Hello students, welcome to today's science class. I'm so happy to see you all here ready to learn something new and exciting. Today, we are going to study a very interesting chapter from your science textbook - Chapter 6: Pressure, Winds, Storms, and Cyclones. This chapter is all around us, students. Every day you experience pressure, you feel the wind, and during monsoon season, you definitely experience storms and cyclones if you live in coastal areas. So let's begin our journey of understanding these natural phenomena.
Before we start, let me ask you some questions. Have you ever wondered why winds are stronger on some days than on others? Have you noticed that water tanks are always placed at a height in our houses? Can you imagine that air pressure can actually crush us? What causes storms and cyclones? And here's an interesting question - if the Earth stopped rotating, would cyclones still form? These are some really thought-provoking questions, and by the end of this chapter, you will be able to answer all of them.
Let's start with something very familiar to all of you. Have you ever seen fallen leaves on the ground swirling in the air or being swept away when the wind blows? Have you seen trees swaying or even bending when there's a strong wind? Have you noticed how doors slam shut during a windy day, or windows rattle, or clothes flutter wildly on a clothesline? What makes all this happen? The answer is wind pressure. The force exerted by wind creates wind pressure which causes all these effects. So students, in this chapter, we will explore the relationship between force and pressure, and understand how they shape powerful natural events like thunderstorms and cyclones.
Now let's start with the first concept - Pressure.
## 6.1 Pressure
Let me tell you a story about two siblings, Megha and her brother Pawan, who were going on a picnic. They were carrying identical items in their bags. But on the way, Pawan kept adjusting his bag and looked very uncomfortable. Megha asked him, "Is there a problem with your bag?" Pawan replied, "Yes, it is hurting my shoulders." Now here's the interesting part - both their bags were equally heavy. So Megha asked, "Both our bags are equally heavy. Why does your bag hurt, and mine doesn't?" Pawan thought for a minute and said, "Perhaps, it is because of the difference in the straps of our bags. My bag has narrow straps while your bag has broad straps."
Now students, can the shape or size of the straps really make a difference? Let's find out.
When we carry a bag, we feel its weight because of the force of gravity acting on our shoulders. The weight of the bag with narrow straps acts on a smaller area of our shoulders, whereas the weight of the bag with broad straps is spread out over a larger area of our shoulders. It is due to this reason that we feel more comfortable carrying a bag with broader straps than one with narrow straps, although both bags have the same weight.
Now students, this is a very important concept. Since the area over which the force acts is involved, we define a quantity called pressure. Pressure is defined as the force per unit area. So mathematically, we write:
Pressure equals Force divided by Area.
At this stage, we will consider only those forces which act perpendicular to the surface on which the pressure is to be computed.
So students, broad straps reduce the pressure exerted by the bag on our shoulders as compared to narrow straps. That's why we feel more comfortable carrying a bag with broad straps.
Can you now understand why it feels easier to lift a water-filled bucket with a broad handle than with a narrow handle? Similarly, you must have seen people carrying loads like pots or vegetable baskets on their heads, and they often place a round piece of cloth under the loads. In both cases, the objective is to reduce pressure by increasing the area over which the weight acts.
Now let's talk about the unit of pressure. Pressure is defined as the force per unit area. The SI unit of force is newton and that of area is metre squared. Therefore, the SI unit of pressure is newton per metre squared. This unit is also called a pascal, and it is denoted by the symbol Pa.
Let me give you an example. If a force of 100 newton is applied on a cardboard of area 2 metre squared, then the pressure applied on the cardboard will be:
Pressure equals Force divided by Area, which is 100 N divided by 2 m², which equals 50 N/m². This is also equal to 50 pascal.
Now students, there are many situations in daily life where pressure plays a role. Let me tell you about an activity you should conduct. You need to record your observations in a table. The activity involves driving an iron nail - you need to try driving it by the head of the nail and then by the pointed end of the nail. You also need to cut an apple with a knife - first using the blunt edge, and then using the sharp edge. For each activity, you need to note whether it is easy or difficult to perform and give reasons.
What can you conclude from these observations? We conclude that when the area over which a force is applied is smaller, the resulting pressure is higher, making it easier to do certain tasks. This is why it is easier to drive a nail using its pointed end, and it is easier to cut an apple with the sharp edge of a knife. This is exactly why knives have sharp edges, why needles are pointed, and why nails have pointed tips. All of this is to increase pressure by decreasing the area.
Now students, you must have seen overhead water tanks in your locality or on the rooftops of houses. Why are these tanks always placed at a height? Let me explain this to you.
Do liquids also exert pressure? Let's find out through an activity.
Take two transparent glass or plastic pipes of the same length, about 25 centimetres, but of different diameters. Take two good-quality rubber balloons and attach them to one end of each pipe. Clamp the pipes on a stand. Now, fill both the pipes with water up to the same level, about halfway. Observe what happens to the balloons. Do both balloons bulge? Do they bulge to the same extent?
What can you infer from this activity? You must have observed that the two balloons bulge to the same extent. Why is it so? Notice that because of the different diameters, the weight of water in the two pipes is different. However, the bulge in both the balloons is the same. This means that the weight of water in the pipes could not be responsible for the extent of the bulge of the balloons.
