Page 1: Probe and ponder
Why are winds stronger on some days than on others?
Answer: Winds are caused by differences in air pressure. Air moves from a place with high pressure to a place with low pressure. When the difference in pressure between two places is very large, the air moves very fast, which we feel as strong winds. On calm days, the pressure difference is very small.
Why are water tanks usually placed at a height?
Answer: Water tanks are placed at a height to increase the water pressure in the taps below. The pressure exerted by a liquid increases with the height of its column. Placing the tank high up creates a tall column of water, which pushes the water out of the taps with greater force.
Can air pressure really crush us?
Answer: No, air pressure cannot crush us, even though it is very strong. This is because the pressure inside our bodies (from our blood and other fluids) is equal to the atmospheric pressure outside. These two pressures balance each other out.
What causes storms and cyclones?
Answer: Storms and cyclones are caused by differences in air pressure, which are created by differences in temperature. When air in one place gets very warm and moist, it rises, creating an area of very low pressure. Cooler air from surrounding high-pressure areas rushes in very fast to fill this space, creating the strong winds, rain, and clouds that we see as storms and cyclones.
If the Earth stopped rotating, would cyclones still form?
Answer: No, cyclones as we know them would not form. The Earth’s rotation is what causes the air rushing into a low-pressure area to spin. Without this spin, you might still have strong winds and storms, but they would not form the large, spinning circular pattern that we call a cyclone.
Page 2: In-text Questions
Have you ever wondered why fallen leaves rise in the air or trees sway or bend?
Answer: This is a question to make us think. The answer is that the wind is pushing them.
Does the wind exert force on fallen leaves to make them rise or on trees to bend?
Answer: Yes, the wind exerts a force on all these objects.
Recall other similar effects of the force exerted by wind like slamming of doors or rattling of windows, or fluttering of clothes?
Answer: Other effects include:
- Flying a kite.
- Making a flag flutter.
- Moving the blades of a windmill.
- Causing waves on a lake or ocean.
How does the force exerted by wind make this happen?
Answer: The force exerted by the moving air (wind) creates wind pressure, and it is this pressure that pushes on objects and causes these effects.
Can the shape or size of the straps really make a difference?
Answer: Yes, the shape and size of the straps can make a big difference in how comfortable the bag feels, even if the weight inside is the same.
Page 3: In-text Questions & Table 6.1
Can you now understand why it feels easier to lift a water-filled bucket with a broad handle than with a narrow handle (Fig. 6.2)?
Answer: Yes. A broad handle spreads the weight (force) of the bucket over a larger area of your hand. This reduces the pressure on your hand, making it feel more comfortable and easier to lift. A narrow handle concentrates all the force on a small area, which creates high pressure and hurts.
(Table 6.1) Driving an iron nail By the head of the nail: Easy or difficult to perform? Give reasons.
Answer:
- Difficult.
- Reason: The head of the nail has a large area. When you hit it, the force spreads out over this large area, creating a very low pressure. This pressure is not high enough to push the nail into the wood.
(Table 6.1) Driving an iron nail By the pointed end of the nail: Easy or difficult to perform? Give reasons.
Answer:
- Easy.
- Reason: The pointed end has a very small area. The same force from the hammer is concentrated on this tiny point, creating a very high pressure. This high pressure is strong enough to push the wood fibers apart and drive the nail in.
(Table 6.1) Cutting an apple with a knife Using the sharp edge of the knife: Easy or difficult to perform? Give reasons.
Answer:
- Easy.
- Reason: The sharp edge of the knife has a very small area. When you push down, your force is concentrated on this small area, creating a very high pressure that can easily cut the apple.
(Table 6.1) Cutting an apple with a knife Using the blunt edge of the knife: Easy or difficult to perform? Give reasons.
Answer:
- Difficult.
- Reason: The blunt edge of the knife has a larger area. The same force is spread out, so the pressure is too low to cut the apple.
Page 4: In-text Questions & Activity 6.1
What can you conclude from your observations in Table 6.1?
Answer: We can conclude that when the area over which a force is applied is smaller, the pressure is higher. This is why sharp and pointed objects, which have a small area, can pierce or cut things easily.
Why are these tanks always placed at a height?
Answer: They are placed at a height to increase the water pressure in the taps. The pressure from a liquid depends on the height of its column, so a higher tank creates more pressure.
Do liquids also exert pressure?
