply a larger force and why ?
Answer: Seema will have to apply more force because the weight of the box increase and the friction increases. For a heavier box the ridges of the surfaces get interlocked more.

Question 7. Explain why sliding friction is less than static friction.
Answer: Sliding friction is less than static friction because it comes into play once the body has started sliding. To make the body slide, more force is required to overcome the irregularities in the surface of the body.

Question 8. Explain why objects moving in fluids must have special shapes.
Answer: Fluids also exert a frictional force on the body moving through them. Therefore, the shape of the body is streamlined to over the frictional force of fluids and to move smoothly through them.

Question 9. Why does the force of friction exist ?
Answer: Friction is caused by the irregularities on the two surfaces in contact.

Question 10. Can we have surfaces with zero friction ?
Answer: No, the friction can be less in some surfaces, but it can never be zero.

Question 11.Which surface have more friction – rough or smooth ? Why ?
Answer: Rough surfaces have more friction because they have larger number of irregularities.

Question 12. Why is it possible to write on rough blackboard with chalk ?
Answer: The chalk is rubbed on the rough blackboard while writing. In the process soft chalk particles get stuck to the irregularities of rough blackboard susfare making the writing visible.

Question 13. A ball is set rolling on the ground. Will it stop by itself ? Why ?
Answer: The ball will stop after sometime because the force of friction is acting from the opposite direction.

Question 14. Why do the soles of shoes wear out ?
Answer: The soles of shoes wear out due to the friction between the soles and the ground.

Question 15. Why are the soles of shoes grooved ?
Answer: Soles of shoes are grooved so as to provide better grip between the shoes and the ground.

Question 16. Give two examples where rolling friction is utilized.
Answer: Rolling friction is utilized by using ball bearings between the hub and the axle of a ceiling fan and the bicycle.

Question 17. What is function of ridges on your palm and bottom of your feet ?
Answer: The function of the ridges is to increase the friction.

Question 18. Name the two factors on which the magnitude of frictional force depends.
Answer: Frictional force depends on

The weight of the body and

The nature of the surface in contact.

Question 19. Give two methods by which friction is reduced in the wheels of the car.
Answer: The wheels of the car are provided with ball bearings along the axle which reduce the friction. The tube and tyres are inflated which reduce friction.

Questions and Answers

Question 1. Give examples to show that friction is both a friend and a foe.
Answer: Friction is a friend in the following ways — it helps us to walk, to write on the blackboard and to apply the brakes of a car.

Friction is a foe in the following ways — it causes wear and tear of machinery and shoes, it produces heat in the machinery.

Question 2. Why does friction increase if the two surfaces are pressed ?
Answer: Since the friction is due to the interlocking of irregularities in the two surfaces which slide with respect to each other, it increases when the two surfaces are pressed harder.

Question 3. Why it is easier to move a box which is already moving than one which is at rest ?
Answer: When the box starts sliding, the contact points on its surfaces, do not get enough time to lock, into the contact points on the floor. So, the sliding friction is slightly less than static friction.

Question 4. Why does a car stop when brakes are applied ?
Answer: In cars, the friction is increased by using brake pads in the brake system. The brake pads do not touch the wheels but when we press the brake level, these pads arrest the motion of the run due to friction and the wheels stop moving.

Question 5. Give three factors on which the frictional force in fluids depends.
Answer: The frictional force in fluids depends on :

The speed of object with respect to the fluid

The shape of the object.

Nature of the fluid.

Question 6. A labourer has to push a heavy trunk. What should be done to reduce his effort in pushing it and why ?
Answer: The labourer should put some cylindrical logs of wood under the trunk. In this way the area of contact between the two surfaces is reduced. Rolling friction is less than sliding friction.

Question 7. Give three harmful effects of friction.
Answer: Friction generates heat, for e.g., between the parts of machinery.

Friction causes wear and tear of the rubbing surfaces e.g., soles of shoes wear out.

Friction reduces the speed of the body and increases the consumption of energy.

Question 8. Why is friction essential ? Give three examples.
Answer: Friction is necessary between the soles of our feet and ground so that we can walk without slipping.