Could it be that the water column is exerting pressure? Yes, it is the pressure exerted by the water column which is responsible for the bulge. That is why equal water column heights produce equal bulges in the balloons, despite their different diameters.
Now, what will happen to the bulge of the balloon if we increase the height of the water column? Pour some more water in any one of the pipes. Observe the bulge of the balloon. Repeat this process a few times, adding more water each time and noting the extent of bulge.
Do you see any relation between the amount of bulge of the rubber balloon and the height of the water column in the pipe? You must have observed that the bulge of the balloon increases as the height of the water column increases.
Thus, as the height of the water column in the pipe increases, the pressure at the bottom of the pipe also increases, which causes the balloon to bulge more. So, we can say that the pressure exerted by a liquid in a vessel depends on the height of its column. This is the reason why overhead tanks are placed at a height - so that the pressure in the taps is increased, resulting in a good stream of water from the taps.
Now here's a question for you. Suppose you are living on the second floor of a three-storeyed building and an overhead water tank is placed on the top floor. Will you or your friend on the first floor receive a more powerful stream of tap water? Give reasons. Students, think about this - the first floor is closer to the ground, so the water has to travel a shorter distance from the tank. The height of the water column above the tap on the first floor is greater than the height above the tap on the second floor. Since pressure depends on the height of the water column, the pressure at the first floor tap will be higher, and therefore your friend on the first floor will receive a more powerful stream of water.
Now, do liquids also exert pressure on the walls of the container? Let's find out.
Take a used plastic bottle and remove its cap. Make four small holes near the bottom around the sides using a needle or a nail. Make sure that the holes are at the same height from the bottom. Seal the holes with a tape and fill the bottle with water. Now, remove the tape from all holes at the same time. What do you observe?
You observe water flowing out through the holes on the sides of the bottle. What can you infer from this observation? It indicates that water also exerts pressure on the sides of a container. Therefore, we can conclude that liquids exert pressure not only at the bottom of the container, but also on its sides. In fact, liquids exert pressure in all directions.
You must have seen water spurting out like a fountain from leaking joints or holes in water pipes. Can you explain why this happens? Yes, it is due to the pressure exerted by water on the walls of the pipes.
Now students, have you ever heard that the base of a dam is much broader than the top? This is because a broad base not only supports the structure of the dam, but also withstands the horizontal water pressure near the bottom. The water stored in the dam exerts pressure horizontally on the side walls of the dam and vertically on the floor due to the height of the water level. The pressure which acts horizontally is very large near its bottom. Thus, to withstand the pressure, the base of the dam is made broader. This is a very important application of pressure in real life.
Now let's move on to the next section.
## 6.2 Pressure Exerted by Air
You already know that air is all around us. The envelope of air surrounding the Earth is called the atmosphere. The atmospheric air contains nitrogen, oxygen, argon, carbon dioxide, and other gases in small quantities. The atmosphere extends up to many kilometres above the surface of the Earth.
Now let's find out if the atmosphere exerts pressure by performing an activity.
Take a paper plate, invert it and attach a stick to it. Place it on a plain surface. Take two identical sheets of chart paper about 70 centimetres by 56 centimetres each. Fold one sheet twice and make a hole in the centre of the folded chart paper sheet big enough for the stick to come out. Place the folded sheet on top of the inverted paper plate.
Now, try to lift the paper plate covered with a folded sheet using the stick. Observe how much effort is needed to lift it.
Now, place the second unfolded chart paper sheet in place of the folded sheet. Make a hole at the centre of this chart paper for the stick to pass through. Cover the paper plate with the unfolded chart paper. Lift the paper plate again and feel the effort needed in doing so. In which case is the lifting easier, with the folded or the unfolded chart paper covering the paper plate?
You would have observed that more effort is needed to lift the paper plate when it is covered with the unfolded chart paper than with the folded chart paper. When we cover the paper plate with unfolded chart paper, the area of the covering sheet increases. The effort needed to lift the paper plate increases. Notice that the weight of the covering sheet has not changed. What can you infer from this?
We can infer from these observations that air exerts force on the covering sheet, which makes it difficult to lift the paper plate. Moreover, this force increases with increase in the area of covering sheets. It means that the air is exerting a force on the paper plate, which increases as the area of the sheet covering it increases. As force per unit area is pressure, we can conclude that air exerts pressure on the paper sheet. In fact, air exerts pressure on all objects. The pressure exerted by air around us is known as the atmospheric pressure.
You must have experienced that when you blow air into a balloon, it gets inflated. Why? This is because the air being filled inside the balloon exerts pressure on the walls of the balloon. Can we say that air exerts pressure in all directions? Yes, that is why the balloon expands in all directions. What happens when an inflated balloon is kept without closing its mouth? The air escapes from the balloon. Why does the air escape from the balloon? Because the air inside the balloon is at higher pressure than the outside air, so it moves from high pressure to low pressure.
