In-text — Pressure and the Bag Straps — Pressure, Winds, Storms, and Cyclones

1. 14 marksCuriosity Grade 8, Chapter 6, pages 81-82

   Megha's and Pawan's bags are equally heavy, yet Pawan's hurts his shoulders and Megha's does not. Explain why.

   Hint. Pawan works it out himself in the story. Look at the straps.

   Because of the width of the straps. Pawan says it himself: My bag has narrow straps while your bag has broad straps.

   The explanation. 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.

   	Pawan's bag	Megha's bag
   Weight of the bag	The same	The same
   Area of shoulder it acts on	Small (narrow straps)	Large (broad straps)
   Force per unit area	High	Low
   How it feels	Hurts	Comfortable

   ✦ Why this story is where the chapter begins. It isolates the new idea perfectly. The force is identical in the two cases — the bags are equally heavy — so whatever makes the difference cannot be the force. Only the area has changed. That forces you to invent a quantity that involves both, which is exactly what the next paragraph does:

   Since the area over which the force acts is involved, we define a quantity called pressure, which is the force per unit area.

   Broad straps do not reduce the weight — they reduce the pressure. Keeping those two apart is the whole point of the chapter's first section.

2. 23 marksCuriosity Grade 8, Chapter 6, page 81

   Define pressure and write its formula. What restriction does the chapter place on the forces it will consider?

   Hint. The restriction is in a single italicised sentence just after the formula.

   Definition. Pressure is the force per unit area.

   Pressure = Force ÷ Area

   The restriction. At this stage, we will consider only those forces which act perpendicular to the surface on which the pressure is to be computed.

   ✦ Why the restriction is there, and why it matters. A force can meet a surface head-on or at a slant. Only the part of it acting straight into the surface presses on it; a force sliding along the surface does something else entirely — that is friction, from the previous chapter. By restricting itself to perpendicular forces, the chapter makes the simple division Force ÷ Area correct as it stands, with no complications.

   And every example in the chapter obeys it: a bag's weight pressing straight down on a shoulder, water pressing straight out on the wall of a bottle, air pressing straight onto a chart-paper sheet, an elephant's weight pressing straight down on the ground.

   Read the formula the right way round. Pressure goes up when the force goes up, and up when the area goes down. Halving the area doubles the pressure for the same force — which is why a knife is sharpened rather than made heavier.

3. 33 marksCuriosity Grade 8, Chapter 6, page 82

   State the SI unit of pressure and show how it follows from the units of force and area. What other name does it have?

   Hint. Build the unit out of the formula.

   The derivation, exactly as the chapter gives it. The SI unit of force is newton and that of area is metre². Therefore, the SI unit of pressure is newton/metre² (N/m²). This unit is also called a pascal, denoted by Pa.

   Quantity	SI unit
   Force	newton (N)
   Area	metre² (m²)
   Pressure = Force ÷ Area	newton/metre² (N/m²) = pascal (Pa)

   So 1 Pa = 1 N/m² — the pressure produced when a force of one newton acts perpendicular on an area of one square metre.

   ✦ The unit is not something to memorise separately. It falls straight out of the definition. If you can write Pressure = Force ÷ Area, you can always reconstruct N/m² even if you have forgotten the word pascal.

   Two practical units for air pressure, from the A step further box on page 87: 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. So 1 mb = 1 hPa = 100 Pa — and the pressures 994 mb to 1008 mb in Fig. 6.19 are in these units.

4. 43 marksCuriosity Grade 8, Chapter 6, page 82

   A force of 100 N is applied on a cardboard of area 2 m². Calculate the pressure. Then find the pressure if the same force acts on 0.5 m².

   Hint. The chapter works the first one. Do the second yourself and compare.

   The chapter's worked example.

   Pressure = Force ÷ Area = 100 N ÷ 2 m² = 50 N/m²

   The second case.

   Pressure = 100 N ÷ 0.5 m² = 200 N/m²

   Force	Area	Pressure
   100 N	2 m²	50 N/m²
   100 N	0.5 m²	200 N/m²

   ✦ What the comparison shows. The force did not change at all. The area was reduced to a quarter, and the pressure became four times as large. That inverse relationship is the whole content of section 6.1, and it is why the same weight can be comfortable on broad straps and painful on narrow ones.

   Two habits worth forming now, because the exercises test them:

   • Put the area in m² before dividing. If you are given centimetres, convert first.
   • Write the unit on the answer. A pressure without N/m² or Pa after it is an incomplete answer, and it is also how you catch a mistake — if your units come out as N or m², you have divided the wrong way round.
5. 53 marksCuriosity Grade 8, Chapter 6, page 82

   Why is it easier to lift a water-filled bucket with a broad handle than with a narrow one? Why do people carrying pots or baskets on their heads place a round piece of cloth underneath?

