NCERT Solutions for Class 8 Science Chapter 6 Pressure Winds Storms and Cyclones (PDF) – 2027

Because the difference in air pressure between two places is larger on some days than on others, and wind speed depends on that difference. Air always flows from high pressure to low pressure, and the bigger the difference, the faster it flows.

So that the water column above the taps is tall, because the pressure a liquid exerts depends on the height of its column.

Activity 6.1 shows that the balloon bulges more as the water column above it gets taller, and that the width of the pipe makes no difference. The water in a tank behaves the same way: the greater the height of water above a tap, the greater the pressure at that tap. That pressure is what pushes the water out. A tank standing on the ground would give only a weak trickle; the same tank on the roof gives a good, strong stream.

The atmosphere presses on us with an enormous force, but it does not crush us, because the pressure inside our body balances it.

The book gives the figure: the air column standing over an area of just 15 cm × 15 cm pushes down with a force of about 2250 N — the same as the force of gravity on a 225 kg mass. Every patch of our skin carries a load like that. We feel nothing because the fluids and gases moving in our tissues and organs keep the pressure inside the body equal to the atmospheric pressure outside. The two pressures push against each other and cancel.

Storms and cyclones both begin with warm air rising and leaving a low-pressure region behind. A cyclone, however, also needs the Earth’s rotation to make that inrushing air spin — so if the Earth stopped rotating, cyclones as we know them would not form.

Because moving air pushes on them — the wind exerts a force on every surface it meets.

A leaf lying on the ground has a large surface for its very small weight. When wind blows across it, the air pushes on that surface, and even a modest push is enough to lift the leaf and carry it along. A tree is far heavier, but its leaves and branches present a very large area to the wind, so the total force on the crown becomes large enough to bend the trunk.

Yes. Moving air exerts a force on whatever it strikes, and that force is what raises the leaves and bends the trees.

We recognise a force by what it does — it can make a resting object move, change its speed or direction, or change its shape. The wind does all of these: a still leaf starts moving, a swaying branch changes direction, a straight trunk bends. Slamming doors, rattling windows and fluttering clothes are the same force showing itself in other ways. The force exerted by wind creates wind pressure, which causes all these effects.

Yes — not to the weight of the bag, but to the pressure that weight produces on the shoulders.

Megha’s and Pawan’s bags are equally heavy, so gravity pulls both down with the same force. But that force is passed to the shoulders through the straps. Pawan’s narrow straps deliver the whole force over a small area of skin, while Megha’s broad straps spread the same force over a much larger area. Since pressure is force divided by area, the pressure under the narrow straps is much greater, and it is high pressure on the skin that hurts.

Yes. Both buckets pull down on the hand with the same force, but the broad handle spreads that force over a larger area of the palm and fingers, so the pressure on the skin is smaller.

A narrow handle touches the hand along a thin line. The whole weight of the bucket is delivered through that thin strip, so the pressure there is very high and it bites into the skin. A broad handle rests across the whole width of the fingers. The weight is the same, the area is several times larger, and the pressure falls in the same proportion.

Driving the nail by its pointed end is easy; driving it by its head is very difficult.

The sharp edge cuts easily; the blunt edge does not.

When the same force acts over a smaller area, the pressure produced is greater — and a greater pressure makes certain tasks easier.

Both rows of the table say the same thing in different words. The pointed end of a nail and the sharp edge of a knife succeed not because more force is used, but because the force is concentrated into a very small area. The head of the nail and the blunt edge of the knife fail because the same force is spread thin.

Because the pressure a liquid exerts depends on the height of its column, and it is that pressure which drives water out of the taps.

Water in the tank is connected to every tap in the house by pipes. The taller the column of water standing above a tap, the greater the pressure at that tap. Placing the tank on the roof makes this column as tall as possible, so the water leaves the taps in a strong stream and can also reach taps on the upper floors.

Yes. A liquid presses on the bottom of its container, on the walls of the container, and in fact in all directions.

Activity 6.1 makes the pressure at the bottom visible: the balloon tied to the lower end of a water-filled pipe bulges outwards, and it bulges more when the water column is made taller. Activity 6.2 shows the sideways pressure: water spurts out of holes made in the side wall of a bottle. Between them, the two activities establish that a liquid exerts pressure everywhere it touches its container.

Yes, both balloons bulge — and they bulge to exactly the same extent, even though one pipe is broad and the other narrow.