Now students, have you ever wondered how large the atmospheric pressure is? Let me tell you about a simple activity to understand this. Take a good-quality rubber sucker. Press it firmly against a smooth flat surface. Do you realise that it sticks to the surface? Now, try to pull it off. Do you find it difficult to pull it off?
When we press the sucker, most of the air between its cup and the surface on which it is placed is pushed out and the air pressure inside it is reduced. The sucker sticks to the surface because the pressure of air surrounding the sucker is higher than the pressure exerted by the air inside the sucker. To pull the sucker off the surface, the applied force should be strong enough to overcome the pressure difference between outside the sucker and inside the sucker.
Now, do you know how large the atmospheric pressure is? The force exerted by the atmospheric air column over an area 15 centimetres by 15 centimetres is nearly equal to the force of gravity on an object of mass 225 kilograms, which is 2250 newton. That's a huge force! The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure. This balances the pressure exerted from outside. The pressure inside our body is caused by the movement of fluids and gases in tissues and organs of the body. So students, that's why we don't feel the atmospheric pressure crushing us - because our internal pressure balances it out.
Now let me tell you about the units. The SI unit of pressure is newton per metre squared, also known as pascal. However, the practical unit of air pressure is millibar, which is equal to 100 pascal. Air pressure is also expressed in hectopascal, which is equal to 100 pascal.
Now let's move on to the next important concept.
## 6.3 Formation of Wind
You must have noticed that on some days, the wind blows strongly, whereas on other days, it is calm. Sometimes, wind becomes so strong that it causes damage to life and property.
You must have seen that when an inflated balloon is kept without closing its mouth, the air from the balloon escapes. Recall that when there is a puncture in the bicycle tube, the air escapes and the tube collapses. In both of these cases, does air move from a high pressure region to a low pressure region? Yes, it does!
Does the difference in air pressure have anything to do with the formation of winds? Yes, it does! Let me explain how winds form.
Let's do an activity to understand this better. Take two similar balloons made of thin rubber, and a drinking straw. Insert one end of the straw into one balloon and secure it with a rubber band or thread. Now inflate the second balloon and hold its mouth with your fingers, so that air does not escape. Insert the free end of the straw into the neck of the inflated balloon and secure it with a rubber band or thread. Make sure that the air does not leak from the balloon as the straw is inserted in it. Now you have one end of the straw inside the inflated balloon and the other end inside the uninflated balloon.
Predict what would happen to the balloons. Observe what happens to both the balloons. Did it happen as predicted? Do you observe any change in the size of the balloons? Write down your observations.
What can be the reason for the change in the sizes of the balloons? The air pressure in the inflated balloon is higher than that in the uninflated balloon. As a result, some air moves from the inflated balloon to the uninflated balloon, resulting in changes in the size of both the balloons.
Do you notice that after some time both the balloons attain almost the same size and the flow of air stops? Why does the air flow stop? The flow of air continues till the air pressure in the inflated balloon is higher than the air pressure in the uninflated balloon. The air flow stops when the pressure in both balloons becomes equal. At this stage, both balloons are almost of the same size.
Thus, we can conclude that air moves from a region of high air pressure to a region of low air pressure. This is a very important conclusion, students. This is exactly how winds are formed. Wind is basically the movement of air from high pressure regions to low pressure regions.
Now you can relate this conclusion to the directions of the sea breeze and land breeze, which you studied in your previous classes. As land gets heated faster than water during the day, the air above the land becomes warmer and lighter. Hence, it rises, creating an area of low pressure. The air from the high pressure region of the sea blows to the low pressure region which develops on the land, resulting in a sea breeze. At night, the water is warmer than the land. Therefore, a low pressure area develops above the sea. As a result, wind blows from the land to the sea, giving rise to land breeze. Thus, the phenomenon of land breeze and sea breeze is mainly due to the pressure differences over the land and the sea.
If we could measure the speed of the escaping air in this activity, we would find that the speed of the air is higher if the pressure difference is higher. So students, remember - greater the pressure difference, faster the wind!
Now there's an interesting thing I've read - that high-speed winds can blow off roofs. I wonder how? Let me explain this to you.
## 6.4 High-Speed Winds Result in Lowering of Air Pressure
Let's do another activity to understand this. Take two balloons of the same size. Inflate both balloons and tie strings to them. Hang the two balloons from a stick, leaving a gap of 6 to 10 centimetres. Now, blow air into the narrow space between the balloons. What happens to the balloons? Note down your observations. Now blow harder and observe.
When you blow between the two balloons, you observe that they move towards each other. This happens because when you blow air between the balloons, a low pressure area is created between them. The higher air pressure surrounding the balloon pushes them towards each other. You must have observed that blowing harder increases the speed at which the balloons approach each other.
What can you infer from this activity? We infer that high speed winds are accompanied by a reduced air pressure. This is a very important finding!
Now let's understand how this applies to real life. When high-speed winds blow over houses, a low-pressure area is created over them, as high-speed winds are accompanied by a reduced pressure. Therefore, the air pressure above the roofs of the houses is lower than the pressure below them. If the pressure difference is large and the roofs are weak, they may be blown away. That is why it is safer to keep doors and windows of the houses open during storms with high-speed winds. When the same wind moves over the roofs and through the houses, the pressure difference between inside of the houses and over the roofs is reduced to a large extent. This helps prevent the roofs from being blown off.
