Activity 6.2 — Liquids Exert Pressure on the Sides Too — Pressure, Winds, Storms, and Cyclones

1. 14 marksCuriosity Grade 8, Chapter 6, pages 84-85, Activity 6.2

   Describe Activity 6.2. Why must the four holes be at the same height from the bottom, and why are they all uncovered at the same time?

   Hint. The design lets you compare four places at once.

   The activity. Take a used plastic bottle and remove its cap. Using a needle or nail, make four small holes near the bottom, around the sides, making sure they are at the same height from the bottom (Fig. 6.7). If the plastic is hard, the needle can be warmed slightly. Seal the holes with tape, fill the bottle with water, then remove the tape from all holes at the same time and watch.

   What you observe. Water flows out through the holes on the sides of the bottle — in four jets, spurting outwards.

   Why the holes are at the same height. So that the water column above each hole is the same, and therefore the pressure at each is the same. Any difference in the jets would then have to be explained by something else. As it is, the four jets look alike, which is itself a small confirmation of Activity 6.1's result.

   Why all at the same time. So that the four jets can be compared under identical conditions. Opening them one by one would let the water level drop between openings, and the later jets would be weaker simply because less water was left above them.

   ✦ And why the cap is removed. With the cap on, air pressure inside the bottle would change as water left, and the flow would stutter. Removing it keeps the top of the water open to the atmosphere, so the only thing driving the jets is the water column itself.

   The result to take away: water also exerts pressure on the sides of a container.

2. 23 marksCuriosity Grade 8, Chapter 6, page 85

   What is concluded from Activity 6.2? State the full result about the directions in which a liquid exerts pressure.

   Hint. The conclusion is stated in three steps, each broader than the last.

   The three steps, in the chapter's own order:

   1. You observe water flowing out through the holes on the sides of the bottle ... It indicates that water also exerts pressure on the sides of a container.
   2. Therefore, we can conclude that liquids exert pressure not only at the bottom of the container, but also on its sides.
   3. In fact liquids exert pressure in all directions.

   ✦ Notice how far the third statement goes beyond what was observed. The experiment showed pressure downward (Activity 6.1's balloons) and sideways (these jets). The chapter then generalises to all directions, which is a bigger claim than the two observations strictly establish — but it is the standard result, and the four jets pointing out in four different sideways directions make it a reasonable one.

   Where the Snapshot puts it: Liquids and gases exert pressure on the walls of a container. Gases are included because section 6.2 goes on to show that an inflating balloon expands in all directions for exactly the same reason.

   A consequence the chapter asks about immediately. You must have seen water spurting out like a fountain from leaking joints or holes in water pipes. A pipe running under a road has water pressing outwards on its walls everywhere, so a hole anywhere on it — top, bottom or side — produces a jet. That would be impossible if liquids pressed only downwards.

   This is what makes exercise 2(ii) false: Liquids exert pressure only at the bottom of a container.

3. 33 marksCuriosity Grade 8, Chapter 6, page 85, Ever heard of ...

   Why is the base of a dam made much broader than its top?

   Hint. Where along the dam wall is the water pressure greatest?

   Because the water pressure on the dam wall is greatest near the bottom, so that is where the wall must be strongest.

   The chapter's account. 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. A broad base not only supports the structure of the dam, but also withstands the horizontal water pressure near the bottom (Fig. 6.8).

   Why the pressure is largest at the bottom. Liquid pressure depends on the height of the column above the point. Near the water surface there is almost no water above, so the sideways push is small. At the foot of the dam there is the entire depth of the reservoir above, so the sideways push is at its greatest.

   ✦ The dam brings both of this section's results together. Activity 6.1 showed that pressure grows with the height of the column; Activity 6.2 showed that liquids press sideways as well as down. A dam wall has to survive a sideways push that increases steadily from top to bottom — so it is built thin at the top, where the push is small, and thick at the base, where it is large. The tapering shape is a direct drawing of the pressure it must resist.

   And note what does not decide it: the reservoir's volume. A long shallow lake pushes on its dam less hard than a small deep one, because depth is what counts.

4. 43 marksCuriosity Grade 8, Chapter 6, pages 83-85

   The two identical balloons A and B of Fig. 6.24 are attached at the same height on the sides of a bottle. Water is poured in. Predict what happens and justify it from the two activities of this section.

   Hint. One activity tells you *whether* they bulge; the other tells you whether they bulge *equally*.

   Prediction: both balloons bulge, and they bulge equally.

   Why they bulge at all — Activity 6.2. The balloons are attached to the sides of the bottle, not the bottom. They inflate only because liquids exert pressure not only at the bottom of the container, but also on its sides. If liquids pressed only downwards, nothing would happen to either balloon.

   Why they bulge equally — Activity 6.1. The pressure exerted by a liquid in a vessel depends on the height of its column. Both openings are at the same height on the bottle, so the column of water standing above each is the same, so the pressure at each is the same, so the bulges match.

   ✦ Two things the answer must not depend on. Not on which side of the bottle the balloon is on — a liquid exerts pressure in all directions, so left and right are alike. And not on how much water the bottle holds — only on how high the water stands above the openings.

   A prediction you can test. If one balloon were fitted lower on the bottle than the other, that lower one would bulge more, because it would have a taller column above it. And if more water were poured in, both bulges would grow together.

   This is exercise question 7.

5. 53 marksCuriosity Grade 8, Chapter 6, pages 84-85

   Give three everyday observations that are explained by liquids exerting pressure that increases with depth.

   Hint. Look for anything that leaks, bursts or has to be built strong at the bottom.

   Observation	Explanation
   Water spurts like a fountain from a hole in a pipe	The water inside presses outwards on the pipe wall, so it escapes through any opening. The chapter raises this case itself on page 85
   A dam's base is much broader than its top	The horizontal pressure is very large near its bottom, so the wall needs most strength there
   Overhead tanks are built high	A taller column above the taps means more pressure and a better stream
   The ground-floor tap runs harder than the top-floor tap	The column above the lower tap is taller
   A tall drum of water bursts at a seam near its base, not near the top	The pressure pushing outwards is greatest at the bottom

   ✦ Notice what these have in common. In each case the quantity that matters is a vertical distance — the depth below the water surface — and not the amount of water. A hole low down on a full bottle throws a jet further than a hole high up on the same bottle, for the same reason a dam is thick at the base.

   An easy test you can add to Activity 6.2. Make the four holes at different heights up one side of the bottle instead of all at the same height. The lowest hole throws its jet furthest, because it has the tallest column above it. That single change converts the activity from a demonstration of sideways pressure into a measurement of how pressure grows with depth.

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.