Friction helps us in writing on paper.

Friction makes it possible to light a match stick, by rubbing it along the rough surface of the match box.

Question 9. How do trees help reduce friction ? Why do meteors bum on entering the atmosphere ?

Answer: Trees slow down strong winds and save us from wind storms.

The speed of the meteor is very high. Therefore, there is a large amount of friction between the air and meteor. This generates a lot of heat and the meteor starts burning.

Question 10. Gaurav went to the market on his bicycle. He found that he had to apply a lot of force while pedalling. Why ? What can he do ?
Answer: Gaurav had to apply more force because there was more friction between the parts of the bicycle. He should apply oil to the parts to reduce the friction.

Questions and Answers

Question 1. Fill in the blanks :

Friction opposes the ………….. between the surfaces in contact with each other.

Friction depends on the ………….. of surfaces.

Friction produces …………..

Sprinkling of powder on the carom board ……….. friction.

Sliding friction is …………… than the static friction.

Answer: Relative motion, Nature, Heat,  Reduces, Surface area.

Question 2. Give five examples to show that friction is increased deliberately.
Answer: Atheletes wear spiked shoes to help them run fast.

Mountaineers wear spiked shoes while walking on snow.

The tyres of vehicles are grooved.

The roads are made rough.

The blackboard is made rough.

Question 3. Why are heat resistant tiles laid along the surface of spaceships ?
Answer: The spaceships are sent into the space from the rockets with a very high speed. This creates a very high frictional force between the air and spaceship. As result, the spaceship gets heated to red hot. The heat resistant tiles save the spaceship from burning.

Question 4. What is meant by lubrication ? Why do we lubricate the moving parts of our vehicles ? Name some lubricants and where they are used.
Answer: Lubrication means oiling the machines from time to time. Lubrication of the moving parts is done to reduce friction between them. We can use oil or grease in machines. Graphite can be used in machines which reach a very high temperature.

Question 5. What is meant by “streamlined shape” ? Where and why is this shape used ? To whom has nature provided streamlined body ?
Answer: Streamlined shape means they are narrow in front and broader at the back. The bodies of aeroplanes, missiles and rockets are streamlined to reduce friction with air. Ships and boats have a streamlined shape to reduce friction with air. Nature has given birds and fish streamlined bodies.

MCQs

Question 1. Friction, that exists between two surfaces in contact, when there is no relative motion between them, is called
(a) sliding friction            (b) static friction
(c) viscous drag               (d) rolling friction
Answer:  (b)

Question 2. Four students were asked to arrange forces due to rolling, static and sliding frictions in an increasing order. Their arrangements are listed below. Choose the correct arrangement. [JKBOSE]
(a) rolling, static, sliding             (b) rolling, sliding, static
(c) static, sliding, rolling             (d) static, rolling, sliding
Answer: (b)

Question 3. The energy required to overcome friction is mainly converted into
(a) sound energy                  (b) heat energy
(c) light energy                     (d) chemical energy
Answer: (b)

Question 4. Out of the following, the better lubricant to be used in the moving parts of a machine,
(a) water                            (b) air
(c) chalk powder              (d) turpentine oil
Answer:  (d)

Question 5. Force of friction is more in
(a) marble tiles                       (b) wooden floor
(c) playground                        (d) glass table
Answer:  (c)

Question 6. Once a body starts moving on table, the friction which comes into play is
(a) static friction                  (b) sliding friction
(c) limiting friction              (d) none of these
Answer: (b)

Question 7. The force of friction that comes into play when one body rolls over another surface, is
(a) sliding friction                               (b) limiting friction
(c) rolling friction                                (d) static friction
Answer: (c)

Question 8. These days we use suitcases with wheels because
(a) they look smart                  (b) they are easy to carry
(c) they make less noise a      (d) none of these
Answer: (b)

Question 9. Tyres have cut grooves in them
(a) to increase friction                      (b) to decrease friction .
(c) to make them look attractive    (d) to save rubber
Answer: (a)

 

 

Class 8 Science Chapter 1 

Microorganisms: Friends and Foe

This chapter manages destructive and valuable kinds of microbes and their significance in different areas. The places where microorganisms live, microorganisms and us, harmful microorganisms, food preservation, nitrogen fixation, and nitrogen cycle are a portion of the primary topics covered in this chapter.