Now let's move on to the next exciting topic.
## 6.5 Storms, Thunderstorms, and Lightning
Have you heard the sound of thunder and seen lightning during the rainy season? Yes, the sound of thunder is so frightening! Usually there is heavy rainfall too.
Let me explain how storms and thunderstorms are formed. When land gets heated, the warm and moist air, being lighter, rises, thereby creating a low pressure area. Cooler air from the surrounding high-pressure areas flows to take its place. This air, in turn, gets heated and rises. This results in a continuous process of wind circulation. As the rising air expands, it cools and moisture in it condenses to form water droplets, creating clouds. The water droplets merge to form heavier drops, which come down as rain, hail, or snow. The strong winds accompanied by rain is called a storm. In hot, humid, and tropical regions like India, storms are more frequent.
Now, under certain conditions, warm air rises to great heights where the low temperature converts water droplets into ice particles. Strong winds blowing upwards and downwards facilitate rubbing between water droplets and ice particles. You have learnt in earlier chapters that when two objects are rubbed against each other, they get charged. In this case, strong winds blowing upwards and downwards and rubbing against each other cause static electric charges to develop within the clouds.
The positively charged lighter ice particles move upwards and occupy the upper part of the clouds. The negatively charged heavier water droplets occupy the lower part of the clouds. Thus, a charge separation within the cloud takes place. Also, when the negatively charged lower part of the cloud moves closer to the ground, it causes the ground and nearby objects, such as trees or buildings, to become positively charged.
Normally, air acts as an electrical insulator and does not let opposite charges meet. But when the build up of charges becomes very large, the insulating property of air breaks down. A sudden flow of charges takes place, producing a bright flash of light called lightning.
Lightning can occur as opposite charges collide within a cloud, between clouds, or between clouds and the ground. Lightning rapidly heats up the air around it, causing the air to expand and produce a loud sound known as thunder. A storm accompanied by lightning and thunder is called a thunderstorm.
Now students, let me tell you some interesting facts about thunderstorms in India. Isolated and localised thunderstorms can sometimes occur in various regions of India. These thunderstorms are known by various names such as Kalboishakhi in West Bengal, Bihar, and Jharkhand and Bordoilsila in Assam. They occur before the arrival of the monsoon, thereby helping kharif crops to grow. In Kerala, Karnataka, and Tamil Nadu, they are known as mango showers as they support the ripening of mangoes. Local thunderstorms in Karnataka help in the growth of coffee plants. Isn't that interesting, students? Our country has such diverse weather phenomena!
Now, lightning can be dangerous! It can ignite fires, damage buildings, and cause severe burns or death in humans and animals. We must take necessary precautions and protect ourselves from lightning. During lightning, stay away from tall objects, find a low-lying open area and crouch down, and minimise contact with the ground. Do not lie down flat. Avoid using an umbrella with a metallic rod. If you are in water, get out of it. If you are inside a bus or a car, you are comparatively safer.
Now, have you heard of a lightning conductor? It is a metallic rod installed along the walls of buildings during their construction. One end of the rod is pointed. This end is kept higher than the highest point of the building. The other end of the rod is buried deep in the ground. The rod provides easy path for the transfer of electric charges into the ground. This protects the building from lightning damage.
Now let's move on to the last and most important topic of this chapter - Cyclones.
## 6.6 Cyclone
Cyclones are large storms that form over warm ocean waters. As the ocean water gets heated, the warm and moist air above it rises. As the moist air rises, the water vapour condenses to form raindrops. We know that during evaporation, water takes up heat to change into vapour. When this water vapour condenses into raindrops, heat is released back into the atmosphere. This causes further warming of the ascending air leading it to rise even further, creating an even lower pressure. Air from the surrounding regions rushes in and it also starts rising. Earth's rotation causes the moving air to spin. This cycle is repeated, resulting in the creation of a very low-pressure area with high-speed winds revolving around it. This spinning system of clouds, winds, and rain is called a cyclone.
In a cyclone, the region of lowest pressure is at the centre, known as the eye of the cyclone. At the eye of the cyclone, the wind is calm, but the surrounding region experiences strong winds and heavy rainfall. As a cyclone moves from the ocean towards the land, it generates higher wind speeds compared to the wind speeds produced by regular thunderstorms. Once the cyclone reaches land, the source of moist air is cut off and it gradually loses its strength.
Even as a cyclone loses its strength while travelling over land, it leaves behind a trail of destruction that can take months or even years to repair. Cyclones can be extremely destructive. For example, the Amphan cyclone in 2020 had peak wind speeds of 270 kilometres per hour.
Now let's understand the destruction caused by cyclones. Strong winds during a cyclone push ocean water towards the shore, creating a wall of water that can be as high as 3 to 12 metres. This surge of water can flood coastal areas and even areas far from the sea. The heavy rainfall which accompanies a cyclone may cause rivers to overflow and can also trigger landslides.