Both pipes are about 25 cm long and are filled with water to the same level, roughly half way. The balloon at the lower end of each pipe pushes out, and the two bulges look alike. This is surprising at first, because the broad pipe holds much more water than the narrow one, and therefore a much greater weight of water.

That the bulge is produced by the pressure of the water column, and that this pressure depends on the height of the column, not on the weight of water above the balloon.

Reason it through. The two pipes have different diameters, so they hold different weights of water. If the weight of water were responsible for the bulge, the broad pipe’s balloon would have bulged much more. It does not — the two bulges are equal. So weight cannot be the cause. What is the same in the two pipes is the height of the water column, and that is what the bulge follows.

The bulge will get bigger, and it will keep growing each time more water is poured in.

Pour a little more water into one of the pipes of Fig. 6.5 and the balloon at its lower end pushes out further. Repeat it a few times and the bulge increases every time. The reason is that the pressure at the bottom of the pipe depends on the height of the water standing above it: a taller column presses harder on the rubber, and the rubber stretches more.

Yes — the bulge increases as the height of the water column increases. The two go up together.

Taken with Activity 6.1, this gives the complete rule for the pressure of a liquid in a vessel: it does not depend on the width of the vessel or on how much liquid it holds, but it does depend on the height of the liquid column. The bulge of the balloon is simply a way of seeing that pressure.

Your friend on the first floor gets the more powerful stream.

The tank stands on the top floor, so the height of the water column above a tap is measured downwards from the tank. The first floor is further below the tank than the second floor is, so the column of water above the first-floor tap is taller. A taller column means greater pressure at the tap, and greater pressure means a faster, stronger stream.

Yes. Activity 6.2 is designed to show exactly this, and it does — water shoots out sideways through holes made in the wall of a bottle.

If the water only pushed downwards, it would simply sit in the bottle and nothing would come out of a hole in the side. The fact that it spurts out horizontally proves that the water is pressing outwards on the wall as well.

Water flows out through all four holes at the same time, in streams that shoot sideways away from the bottle.

The four holes are made at the same height near the bottom, sealed with tape, and the bottle is filled with water. When all the tapes are removed together, water jets out of each hole. Because the holes are at the same height, the four streams are alike; and as the water level in the bottle falls, all four streams weaken together.

That liquids exert pressure on the sides of a container as well as on its bottom — in fact, a liquid exerts pressure in all directions.

The holes are in the vertical wall of the bottle. Water can only be pushed out through them if it is pressing horizontally, outwards, against that wall. And since all four holes at the same height give similar streams, the sideways pressure is the same all round at a given depth.

Yes — it is the pressure of the water on the pipe wall that drives the fountain.

Water in a supply pipe is under pressure, because the pipe is fed from a tank standing high above it. That water presses outwards on the pipe wall everywhere, exactly as it pressed on the wall of the bottle in Activity 6.2. Where the wall is unbroken, the metal or plastic pushes back and holds the water in. At a crack or a loose joint there is nothing to push back, so the water is driven out through the gap.

Lifting is easier when the plate is covered with the folded chart paper. With the unfolded sheet, distinctly more effort is needed.

The two chart-paper sheets are identical — about 70 cm × 56 cm each — so their weights are the same. Folding one twice does not make it lighter; it only makes it cover a smaller area of the plate. Yet the folded sheet lifts easily and the spread-out sheet resists. The difference cannot come from weight, so it must come from area.

That air presses down on the covering sheet, and that the total force it exerts grows as the area of the sheet grows. Force per unit area is pressure — so air exerts pressure.

Set the two cases side by side. Same sheet, same weight, same stick, same plate. The only thing that changed was how much area the sheet presented to the air above it. When that area was made larger, the effort needed to lift the plate became larger. Something above the sheet must therefore be pushing down on it, and the push must be proportional to the area — which is precisely how a pressure behaves.

Because the air you blow in presses outwards on the inner walls of the balloon, and the rubber stretches under that pressure.

Every breath you blow adds more air into the same small space. The air inside pushes on the rubber from within; the atmosphere outside pushes back. As long as the inside pressure is greater, the rubber is stretched and the balloon grows. It stops growing when the stretched rubber is taut enough to balance the extra pressure inside.

Yes — and the balloon itself is the proof, because it swells in every direction, not just upwards or sideways.

