Activities 6.3 and 6.4 — Air Exerts Pressure — Pressure, Winds, Storms, and Cyclones

1. 13 marksCuriosity Grade 8, Chapter 6, page 85, section 6.2

   What is the atmosphere? What does the chapter say it contains and how far it extends?

   Hint. Three facts, all in one short paragraph.

   Definition. The envelope of air surrounding the Earth is called atmosphere.

   What it contains. The atmospheric air contains nitrogen, oxygen, argon, carbon dioxide, and other gases in small quantities.

   How far it extends. The atmosphere extends up to many kilometres above the surface of the Earth.

   ✦ Note how carefully the chapter words the last two points, and match it. It gives the gases in order but assigns them no percentages, and it says many kilometres rather than naming a height. Both are deliberate — the atmosphere has no sharp upper edge, and the composition figures belong to a later grade. Adding '78% nitrogen, 21% oxygen' or 'about 100 km thick' would be going beyond what this book gives you.

   Why the section opens this way. Before asking whether air exerts pressure, the chapter has to establish that there is a great deal of air, standing in a column above every point on the Earth's surface. That column is what will turn out to press on everything — including, as Activity 6.4 shows, with a force equivalent to 225 kg on a patch 15 cm square.

   You already know from Exploring Forces that air exerts a force of friction on things moving through it. This chapter adds that it presses on things that are standing still.

2. 24 marksCuriosity Grade 8, Chapter 6, pages 85-86, Activity 6.3

   Describe Activity 6.3 with the paper plate and the chart paper. What is compared, and what is deliberately kept the same?

   Hint. Two sheets are used, and the pair is chosen with great care.

   The setup. Invert a paper plate, attach a stick to it, and stand it on a flat surface (Fig. 6.9a). Take two identical sheets of chart paper, about 70 cm × 56 cm each.

   1. Fold one sheet twice, make a hole in the centre big enough for the stick, and lay it over the inverted plate (Fig. 6.9b). Lift the plate by the stick and note the effort needed.
   2. Replace it with the second, unfolded sheet, holed at the centre for the stick (Fig. 6.9c). Lift again and compare the effort.

   What is compared: how hard it is to lift, with a small covering area versus a large one.

   What is kept the same — and this is the crux: the two sheets are identical, so the weight of the covering sheet has not changed. Folding a sheet twice does not make it lighter; it only makes it cover a quarter of the area.

   ✦ Why that control is everything. Without it, the obvious explanation for 'harder to lift' would be 'the covering is heavier'. By using the same sheet folded and unfolded, the chapter removes weight from the discussion entirely, and the only thing left that differs is the area.

   The result. More effort is needed to lift the paper plate when it is covered with the unfolded chart paper, than with the folded chart paper.

3. 34 marksCuriosity Grade 8, Chapter 6, page 86

   What is inferred from Activity 6.3? Define atmospheric pressure.

   Hint. Follow the chapter's chain from force to pressure.

   The chain of inference, as the chapter builds it:

   1. More effort is needed with the unfolded sheet, which covers a larger area.
   2. The weight of the covering sheet has not changed, so weight cannot be the explanation.
   3. We can infer from these observations that air exerts force on the covering sheet, which makes it difficult to lift the paper plate.
   4. Moreover, this force increases with increase in the area of covering sheets.
   5. 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.

   Definition. The pressure exerted by air around us is known as the atmospheric pressure.

   ✦ Step 4 is what earns the word 'pressure' rather than just 'force'. If air simply pushed down with some fixed force, the folded and unfolded sheets would be equally hard to lift. The force scaling in proportion to the area is exactly what a constant pressure acting over that area would give — so the conclusion is not merely that air pushes, but that it pushes with a definite force on every square metre.

   And it presses in all directions, not just downwards. When you blow air into a balloon, it gets inflated ... because the air being filled inside the balloon exerts pressure on the walls of the balloon — and that is why the balloon expands in all directions, in every direction at once, exactly like the liquid of Activity 6.2.

4. 43 marksCuriosity Grade 8, Chapter 6, page 86

   Why does an inflated balloon expand in all directions? And why does the air rush out when its mouth is left open?

   Hint. Two questions the chapter asks and only half answers on the spot.

   Why it expands in all directions. The air being filled inside the balloon exerts pressure on the walls of the balloon (Fig. 6.10), and it exerts that pressure in every direction, not just downwards or towards one side. Since every part of the rubber is being pushed outwards equally, the balloon swells into a rounded shape rather than bulging in one place. As the chapter puts it: Can we say that air exerts pressure in all directions? Yes, that is why the balloon expands in all directions.

   Why the air escapes when the mouth is left open. The chapter asks Why does the air escape from the balloon? and leaves it hanging — deliberately, because the answer is the subject of the very next section. The air inside a stretched balloon is at a higher pressure than the air outside, and air moves from a region of high air pressure to a region of low air pressure. Open the mouth and it does exactly that.

   ✦ The unanswered question is a signpost. Section 6.3 opens by recalling this same balloon and the punctured bicycle tube, and asks: In both of these cases, does air move from a high pressure region to a low pressure region? Activity 6.5 then settles it.

   This is also why a balloon stops expanding. It swells until the stretched rubber pushes back as hard as the air inside pushes out. Blow harder and it either stretches further or bursts.

5. 54 marksCuriosity Grade 8, Chapter 6, page 87, Activity 6.4

   Describe Activity 6.4 with the rubber sucker. Why does it stick, and why is it hard to pull off?

   Hint. Something is removed when you press it, and that is the key.

   The activity. Take a good-quality rubber sucker and press it firmly against a smooth flat surface (Fig. 6.11). It sticks. Now try to pull it off — it is difficult.

   Why it sticks. 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.

   Why it is hard to pull off. 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.