Seawater that rushes inland can contaminate drinking water sources and damage farmland. The salt in seawater can make soil less fertile, affecting crops. Roads may get blocked due to fallen trees and debris, making it difficult for help to reach the affected areas. Power outages can last for days, disrupting emergency services and daily life.
Now, how can we protect ourselves during cyclones? It is important to stay updated on weather reports and periodic alerts and warnings issued by the India Meteorological Department, also known as IMD. Thanks to the weather monitoring satellites, today we can track cyclones and predict their path, helping us reduce their impact on life and property. Several national and international organisations work together to monitor cyclone-related disasters. If you live in a cyclone-prone area, keep an emergency kit ready with essential items. During a cyclone, quickly move to a nearby designated cyclone shelter.
Now let me summarize what we have learned so far in this chapter.
We learned that warm air rises, creating a low-pressure area. Cool air rushes to occupy the low-pressure area. Warm air rises, cools, and the water vapour condenses to form clouds. Bigger water drops in the clouds fall to the ground as rain, hail, or snow. Positive and negative charges are created in the clouds by strong winds blowing upwards and downwards. When positive and negative charges meet, they cause lightning. Lightning may occur within a cloud, between clouds, or between a cloud and the ground. Under certain weather conditions, storms may develop into cyclones.
Now students, it's time to solve the exercises and questions given at the end of the chapter. Let me go through each one carefully.
## Keep the curiosity alive - Question 1
Let's solve question 1 part by part.
Part (i): Look at the figure carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be ____________.
The options are: (a) the highest in vessel P, (b) the highest in vessel Q, (c) the highest in vessel R, (d) equal in all three vessels.
Now students, think about this. We have three vessels P, Q, and R. Vessel R is filled with water. When we stop pouring water, the water will flow to achieve the same level in all connected vessels. This is because of the principle of communicating vessels. The water level will be equal in all three vessels. So the correct answer is (d) equal in all three vessels.
Part (ii): A rubber sucker M is pressed on a flat smooth surface and an identical sucker N is pressed on a rough surface. The options are: (a) Both M and N will stick to their surfaces, (b) Both M and N will not stick to their surfaces, (c) M will stick but N will not stick, (d) M will not stick but N will stick.
Now students, think about this. When we press a rubber sucker on a surface, we push out most of the air between the sucker and the surface. This creates a low pressure inside the sucker compared to the outside atmospheric pressure, which makes the sucker stick to the surface. However, on a rough surface, it is difficult to create a perfect seal because the air can still leak through the gaps. Therefore, the sucker on the rough surface will not stick properly. So the correct answer is (c) M will stick but N will not stick.
Part (iii): A water tank is placed on the roof of a building at a height 'H'. To get water with more pressure on the ground floor, one has to: (a) increase the height 'H' at which the tank is placed, (b) decrease the height 'H' at which the tank is placed, (c) replace the tank with another tank of the same height that can hold more water, (d) replace the tank with another tank of the same height that can hold less water.
Now students, we learned that pressure exerted by a liquid depends on the height of the liquid column. The higher the tank is placed, the greater the height of the water column, and therefore the greater the pressure at the ground floor. So to get water with more pressure, we need to increase the height 'H'. The correct answer is (a) increase the height 'H' at which the tank is placed.
Part (iv): Two vessels, A and B contain water up to the same level as shown in the figure. P_A and P_B is the pressure at the bottom of the vessels. F_A and F_B is the force exerted by the water at the bottom of the vessels A and B. The options are: (a) P_A = P_B, F_A = F_B, (b) P_A = P_B, F_A < F_B, (c) P_A < P_B, F_A = F_B, (d) P_A > P_B, F_A > F_B.
Now students, we learned that pressure exerted by a liquid depends on the height of the liquid column, not on the shape or size of the container. Since both vessels contain water up to the same level, the pressure at the bottom of both vessels will be the same. So P_A = P_B. Now, force equals pressure times area. The force at the bottom depends on both pressure and area. If vessel A has a larger bottom area than vessel B, then the force on the bottom of A will be greater than the force on the bottom of B. Without the figure, we cannot determine which vessel has a larger area. But typically, if the vessels have different shapes, the force could be different. However, since the question doesn't specify the areas, and given that both have the same water level, the most common answer would be (a) P_A = P_B, F_A = F_B, assuming they have the same bottom area. But actually, the force depends on the area. Let me reconsider - if we assume the vessels have different shapes but the same water level, the pressure at the bottom is the same, but the force depends on the area. Since we don't know the areas, the safest answer is (a) assuming they have equal areas, or we need to see the actual figure. But based on the general principle, the pressure depends only on height, so P_A = P_B. The force F = P × A. If we assume the vessels have the same base area, then F_A = F_B. So I'll go with (a).
## Question 2 - True or False
Now let's solve question 2. We need to state whether the following statements are True or False.
(i) Air flows from a region of higher pressure to a region of lower pressure.
This is TRUE. We learned that air moves from high pressure to low pressure regions. This is how winds are formed.
(ii) Liquids exert pressure only at the bottom of a container.
This is FALSE. We learned that liquids exert pressure not only at the bottom but also on the sides and in all directions. This was demonstrated in Activity 6.2 where water flowed out through holes on the sides of the bottle.