If the air inside pushed only downwards, the balloon would sag at the bottom and stay flat elsewhere. Instead it rounds out equally all over, which tells us the air inside is pressing equally on every part of the wall. The same is true of the air outside: it presses on the top, the sides and the underside of every object around us.

The air rushes out and the balloon collapses. It escapes because the pressure of the air inside the balloon is greater than the atmospheric pressure outside.

The stretched rubber squeezes the air within, so the air inside is at a higher pressure than the air in the room. Air always moves from a region of higher pressure to a region of lower pressure. The open mouth gives it a path, so it flows out until the pressures inside and outside are equal — by which time the balloon has shrunk back to its original size.

Yes — once pressed firmly on a smooth flat surface, the sucker stays there on its own.

Pressing the cup down pushes almost all the air out from between the cup and the surface. Very little air is left inside, so the air pressure inside the sucker becomes much lower than the atmospheric pressure outside. The outside air now presses the sucker against the surface from every side, and it is held in place.

Yes, it is surprisingly hard to pull off — and how hard tells us how large the atmospheric pressure is.

To lift the sucker you must overcome the pressure difference between the air outside it and the thin air inside it. Your pull has to be greater than the force with which the atmosphere is holding the cup down. Since the atmosphere presses on every square centimetre with a large force, even a small sucker takes a firm tug.

It is very large. Over an area of 15 cm × 15 cm, the atmospheric air column pushes with a force of about 2250 N — nearly the force of gravity on an object of mass 225 kg.

Work out what that means for the area itself:

Yes. In both cases the air inside is at a higher pressure than the air outside, and it flows out until the two pressures are equal.

In the balloon, the stretched rubber squeezes the air within. In the bicycle tube, the pump has forced a large amount of air into a small volume. Both hold air at a pressure well above atmospheric. Open the balloon’s mouth or puncture the tube, and you give that high-pressure air a path to the low-pressure air outside — so out it goes, and the balloon or tube collapses.

Wind is air on the move from a region of higher pressure to a region of lower pressure. Wherever such a pressure difference appears, air begins to flow, and that flow is what we call wind.

The pressure difference is usually created by uneven heating. When one region is heated more than its surroundings, the air over it becomes warm and lighter and rises, leaving a region of low pressure behind. Air from the cooler, higher-pressure surroundings moves in horizontally to take its place. That horizontal movement is the wind we feel.

Yes — the pressure difference is the whole cause of wind, and its size decides how strong the wind is.

Activity 6.5 settles both points. Air flows from the inflated balloon to the uninflated one only while their pressures differ; the moment the pressures become equal, the flow stops and both balloons stay the same size. And as the book adds, if we could measure the speed of the escaping air we would find it higher when the pressure difference is higher.

Prediction: the inflated balloon will shrink and the uninflated one will swell, until both are of about the same size.

The reason for predicting this is the rule we already have: air moves from a region of high pressure to a region of low pressure. The inflated balloon holds air at a pressure higher than that in the limp balloon at the other end of the straw. As soon as the mouth is released, the straw becomes a path between them, so air should travel through it from the inflated balloon to the uninflated one.

Yes, it happens as predicted. The inflated balloon starts getting smaller and the uninflated one starts filling up.

Air travels along the straw from the inflated balloon into the limp one. You can often hear the rush of air and see both balloons changing at the same moment. After a short while the movement slows and stops, and the two balloons are left almost equal in size.

Yes. Both balloons change size, and they change in opposite ways.

• The inflated balloon shrinks — it becomes visibly smaller as air leaves it.
• The uninflated balloon swells — it fills out as air arrives in it.
• After some time both balloons are almost the same size, and no further change is seen.

The air pressure in the inflated balloon is higher than the air pressure in the uninflated balloon, so air moves from the inflated balloon into the uninflated one through the straw.

The balloon that loses air shrinks; the balloon that gains air expands. Nothing is being blown by mouth at this stage and no rubber is being stretched by hand — the only cause available is the difference in pressure between the two ends of the straw.

The flow stops because the pressures in the two balloons have become equal. With no pressure difference left, there is nothing to drive the air along the straw.

Follow the process through. At the start the pressure difference is large and air moves quickly. As air leaves the inflated balloon its pressure falls; as air enters the other balloon its pressure rises. The gap between the two narrows, the flow slows, and when the two pressures match, the movement ceases altogether. At that stage both balloons are of almost the same size.