Answer: Yes, liquids also exert pressure.
(Activity 6.1) Do both balloons bulge? Do they bulge to the same extent?
Answer: Yes, both balloons bulge. Yes, they bulge to the same extent.
(Activity 6.1) What can you infer from this activity?
Answer: We can infer that the pressure exerted by a liquid (water) at a certain depth depends only on the height of the liquid column above it, not on the width of the pipe or the total weight of the water.
(Activity 6.1) Could it be that the water column is exerting pressure?
Answer: Yes, it is the pressure from the water column that causes the balloons to bulge.
Page 5: In-text Questions & Activity 6.2
What will happen to the bulge of the balloon if we increase the height of the water column?
Answer: The bulge of the balloon will increase as the height of the water column increases.
Do you see any relation between the amount of bulge of the rubber balloon and the height of the water column in the pipe?
Answer: Yes. The relation is that as the height of the water column increases, the bulge of the balloon also increases. This shows that liquid pressure increases with height (or depth).
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.
Answer: My friend on the first floor will receive a more powerful stream of tap water.
Reason: The pressure of a liquid depends on the height of the water column above the tap. The tap on the first floor is lower down than the tap on the second floor. This means there is a taller column of water above the first-floor tap, which creates more pressure.
Do liquids also exert pressure on the walls of the container?
Answer: Yes, liquids also exert pressure on the walls (sides) of the container.
(Activity 6.2) What do you observe?
Answer: We observe water flowing out from all the holes on the sides of the bottle.
Page 6: In-text Questions
What can you infer from this observation?
Answer: We can infer that the liquid (water) exerts pressure on the sides (walls) of the container, not just at the bottom.
Can you explain why this happens? Is it due to the pressure exerted by water on the walls of the pipes?
Answer: Yes, this happens because water exerts pressure in all directions, including on the walls of the pipes. When there is a leak, this pressure pushes the water out sideways.
(Ever heard of…) Do you know that the base of a dam is much broader than the top? [Why?]
Answer: The base of a dam is much broader than the top to withstand the huge amount of water pressure at the bottom. Liquid pressure increases with depth (height of the column). This means the water exerts the largest horizontal pressure near the bottom of the dam. A broad base is needed to be strong enough to hold back this large force.
Page 7: Activity 6.3 & In-text Questions
(Activity 6.3) In which case is the lifting easier, with the folded or the unfolded chart paper covering the paper plate?
Answer: It is easier to lift the paper plate when it is covered with the folded (smaller) chart paper. It takes more effort to lift it when covered with the unfolded (larger) chart paper.
(Activity 6.3) What can you infer from this?
Answer: We can infer that air exerts a force, and this force increases as the area increases. Since pressure is force per unit area, we can conclude that the air around us exerts pressure. This is called atmospheric pressure.
You must have experienced that when you blow air into a balloon, it gets inflated. Why?
Answer: This is because the air being blown inside the balloon exerts pressure on the inner walls of the balloon, pushing them outwards.
Can we say that air exerts pressure in all directions?
Answer: Yes, we can. This is why the balloon expands in all directions (it gets round) when you inflate it.
What happens when an inflated balloon is kept without closing its mouth?
Answer: The air inside the balloon escapes, and the balloon deflates.
Why does the air escape from the balloon?
Answer: The air escapes because the air pressure inside the inflated balloon is higher than the air pressure outside it. Air always moves from a region of high pressure to a region of low pressure.
Page 8: Activity 6.4 & In-text Questions
(Activity 6.4) Do you find it difficult to pull it off [the rubber sucker]? [Why?]
Answer: Yes, it is difficult to pull the sucker off a smooth surface.
Why: When you press the sucker, most of the air is pushed out from under it, creating a low-pressure area inside the cup. The normal atmospheric pressure outside the cup is now much higher, and it pushes the sucker firmly onto the surface.
Do you know how large the atmospheric pressure is?
Answer: The atmospheric pressure is very large. The force from the atmosphere over an area of just 15 cm × 15 cm is nearly equal to the weight of a 225 kg mass.
In both of these cases [balloon deflating, tire puncture], does air move from a high pressure region to a low pressure region?
Answer: Yes, in both cases, the air moves from the high-pressure region (inside the balloon or tire) to the low-pressure region (outside).
Page 9: In-text Questions & Activity 6.5
Does the difference in air pressure have anything to do with the formation of winds?