(iii) Weather is stormy at the eye of a cyclone.
This is FALSE. At the eye of the cyclone, the wind is calm and the weather is actually quiet. It is the surrounding region that experiences strong winds and heavy rainfall.
(iv) During a thunderstorm, it is safer to be in a car.
This is TRUE. If you are inside a bus or a car, you are comparatively safer during lightning because the metal body of the vehicle acts as a shield.
## Question 3
Now let's solve question 3. Fig. 6.23a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on a loose sand bed. In which case does the boy sink more in sand? Give reasons.
Now students, think about this. When the boy lies horizontally on the sand, his weight is distributed over a larger area. When he stands vertically, his weight is concentrated on a smaller area - his feet. Since pressure equals force divided by area, when the area is smaller, the pressure is higher. Therefore, the boy will sink more in the sand when he stands vertically compared to when he lies horizontally. So the boy sinks more in case (b) when he is standing vertically.
## Question 4
Now let's solve question 4. An elephant stands on four feet. If the area covered by one foot is 0.25 m², calculate the pressure exerted by the elephant on the ground if its weight is 20000 N.
Now students, this is a simple calculation. First, we need to find the total area of all four feet. Area of one foot is 0.25 m², so total area of four feet is 0.25 × 4 = 1.0 m².
Now, pressure equals force divided by area. The force is the weight of the elephant, which is 20000 N. So pressure = 20000 N ÷ 1.0 m² = 20000 N/m² or 20000 pascal.
So the pressure exerted by the elephant on the ground is 20000 pascal.
## Question 5
Now let's solve question 5. There are two boats, A and B. Boat A has a base area of 7 m², and 5 persons are seated in it. Boat B has a base area of 3.5 m², and 3 persons are seating in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much?
Now students, first we need to calculate the total weight on each boat.
For boat A: 5 persons × 700 N = 3500 N For boat B: 3 persons × 700 N = 2100 N
Now, pressure equals force divided by area.
Pressure on boat A = 3500 N ÷ 7 m² = 500 N/m² = 500 Pa Pressure on boat B = 2100 N ÷ 3.5 m² = 600 N/m² = 600 Pa
So boat B experiences more pressure on its base. The difference in pressure is 600 - 500 = 100 Pa.
So boat B experiences 100 pascal more pressure than boat A.
## Question 6
Now let's solve question 6. Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
Now students, this is a very interesting question. Lightning occurs because air acts as an insulator. When charges build up in the clouds, the air prevents the opposite charges from meeting. But when the charge build-up becomes very large, the insulating property of air breaks down, and we get lightning - a sudden flow of charges.
If air and clouds were good conductors of electricity, the charges would continuously flow and would not accumulate. There would be no sudden build-up of charges, and therefore no sudden discharge we call lightning. So lightning would not occur if air and clouds were good conductors.
## Question 7
Now let's solve question 7. What will happen to the two identical balloons A and B as shown in the figure when water is filled into the bottle up to a certain height. Will both the balloons bulge? If yes, will they bulge equally? Explain your answer.
Now students, think about this. When we fill water into the bottle, the water exerts pressure on both balloons. Since both balloons are identical and attached to the bottle at the same level, they will experience the same pressure from the water column. Therefore, both balloons will bulge, and they will bulge equally. This is because the pressure exerted by a liquid depends on the height of the liquid column, not on the width of the container. We learned this in Activity 6.1.
## Question 8
Now let's solve question 8. Explain how a storm becomes a cyclone.
Now students, let me explain this step by step.
A storm becomes a cyclone under specific conditions over warm ocean waters. Here's how it happens:
First, when the ocean water gets heated, the warm and moist air above it rises. As the moist air rises, the water vapour condenses to form raindrops. During evaporation, water takes up heat to change into vapour. When this water vapour condenses into raindrops, heat is released back into the atmosphere. This heat release causes further warming of the ascending air, making it rise even further and creating an even lower pressure.
Air from the surrounding regions rushes in to fill this low-pressure area, and this air also starts rising. Due to Earth's rotation, the moving air begins to spin. This cycle repeats, resulting in the creation of a very low-pressure area with high-speed winds revolving around it. This spinning system of clouds, winds, and rain is called a cyclone.
So essentially, a storm becomes a cyclone when it forms over warm ocean waters, gets organized with a rotating wind pattern, and develops a well-defined low-pressure centre called the eye.
## Question 9
Now let's solve question 9. Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land - A or B. Explain your answer.
Now students, this is about sea breeze. During a summer afternoon, the land gets heated faster than the water. The air above the land becomes warmer and lighter, and rises, creating a low-pressure area. The cooler air from the sea, which is at higher pressure, blows towards the land. This is the sea breeze.
So in the figure, the side where the wind is blowing from the sea towards the land is the land side. Typically, in a summer afternoon, the wind blows from the sea to the land, so the side where the trees are bending away from the sea is the land side. Without seeing the figure, I would say that the side towards which the wind is blowing is the land side. So if the wind is blowing from left to right, then the right side is land.
## Question 10
Now let's solve question 10. Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
Now students, we already did this activity in class. Let me describe it again.