Because high-speed winds are accompanied by reduced air pressure. When a fast wind sweeps over a roof, the pressure above the roof falls below the pressure of the still air inside the house, and the roof is pushed upwards from below.

The two balloons move towards each other and the gap between them closes, even though you are blowing air into that gap.

Two equally sized inflated balloons hang from a stick with a gap of 6–10 cm between them. Most people expect the blast of air to push them apart. Instead, the moment you blow into the narrow space, the balloons swing inwards and may even touch.

Blowing harder makes the balloons come together faster and more strongly.

The harder you blow, the higher the speed of the air rushing between the balloons, and the lower the pressure it creates there. The difference between the outside pressure and the pressure in the gap therefore grows, so the inward push on each balloon grows too, and they approach each other more quickly.

That high-speed winds are accompanied by a reduced air pressure.

Blowing between the balloons creates a region of fast-moving air, and therefore a region of low pressure, in the gap. The air surrounding the balloons on their outer sides is still at ordinary atmospheric pressure, which is now higher. That higher pressure pushes each balloon inwards, towards the low-pressure region — so they meet in the middle.

Yes. In the rainy season, especially just before the monsoon rains, we often see bright flashes of lightning and then hear the loud sound of thunder. Usually there are strong winds and heavy rainfall too.

Lightning is a sudden flow of the electric charges that build up in clouds. It heats the air around it so quickly that the air expands and produces the loud sound we call thunder. A storm accompanied by lightning and thunder is called a thunderstorm.

(i) (d) equal in all three vessels. The vessels are joined at the bottom, and liquid pressure depends only on the height of the column, so the water settles at the same level in all of them.

(ii) (c) M will stick but N will not stick. On the smooth surface the air pushed out cannot get back in; on the rough surface air leaks in through the gaps, so no pressure difference remains.

(iii) (a) increase the height ‘H’ at which the tank is placed. A taller water column gives a greater pressure; the amount of water in the tank does not matter.

(iv) (b) P_A = P_B, F_A < F_B. The same water level gives the same pressure, but B has the larger base, and force = pressure × area.

(i) True  •  (ii) False  •  (iii) False  •  (iv) True

The boy sinks more when he is standing vertically (Fig. 6.23 b).

His weight is exactly the same in both pictures — lying down does not make him lighter. What changes is the area of sand supporting that weight. Standing, he touches the sand only with the soles of his two feet, a very small area. Lying down, his back, legs, arms and head all rest on the sand, an area many times larger. Since pressure is force divided by area, the pressure on the sand is far greater when he stands, and loose sand grains give way under high pressure — so he sinks in deeper.

The elephant exerts a pressure of 20000 N/m², that is 20000 Pa, on the ground.

The elephant stands on all four feet, so its whole weight is shared over the total area of the four feet. First find that total area, then divide the weight by it.

Boat B experiences the greater pressure on its base — greater by 100 N/m² (100 Pa).

For each boat, first add up the weight of the persons sitting in it, then divide by the base area.

No, lightning would not occur. Lightning depends on air being an insulator, and on the cloud being able to hold separated charges.

If the clouds were good conductors, the positive and negative charges could not stay apart within the cloud; they would move through it and neutralise each other as fast as they were produced, so no large build-up could ever form. If the air were a good conductor, the charge would leak away steadily and continuously into the surrounding air and the ground, instead of being stored until air suddenly gives way. Either way there would be no huge accumulation and no sudden discharge — and lightning is precisely that sudden discharge.

Yes, both balloons will bulge — and they will bulge equally.

Water presses on the walls of its container in all directions, so it presses outwards into each side tube and stretches the rubber — that is why both balloons bulge. How much each bulges is decided by the pressure at the tube’s mouth, and that pressure depends only on the height of the water column standing above it. Since the two tubes are at the same height, the column above each is the same, the pressure at each is the same, and the two identical balloons stretch by the same amount.

A storm becomes a cyclone when it forms over warm ocean water and keeps growing stronger:

1. Warm, moist air over the heated ocean rises, creating a low-pressure area.
2. As the air rises, its water vapour condenses into raindrops and releases the heat that the water took up during evaporation.
3. This heat warms the rising air further, so it rises even higher and the pressure at the centre becomes even lower.
4. Air from the surrounding high-pressure regions rushes in and also starts rising.
5. Earth’s rotation makes this moving air spin.
6. The cycle repeats, giving a very low-pressure centre (the eye) with high-speed winds revolving around it. This spinning system of clouds, winds and rain is a cyclone.