Answer: Yes, the difference in air pressure is the main cause of winds.
How do winds form?
Answer: Winds form when air moves from a region of high air pressure to a region of low air pressure. These pressure differences are usually created by differences in temperature.
(Activity 6.5) Predict what would happen to the balloons.
Answer: I would predict that the inflated balloon (high pressure) will get smaller, and the uninflated balloon (low pressure) will get bigger, as air moves from one to the other.
(Activity 6.5) Observe what happens to both the balloons. Did it happen as predicted?
Answer: Yes, the inflated balloon gets smaller and the uninflated balloon gets partly inflated.
(Activity 6.5) Do you observe any change in the size of the balloons?
Answer: Yes, the size of both balloons changes.
(Activity 6.5) What can be the reason for the change in the sizes of the balloons?
Answer: The reason is that air moved from the inflated balloon (which had high air pressure) to the uninflated balloon (which had low air pressure).
(Activity 6.5) Why does the air flow stop?
Answer: The air flow stops when the air pressure inside both balloons becomes equal. Once there is no pressure difference, there is no reason for the air to move.
Page 10: In-text Question & Activity 6.6
I have read that high-speed winds can blow off roofs. I wonder how?
Answer: This happens because high-speed winds are accompanied by reduced (low) air pressure. When fast wind blows over a roof, it creates an area of low pressure above the house. The normal, higher air pressure inside the house then pushes the roof upwards from below, which can blow it off if it is weak.
(Activity 6.6) What happens to the balloons?
Answer: When you blow air between the two balloons, they move towards each other.
(Activity 6.6) What can you infer from this activity?
Answer: We can infer that high-speed winds are accompanied by reduced air pressure. The high-speed air blown between the balloons creates a low-pressure area, and the higher pressure of the surrounding air pushes the balloons into that space.
Page 11: In-text Questions
Have you heard the sound of thunder and seen lightning during the rainy season?
Answer: Yes, most people have heard the loud sound of thunder and seen the bright flash of lightning during rainy-season storms.
Page 15 & 16: Keep the curiosity alive (Exercises)
Question 1: Choose the correct statement.
(i) Look at Fig. 6.21 carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be
(a) the highest in vessel P
(b) the highest in vessel Q
(c) the highest in vessel R
(d) equal in all three vessels
Answer 1(i): (d) equal in all three vessels. (This is because liquid pressure only depends on the height, and a liquid will always find its own level in connected containers.)
(ii) A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker (N) is pressed on a rough surface:
(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.
Answer 1(ii): (c) M will stick but N will not stick. (The sucker works because it pushes air out to create a low-pressure seal. A smooth surface allows a good seal. A rough surface will let air leak back in, breaking the seal.)
(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.
Answer 1(iii): (a) increase the height ‘H’ at which the tank is placed. (Liquid pressure depends on the height of the column, not the width or total amount of water. Increasing ‘H’ increases the pressure.)
(iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. Pₐ and Pₛ is the pressure at the bottom of the vessels. Fₐ and Fₛ is the force exerted by the water at the bottom of the vessels A and B.
(a) Pₐ = Pₛ, Fₐ = Fₛ
(b) Pₐ = Pₛ, Fₐ < Fₛ
(c) Pₐ < Pₛ, Fₐ = Fₛ
(d) Pₐ > Pₛ, Fₐ > Fₛ
Answer 1(iv): (b) Pₐ = Pₛ, Fₐ < Fₛ
Reason:
- Pressure (P): Liquid pressure only depends on the height of the water. Since the water level is the same in both, the pressure at the bottom is equal: Pₐ = Pₛ.
- Force (F): Force is equal to Pressure × Area. Since the pressure (Pₐ and Pₛ) is the same, the force will be greater on the vessel with the larger bottom area. Vessel B has a wider base (larger area) than Vessel A. Therefore, the force on the bottom of B is greater: Fₐ < Fₛ.
Question 2: State whether the following statements are True [T] or False [F].
(i) Air flows from a region of higher pressure to a region of lower pressure. [ ]
(ii) Liquids exert pressure only at the bottom of a container. [ ]
(iii) Weather is stormy at the eye of a cyclone. ( )
(iv) During a thunderstorm, it is safer to be in a car. [ ]
Answer 2:
(i) [T]
(ii) [F] (Liquids also exert pressure on the sides/walls of a container.)