Take two similar balloons made of thin rubber, and a drinking straw. Insert one end of the straw into one balloon and secure it with a rubber band or thread. Now inflate the second balloon and hold its mouth with your fingers, so that air does not escape. Insert the free end of the straw into the neck of the inflated balloon and secure it with a rubber band or thread. Make sure that the air does not leak from the balloon as the straw is inserted in it.
Now you have one end of the straw inside the inflated balloon and the other end inside the uninflated balloon. Observe what happens to both the balloons.
You will observe that the inflated balloon becomes smaller and the uninflated balloon becomes bigger. This happens because the air pressure in the inflated balloon is higher than that in the uninflated balloon. As a result, air moves from the inflated balloon (high pressure) to the uninflated balloon (low pressure).
This activity clearly demonstrates that air flows from a region of high pressure to a region of low pressure.
## Question 11
Now let's solve question 11. What is a thunderstorm? Explain the process of its formation.
Now students, a thunderstorm is a storm accompanied by lightning and thunder. Let me explain the process of its formation.
When land gets heated, especially during hot and humid days, the warm and moist air near the surface becomes lighter and rises. This creates a low-pressure area. Cooler air from surrounding high-pressure areas flows to take its place. This incoming air also gets heated and rises. This results in a continuous process of wind circulation.
As the rising air expands, it cools at higher altitudes. The moisture in this cooling air condenses to form water droplets, creating clouds. These water droplets merge to form heavier drops, which come down as rain. This whole process, accompanied by strong winds and rain, is called a storm.
Under certain conditions, particularly in hot, humid, and tropical regions, the warm air rises to great heights where the low temperature converts water droplets into ice particles. Strong winds blowing upwards and downwards facilitate rubbing between water droplets and ice particles. This rubbing causes static electric charges to develop within the clouds.
The positively charged lighter ice particles move upwards to the upper part of the clouds, while the negatively charged heavier water droplets occupy the lower part of the clouds. This creates a charge separation within the cloud.
When the charge build-up becomes very large, the insulating property of air breaks down. A sudden flow of charges takes place, producing a bright flash of light called lightning. This lightning rapidly heats up the air around it, causing the air to expand and produce a loud sound known as thunder.
A storm accompanied by lightning and thunder is called a thunderstorm.
## Question 12
Now let's solve question 12. Explain the process that causes lightning.
Now students, let me explain the process that causes lightning step by step.
First, we need to understand how charges develop in clouds. During a thunderstorm, warm moist air rises to great heights. As it rises, it cools and the water vapour condenses to form water droplets. At very high altitudes, where the temperature is very low, these water droplets freeze into ice particles.
Now, strong winds blowing upwards and downwards cause these water droplets and ice particles to rub against each other. Just like when you rub a balloon on your hair and it becomes charged, this rubbing causes static electric charges to develop in the clouds.
The positively charged lighter ice particles move upwards and occupy the upper part of the clouds. The negatively charged heavier water droplets occupy the lower part of the clouds. Thus, a charge separation takes place within the cloud.
Also, when the negatively charged lower part of the cloud moves closer to the ground, it induces positive charges on the ground and nearby objects like trees and buildings.
Now, normally air acts as an electrical insulator and does not let opposite charges meet. But when the build-up of charges becomes very large, the insulating property of air breaks down. A sudden flow of charges takes place between the oppositely charged regions. This produces a bright flash of light called lightning.
Lightning can occur in different ways - when opposite charges collide within a cloud, between clouds, or between a cloud and the ground. The lightning flash heats up the air around it very rapidly, causing the air to expand explosively and produce the loud sound we call thunder.
## Question 13
Now let's solve question 13. Explain why holes are made in banners and hoardings.
Now students, you must have seen banners and hoardings along roads, especially during elections or festivals. Have you noticed that they often have holes in them? Let me explain why.
When strong winds blow, they create a low-pressure area on the side of the banner facing the wind. If the banner is a solid sheet without holes, there would be a large pressure difference between the front and back of the banner. This large pressure difference would create a strong force on the banner, potentially tearing it apart or blowing it away.
By making holes in the banner, some wind can pass through these holes. This reduces the pressure difference between the front and back of the banner. The wind can flow through the holes, reducing the force on the banner. This helps to prevent the banner from being torn or blown away easily.
So holes are made in banners and hoardings to allow wind to pass through and reduce the pressure difference, making them more stable during windy conditions.
Now let's move on to the Discover, design, and debate section.
## Discover, design, and debate - Activity 1
Hold a strip of paper, 18 cm long and 2 cm wide, between your thumb and forefinger so that it hangs freely. Predict what you will observe if you blow over the paper. Perform the activity now. Note down your observations and interpret your results.
Now students, let's do this activity. Hold a strip of paper, 18 centimetres long and 2 centimetres wide, between your thumb and forefinger so that it hangs freely. Now, predict what will happen if you blow over the paper.
When you blow over the top of the paper, you create high-speed winds above the paper. We learned that high-speed winds are accompanied by reduced air pressure. So above the paper, the air pressure decreases. The air below the paper is at normal atmospheric pressure, which is higher. This higher pressure below pushes the paper upwards. So the paper will rise up towards the blowing air.