   ✦ Nothing is pulling the sucker in — something is pushing it in. It is easy to imagine the sucker gripping or gluing itself to the wall. It does neither. The outside air is simply pushing on it harder than the thin remaining air inside can push back, and that difference holds it there. Remove the atmosphere and a sucker would fall off at once.

   Why the surface must be smooth and flat. On a rough surface, air leaks back in through the gaps between the cup and the surface, the pressure inside rises to match the outside, the difference disappears — and the sucker falls. That is exactly exercise 1(ii), whose answer is (c) M will stick but N will not stick.

   And why it must be good-quality rubber: a cracked or stiff sucker cannot seal, so the same leak occurs.

6. 64 marksCuriosity Grade 8, Chapter 6, page 87

   How large is atmospheric pressure, according to the chapter? Verify the chapter's figure by calculation, and explain why we are not crushed.

   Hint. Convert the area to m² first, then divide.

   The chapter's statement. The force exerted by the atmospheric air column over an area 15 cm × 15 cm is nearly equal to the force of gravity on an object of mass 225 kg (2250 N).

   Verifying it.

   Area = 15 cm × 15 cm = 0.15 m × 0.15 m = 0.0225 m²

   Pressure = Force ÷ Area = 2250 N ÷ 0.0225 m² = 1,00,000 N/m²

   That is 1,00,000 Pa. Using the A step further box (1 hPa = 1 mb = 100 Pa), it is 1000 hPa = 1000 mb — which sits right in the middle of the pressures marked on Fig. 6.19, from 994 mb to 1008 mb. The chapter's figure is internally consistent, which is worth checking rather than assuming.

   Why we are not crushed. 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.

   ✦ What actually causes damage is a pressure difference, never pressure by itself. The sucker of Activity 6.4 is held on by a difference. A roof is blown off, later in this chapter, by a difference. We stand comfortably under a hundred thousand pascals because there is the same pressure pushing outwards from inside us.

   The chapter gives 2250 N for 225 kg, so it is using about 10 N per kilogram — the same figure as the planet table in Exploring Forces.

7. 73 marksCuriosity Grade 8, Chapter 6, page 87, A step further

   What practical units are used for air pressure, and how do they relate to the pascal?

   Hint. Two names, one value.

   The chapter's box. The SI unit of pressure is N/m², also known as pascal (Pa). However, the practical unit of air pressure is millibar (mb), which is equal to 100 Pa. Air pressure is also expressed in hectopascal (hPa), which is equal to 100 Pa.

   1 millibar (mb) = 1 hectopascal (hPa) = 100 Pa

   Unit	Value	Where you meet it
   pascal (Pa)	1 N/m²	The SI unit — used for all the pressure calculations in this chapter
   millibar (mb)	100 Pa	Weather maps; Fig. 6.19 marks 994, 996, 998 and 1008 mb
   hectopascal (hPa)	100 Pa	Weather reports and forecasts — the same size as a millibar

   ✦ Why weather uses a bigger unit. Ordinary atmospheric pressure is about 1,00,000 Pa, and the differences that drive weather are only a few hundred pascals. Written in pascal, a weather map would read 100400, 100600, 100800 — long numbers whose interesting digits are at the far end. In millibars the same map reads 1004, 1006, 1008, and the differences leap out.

   The two names are for the same size of unit, which can be confusing. Hectopascal says what it is (a hundred pascal); millibar is the older name, still used because generations of weather charts were drawn in it.

   Reading Fig. 6.19 with this in mind: the pressure falls from 1008 mb at the outside to 994 mb at the centre — a drop of 14 mb, or 1400 Pa. That small-looking difference is what drives a cyclone's winds.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 6 'Pressure, Winds, Storms, and Cyclones', book pages 80-97 (hecu106.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. THREE FIGURES WERE MEASURED, NOT EYEBALLED, because three exercise answers turn on them. Fig. 6.22 (exercise 1(iv)) was rendered at 900 dpi and the vessel walls located from the dark outlines: vessel A is 519 px wide and vessel B is 726 px, so B is 1.40x wider, while the water columns measure 660 px and 671 px - equal to within 1.7%, matching the question's premise of equal levels. Hence P_A = P_B but F_A < F_B, answer (b). Fig. 6.24 (exercise 7) was segmented by colour: the two balloons occupy identical vertical ranges (rows 1260-1403, centroids both at row 1334), so they are at exactly the same height and bulge equally. Fig. 6.25 (exercise 9) was measured by tracking the palm trunks: the leftmost trunk's centre moves from x=655 at the top to x=763 near the base, so the crown sits about 108 px LEFT of the base, and all four crowns stream leftwards - the wind blows from B to A. Since a summer afternoon gives a SEA BREEZE (sea to land), B is the sea and A is the land. Fig. 6.21 was also rendered and confirmed to show the three vessels JOINED BY TUBES near their bases, which is what makes answer (d) correct. The chapter's own atmospheric-pressure figure was checked rather than assumed: 2250 N over 15 cm x 15 cm = 0.0225 m^2 gives 1,00,000 Pa = 1000 hPa = 1000 mb, which sits squarely inside the 994-1008 mb range marked on Fig. 6.19, so the book's number is internally consistent. Two deliberate restraints. (1) The chapter names exactly ONE cyclone (Amphan 2020, peak winds 270 km/h) and one surge range (3-12 m), and gives no cyclone categories, no casualty figures, no monsoon rainfall percentages and no atmospheric composition percentages. None are supplied here; the research projects tell the student to cite IMD or an equivalent checked source and to report disagreement between sources rather than pick a number. (2) Lightning-safety advice is reproduced exactly as the book gives it, with nothing added, since an invented extra rule could put someone at risk.. Questions are referenced from the NCERT textbook for identification.