(iii) [F] (The eye is the calm, low-pressure center. The surrounding region has strong winds.)
(iv) [T]
Question 3: Fig. 6.23 a 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.
Answer: The boy will sink more in sand when he is standing vertically (Fig. 6.23b).
Reason: In both cases, the boy’s weight (the force) is the same.
- When lying down, his weight is spread over a large area (his entire back). This creates low pressure on the sand.
- When standing, his entire weight is concentrated on the small area of his feet. This creates a very high pressure. Since high pressure pushes harder, he will sink more when standing.
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.
Answer:
- Weight of the elephant (Force) = 20000 N
- Area of one foot = 0.25 m²
- Total area covered by four feet = 4 × 0.25 m² = 1.0 m²
- Pressure = Force / Area
- Pressure = 20000 N / 1.0 m²
- Pressure = 20000 N/m² (or 20000 Pa)
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?
Answer:
- For Boat A:
- Total Weight (Force) = 5 persons × 700 N/person = 3500 N
- Area = 7 m²
- Pressure A = Force / Area = 3500 N / 7 m² = 500 N/m²
- For Boat B:
- Total Weight (Force) = 3 persons × 700 N/person = 2100 N
- Area = 3.5 m²
- Pressure B = Force / Area = 2100 N / 3.5 m² = 600 N/m²
- Comparison:
- Boat B will experience more pressure on its base.
- By how much:
- Difference = 600 N/m² – 500 N/m² = 100 N/m²
- Boat B experiences 100 N/m² (or 100 Pa) more pressure than Boat A.
Question 6: Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
Answer: No, lightning would not occur.
Reason: Lightning is a sudden flow of electricity caused by a large build-up of static charges in the clouds (positive at the top, negative at the bottom). This build-up happens because air is normally an insulator (a bad conductor) and does not let the charges move. If air and clouds were good conductors, the charges would just flow gently and continuously as soon as they formed, and they would never build up to the huge amount needed to create a giant spark like lightning.
Question 7: What will happen to the two identical balloons A and B as shown in Fig. 6.24 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.
Answer:
- Yes, both balloons will bulge.
- Yes, they will bulge equally.
- Explanation: This setup shows that a liquid exerts pressure on the sides of its container. However, liquid pressure also increases with depth (or the height of the column above it). Since the two balloons are at the same height from the bottom, they are at the same depth below the water surface. Therefore, the water pressure at both holes will be equal, and both balloons will bulge equally.
(Self-correction: The first “No” was incorrect. The text clearly states pressure depends on the height of the column. Since both balloons are at the same height, they are at the same depth. My final answer is that they will bulge equally.)
Question 8: Explain how a storm becomes a cyclone.
Answer: A storm becomes a cyclone when a very low-pressure area forms over warm ocean water.
- Warm, moist air rises from the ocean, creating a low-pressure area.
- As the air rises, the water vapor condenses, forming clouds and releasing heat.
- This released heat warms the air even more, making it rise faster and creating an even lower pressure area.
- Air from the surrounding high-pressure areas rushes in to fill the space.
- Because the Earth is spinning, this rushing air begins to spin (or rotate). This repeating cycle of rising warm air and spinning winds creates the massive, rotating storm system called a cyclone.
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.
Answer:
- Side A is the land.
- Explanation: The picture shows the wind blowing from B towards A (the trees are bent towards A). This is a sea breeze.
- During a summer afternoon, the land (Side A) heats up much faster than the sea (Side B). The warm air above the land rises, creating a low-pressure area. The air above the cooler sea is at a higher pressure.
- Wind always blows from a high-pressure region to a low-pressure region. Therefore, the wind blows from the sea (B) to the land (A).
Question 10: Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
Answer: You can use two balloons and a straw (Activity 6.5).
- Take one balloon and inflate it (this balloon now has high-pressure air inside).
- Take a second balloon and leave it uninflated (this balloon has low-pressure air inside).
- Carefully connect the mouth of the inflated balloon to one end of a straw and secure it with a rubber band.
- Connect the mouth of the uninflated balloon to the other end of the straw and secure it.
- Observation: You will see the inflated (high-pressure) balloon get smaller, while the uninflated (low-pressure) balloon gets bigger. This shows that the air flowed from the region of high pressure to the region of low pressure until the pressure became equal in both.
Question 11: What is a thunderstorm? Explain the process of its formation.