Class 8 Science Chapter 2 

Coal and Petroleum

Coal and petroleum manage the two most imperative fuels ever found by humankind. The chapter additionally explains natural gas and how the characteristics assets are getting depleted because of huge and uncontrolled usage.

Class 8 Science Chapter 3 

Conservation of Plants and Animals

Conservation of plants and animals is a critical subject of science. This part will push the students to not just to understand the human exercise that negatively affects the nature’s abundance yet in addition to manners by which they can ensure the fauna and flora of the ecosystem.

Class 8 Science Chapter 4 

Reproduction in Animals

It explains the modes and procedure of reproduction in creatures. It is fundamental for the continuation of a species. In this chapter, u will figure out how reproduction happens in animals is talked about in detail.

Class 8 Science Chapter 5 

Sound

Sound talks about the distinction between music and noise. Noise pollution, its causes, impacts, and control measures are explained towards the finish off the chapter. This chapter will strengthen your base for future complex topics.

Class 8 Science Chapter 6

Food Production and Management

This chapter explains rural practices related to the creation of yield and their administration.It includes the presentation of various harvest, their climatic and nourishing necessities and the preparation of soil for comprehensive improvement of the crops 

Class 8 Science Chapter 7 

Combustion and Flame

In this chapter, you will learn about different sorts of fuels are utilized for a different purpose at home, in industry and for running autos. Powers like cow dung, wood, coal, charcoal, petroleum, diesel, packed gaseous petrol (CNG) and so on are known to the students. In combustion and flame, the chemical procedure of burning and the sorts of flames created amid this procedure are considered in detail.

Class 8 Science Chapter 8 

Chemical Effects of Electric Current

In this chapter, we will discover that metals, for example, copper and aluminum conducts electricity while materials, for example, rubber, plastic, and wood don’t conduct electricity.

Class 8 Science Chapter 9 

Force and Pressure

This chapter will take the students through the brilliant world of force and pressure. It will push them to build u uphold of the point as well as ace the ideas that will assist them with performing better.

Class 8 Science Chapter 10 

Reaching the Age of Adolescence

In this chapter, the student will find out about changed that happen in the human body after which an individual ends up fitting for reproduction.in this chapter we will learn about the human conceptive organs and the role of hormones in the development of an individual.

Class 8 Science Chapter 11

Materials: Metals and Non-metals

This chapter explains the world of metals, non-metals, their properties, and employment. This section will assist the students with classifying the components into metals and non-metals based on their properties. Physical properties of metals and non-metals, chemical properties of metals and non-metals, use of metals and non-metals are a portion of the vital topics talked about in this chapter. 

Class 8 Science Chapter 12

Light

Light talks about the key properties, impacts, and utilization of light and its subordinates. This chapter also gives information about regular reflection, diffused reflection, laws of reflection, etc.

 

Class 8 Science Chapter 13 

 Pollution of Air and Water

This chapter addresses the solution for the unsafe wonder of pollution and how might one add to its decrease in the individual dimension. Air pollution, how does air get polluted are some of the significant topics discussed in this chapter.

Class 8 Science Chapter 14

Friction

It is an essential section of physics that manages the backing off or opposition of movement. Its causes, impacts, applications and other key properties are talked about in his chapter.

 

Class 8 Science Chapter 15 

Stars and Solar System

This chapter explains about the celestial objects. Position of those objects, their structure and centrality are portrayed in this chapter. This part also discusses the moon, periods of the moon its situation in the planetary system and surface of the moon.

Class 8 Science Chapter 16

Cell: Structure and Functions

It explains the inside and out information about the major unit of life -cell. It enlightens the students regarding the discovery of the cell, its inclination, properties, and composition

 

Class 8 Science Chapter 17 

Some Natural Phenomenon

This chapter throws light on the wonderful phenomenon of nature, for example, lightning and earthquake. Causes, impacts, magnitude and properties of lightning are discussed in this chapter.