This demonstrates the concept we learned earlier - that high-speed winds result in lowering of air pressure. This is the same principle that can blow off roofs during cyclones.
## Discover, design, and debate - Activity 2
List three major cyclones which have occurred in India in the last 20 years. List two major destruction caused by each of the cyclones. What measures were taken by the local government and communities to reduce the loss of life and destruction of property? Mention two suggestions you would like to propose to the local government.
Now students, let me tell you about some major cyclones that have occurred in India in the last 20 years.
One of the most devastating cyclones was Cyclone Amphan in 2020. It affected West Bengal and Odisha. The major destruction caused included widespread damage to infrastructure with wind speeds of up to 270 kilometres per hour, and severe flooding in coastal areas due to the storm surge. Many houses were destroyed, and there was massive damage to crops and agricultural land.
Another major cyclone was Cyclone Fani in 2019, which also affected Odisha and also impacted West Bengal, Andhra Pradesh, and Tamil Nadu. The destruction included widespread power outages lasting for days, damage to houses and infrastructure, and disruption of transportation and communication services.
Cyclone Gaja in 2018 affected Tamil Nadu and Andhra Pradesh. The destruction included damage to houses and huts, destruction of crops especially coconut and banana plantations, and loss of livestock.
Now, the measures taken by the local government and communities included evacuation of people from low-lying coastal areas to safer shelters, deployment of rescue and relief teams, distribution of food and essential supplies, and restoration of power and transportation.
Two suggestions I would like to propose are: first, to build more robust cyclone shelters in vulnerable coastal areas, and second, to develop better early warning systems and ensure proper dissemination of weather information to all communities, including those in remote areas.
## Discover, design, and debate - Activity 3
Collect data on the strength of thunderstorms for various regions of India. Compare your findings and identify which regions are more prone to thunderstorms. Can you give reasons for your findings?
Now students, this is a research activity. You need to collect data on thunderstorms from different regions of India. Based on general knowledge, the regions that are more prone to thunderstorms include the northeastern states like Assam, West Bengal, Bihar, and Jharkhand, as well as the coastal states like Odisha, Andhra Pradesh, and Tamil Nadu.
The reasons for this are: first, these regions are close to the sea and have access to moisture-laden winds, which is essential for thunderstorm formation. Second, during the summer months, the land in these regions gets heated, creating conditions suitable for convection and thunderstorm development. Third, the presence of hills and mountains in some regions like the Northeast helps in forcing moist air to rise, leading to thunderstorm formation.
Now students, we have covered the entire chapter. Let me give you a comprehensive summary of everything we have learned.
## Summary
In this chapter, we learned about pressure, winds, storms, and cyclones. Let me summarize all the key points.
First, we learned about pressure. Pressure is defined as force per unit area. The SI unit of pressure is newton per metre squared, also called pascal. We learned that pressure depends on both the force applied and the area over which it is applied. Smaller area results in higher pressure, which is why sharp knives cut better, pointed nails are easier to drive, and broad straps on bags are more comfortable.
We learned that liquids exert pressure, and this pressure depends on the height of the liquid column, not on the shape of the container. This is why water tanks are placed at heights - to increase the pressure of water at the taps. We also learned that liquids exert pressure in all directions, not just at the bottom, which is why dams have broad bases - to withstand the horizontal pressure exerted by water.
We learned that air exerts pressure called atmospheric pressure. This pressure is quite large - the force exerted by the atmospheric air column over an area of 15 centimetres by 15 centimetres is nearly equal to the force of gravity on a 225 kilogram object. We don't feel crushed because the pressure inside our bodies balances the external atmospheric pressure.
We learned that winds are formed due to differences in air pressure. Air flows from high pressure regions to low pressure regions. This is also the principle behind sea breezes and land breezes.
We learned that high-speed winds are accompanied by reduced air pressure. This is why strong winds can blow off weak roofs - the pressure above the roof becomes lower than the pressure inside the house.
We learned about storms and thunderstorms. Storms are strong winds accompanied by rain. Thunderstorms are storms accompanied by lightning and thunder. Lightning is caused by the buildup of static electric charges in clouds due to rubbing between ice particles and water droplets. When these charges become large enough, they discharge as lightning.
We learned about cyclones - large storms that form over warm ocean waters. Cyclones have a calm centre called the eye, surrounded by strong winds and heavy rainfall. Cyclones can cause massive destruction through strong winds, storm surges, and flooding.
We learned about safety measures during thunderstorms and cyclones, including the use of lightning conductors on buildings and the importance of following weather warnings from the India Meteorological Department.
We also solved various numerical problems related to pressure calculations and answered conceptual questions about the formation of winds, storms, and cyclones.
Students, this concludes our lesson on Chapter 6: Pressure, Winds, Storms, and Cyclones. I hope you have understood all the concepts clearly. Remember, science is all around us, and these phenomena affect our daily lives, especially if you live in cyclone-prone coastal areas of India. Stay curious, keep learning, and always stay safe during storms and cyclones. Thank you for your attention, and I'll see you in the next class!