Answer: A thunderstorm is a storm that has lightning and thunder. Its formation involves these steps:
- Warm, moist air (often in hot, humid weather) gets heated and rises, creating a low-pressure area.
- Cooler air rushes in from high-pressure areas to take its place.
- As the warm air rises high, it cools, and the moisture condenses to form big clouds. Water droplets fall as rain.
- Strong winds start blowing upwards and downwards inside the cloud.
- This movement causes water droplets and ice particles to rub against each other, building up static electric charges in the cloud (usually positive at the top and negative at the bottom).
- When these charges become very large, they suddenly flow, creating a bright flash of lightning. The lightning rapidly heats the air, causing the loud sound of thunder.
Question 12: Explain the process that causes lightning.
Answer: Lightning is caused by a build-up of static electric charges in storm clouds.
- Inside a tall storm cloud, strong winds blowing up and down cause tiny ice particles and water droplets to rub against each other.
- This rubbing separates the charges. Lighter, positive charges move to the top of the cloud, and heavier, negative charges gather at the bottom.
- The ground below the cloud also becomes positively charged.
- Air is normally an insulator, which keeps these opposite charges apart.
- But when the build-up of charges becomes too large, the air can’t hold them back anymore. Electricity suddenly flows between the opposite charges (like from the negative bottom of the cloud to the positive ground) to get balanced. This sudden, giant spark is what we see as lightning.
Question 13: Explain why holes are made in banners and hoardings.
Answer: Holes are made in banners and hoardings to prevent them from being torn or blown down by strong winds. According to the principle we learned, high-speed winds create an area of low pressure. If a strong wind blows, it creates low pressure on one side of the solid banner. The normal (high) pressure on the other side would push the banner with a very large force, causing it to tear. The holes let the high-speed air pass through the banner. This helps to reduce the pressure difference between the front and the back, which lowers the total force on the banner and keeps it from being damaged.
Page 18: Discover, design, and debate
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.
Answer:
- Prediction: I might predict that blowing over the paper strip will push it down.
- Observation: When you blow hard over the top surface of the paper strip, the strip lifts upwards.
- Interpretation: This happens because the fast-moving air you blew has a lower pressure than the still air. The normal, higher-pressure air that is under the paper strip pushes it upwards into the low-pressure area you created on top. This is the same idea as in Activity 6.6.
List three major cyclones which have occurred in India in the last 20 years. List two major destructions 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?
Answer: (Based on information from 2005-2025)
- Cyclone Amphan (2020):
- Destruction: (1) It was the costliest cyclone ever in the North Indian Ocean, causing over $13 billion in damage. (2) It caused 128 deaths across India and Bangladesh.
- Measures: The India Meteorological Department (IMD) issued accurate warnings. Proactive mass evacuations were carried out, and many rehabilitation centers were set up for people who lost their homes.
- Cyclone Fani (2019):
- Destruction: (1) It was an Extremely Severe Cyclonic Storm that caused over $8.1 billion in damage. (2) It caused 89 deaths in India.
- Measures: This event showed the success of the “Odisha Model.” The government and community groups worked together to evacuate 1.2 million people into thousands of multipurpose cyclone shelters.
- Cyclone Phailin (2013):
- Destruction: (1) It was the most intense cyclone to hit India since 1999. (2) It caused 45 deaths and widespread damage to homes and crops.
- Measures: Learning from the 1999 disaster, the government and local communities performed an effective evacuation of nearly 1 million people, which saved many lives.
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?
Answer:
- Data and Comparison: Data shows that thunderstorm activity is not the same everywhere in India.
- Most Prone Regions (Frequency): The highest frequency (number) of thunderstorms occurs in the northeastern states (like Assam and Meghalaya), Kerala, the Western Ghats (western coast), and along the Himalayan foothills.
- Most Prone Regions (Intensity/Lightning): The strongest thunderstorms, especially those with the most dangerous lightning, are often found along the coastal regions, with the state of Odisha being ranked as one of the most vulnerable to lightning strikes.
- Reasons: Thunderstorms need two main ingredients: moisture and instability (warm air that rises quickly).
- The coastal and northeastern regions get a lot of moisture from the Bay of Bengal and the Arabian Sea.
- The hot, humid climate in these areas, along with the hilly and mountainous terrain (like the Himalayas and Western Ghats), helps the warm, moist air to rise very fast, creating perfect conditions for powerful thunderstorms.