   Hint. Both are the schoolbag problem again, in different clothes.

   Both work the same way: the weight is the same, but it is spread over a larger area, so the pressure is less. The chapter says so directly: In both cases, the objective is to reduce pressure by increasing the area over which the weight acts.

   	What presses	On what area	Effect of the fix
   Broad bucket handle (Fig. 6.2)	The weight of bucket and water	A wide strip of your palm and fingers instead of a thin line	Lower pressure — it does not cut into the hand
   Cloth ring under a head-load (Fig. 6.3)	The weight of the pot or basket	A broad ring of the head instead of the pot's narrow rim	Lower pressure — far more comfortable, and steadier

   ✦ Neither fix makes the load lighter. The bucket weighs exactly the same with a broad handle, and the pot weighs the same with a cloth ring under it. Your arms and neck still carry the whole weight. What changes is how that weight is distributed over your skin — and that is what you feel.

   The pattern to carry forward. Whenever something is uncomfortable or damaging, ask whether the fix is to reduce the force or to spread it over a larger area. In all three examples so far — straps, handles, head-cloths — it is the area.

   The next question turns this round: sometimes you want a very high pressure, and then you make the area as small as possible.

6. 64 marksCuriosity Grade 8, Chapter 6, pages 82-83, Table 6.1

   Carry out the activities of Table 6.1 — driving a nail by its head and by its pointed end, and cutting an apple with the sharp edge and the blunt edge of a knife. Record your observations and state the conclusion.

   Hint. Two pairs, and in each pair only the area changes.

   Safety first. The activities listed in Table 6.1 should be conducted under the supervision of an adult.

   The completed table:

   Activity	Mode of action	Easy or difficult?	Reason
   Driving an iron nail	By the head of the nail	Difficult	The head is broad, so the same force is spread over a large area — the pressure on the wood is low
   	By the pointed end	Easy	The point has a tiny area, so the same force gives a very high pressure, and the nail goes in
   Cutting an apple	Using the sharp edge	Easy	The sharp edge has a very small area of contact, so the pressure is high
   	Using the blunt edge	Difficult	The blunt edge has a much larger area, so the pressure is low and it presses rather than cuts

   The conclusion, in the chapter's words: when the area over which a force 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 the mirror image of the previous question, and that is why the two sit together. Straps, handles and head-cloths increase the area to make the pressure low, because pressure on your body is unwanted. Nails and knives decrease the area to make the pressure high, because pressure on the wood or the apple is exactly what you want. Same formula, opposite intentions.

   Which is why a knife is sharpened, not weighted. Making the blade heavier would raise the force a little; making the edge thinner raises the pressure enormously.

7. 73 marksCuriosity Grade 8, Chapter 6, pages 81-83

   Give three more everyday examples where the area is deliberately made large to reduce pressure, and three where it is made small to increase pressure.

   Hint. Ask in each case whether the high pressure is wanted or unwanted.

   Area made LARGE, to keep the pressure low (the pressure is unwanted):

   Example	Why
   Broad straps on a schoolbag	Spreads the bag's weight over more of the shoulder
   Broad handle on a bucket	Spreads the load over more of the palm
   Cloth ring under a head-load	Spreads the weight over a broad ring of the head
   Wide tyres on a tractor	Keeps it from sinking into soft field soil
   Wide foundations under a wall	Spreads the building's weight over more ground
   Skis and snowshoes	Spread a person's weight so they do not sink into snow

   Area made SMALL, to make the pressure high (the pressure is wanted):

   Example	Why
   The pointed end of a nail	A small area gives enough pressure to enter wood
   The sharp edge of a knife	Cuts instead of merely pressing
   The tip of a needle, a pin, a drawing pin	Pierces cloth, paper or a board easily
   The pointed end of a pen nib	Presses ink onto the paper
   Studs on sports shoes	Dig into the ground and grip

   ✦ A test for sorting any new example. Ask: what is being pressed, and do I want it to give way? If it is your own body — shoulder, palm, head — you want low pressure, so spread the area. If it is something you want to pierce, cut or push into — wood, an apple, soil — you want high pressure, so shrink the area.

   One example that goes both ways. A tractor has wide tyres to avoid sinking, but its plough has a narrow blade to cut into the same soil. The design choice depends entirely on what the part is for.

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.