Side A is the land (and side B is the sea).

Read the wind direction from the trees first. Their trunks lean towards A and their fronds are swept towards A, so the wind must be blowing from B towards A.

Now use the time of day. It is a summer afternoon, when land heats up much faster than water. The air over the land becomes warm and lighter, rises, and leaves a low-pressure area over the land. The air over the sea stays cooler and is at higher pressure. Air therefore blows from the sea to the land — this is the sea breeze. Since the wind here blows from B to A, B must be the sea and A must be the land.

Activity 6.5 (two balloons joined by a straw) shows this.

Materials: two similar balloons of thin rubber, a drinking straw, rubber bands or thread.

Method: Tie one end of the straw into an uninflated balloon. Inflate the second balloon, hold its mouth shut, and tie the free end of the straw into it without letting air escape. Then release your fingers.

Observation: the inflated balloon shrinks and the uninflated one swells until both are almost the same size; then the flow stops.

Conclusion: the air pressure in the inflated balloon was higher, so air flowed through the straw from the region of high pressure to the region of low pressure, and it stopped when the two pressures became equal.

A thunderstorm is a storm accompanied by lightning and thunder.

How it forms:

1. Heated land makes warm, moist air rise, creating a low-pressure area. Cooler air from surrounding high-pressure areas moves in, gets heated and rises, so the wind keeps circulating.
2. The rising air cools; its moisture condenses into droplets that form clouds, and heavier drops fall as rain. Strong winds with rain make a storm.
3. If the warm air rises very high, the low temperature turns droplets into ice particles.
4. Strong up-and-down winds rub ice particles and water droplets together, so charges develop: positive at the top of the cloud, negative at the bottom.
5. When the charge build-up is very large, lightning flashes, and the heated air expands with a loud sound, thunder.

Lightning is caused by a sudden flow of charges between oppositely charged regions:

1. Strong winds blowing up and down in a storm cloud rub ice particles and water droplets together, and they get charged.
2. Lighter, positively charged ice particles move to the top of the cloud; heavier, negatively charged water droplets stay at the bottom.
3. The negatively charged lower part makes the ground and objects on it (trees, buildings) positively charged.
4. Air is normally an insulator, but when the charge build-up is very large, its insulating property breaks down.
5. A sudden flow of charges gives a bright flash, lightning, within a cloud, between clouds, or between a cloud and the ground.

Holes are made so that wind can pass straight through the banner instead of piling up against it — which keeps the pressure difference across the two faces small, and stops the banner from being torn or blown away.

The holes let air flow through from the high-pressure side to the low-pressure side. The two faces are then at nearly the same pressure, the net force on the banner falls sharply, and the banner stays put.

Prediction most students make: the air will push the strip down. What actually happens: the strip rises and flutters up towards the stream of air.

Interpretation: blowing makes the air above the strip move fast, and high-speed air is accompanied by reduced pressure. The air below the strip is still, so it remains at ordinary atmospheric pressure. The pressure below is now greater than the pressure above, and this difference pushes the strip up. Blowing harder increases the speed, lowers the pressure above still more, and lifts the strip higher — which matches the result of Activity 6.6.

Measures taken: the IMD tracked each cyclone by satellite and issued early warnings; lakhs of people were moved to cyclone shelters before landfall; fishermen were kept off the sea; NDRF and state teams stood ready with food, water and medicines, and afterwards cleared the roads and restored power.

Two suggestions: (1) build cyclone shelters within walking distance of every coastal village, linked by all-weather roads; (2) protect and replant mangroves along the coast to slow the storm surge and protect farmland.

Sample findings:

The pattern and the reason: thunderstorms are most frequent where two things occur together — strong heating of the land and plenty of moisture in the air. Heating makes the air rise and creates the low pressure that draws in more air; moisture supplies the water vapour that condenses, forms tall clouds and releases heat that drives the rising further. That is why the eastern and north-eastern states, close to the warm Bay of Bengal, record far more thunderstorm days than the dry interior of the north-west. The chapter states the same conclusion in one line: the important requirements for the formation of thunderstorms are moisture and strong winds.