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Class 8 Science · Chapter 6

6. Pressure, Winds, Storms, and Cyclones

Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).

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Keep the Curiosity Alive

1Choose the correct statement.

(i) Vessels P, Q and R are connected (Fig. 6.21). Vessel R is filled with water. When pouring is stopped, the level of water will be

Answer: (d) equal in all three vessels

Liquids exert pressure in all directions. Water flows between the connected vessels till the pressure at the bottom is equal everywhere. This happens when the water level is the same in all.

(ii) A rubber sucker M is pressed on a flat smooth surface and an identical sucker N on a rough surface.

Answer: (c) M will stick but N will not stick.

On a smooth surface, the air is pushed out from under the sucker. The atmospheric pressure outside holds it in place. On a rough surface, air leaks in through the gaps, so the sucker falls off.

(iii) To get water with more pressure on the ground floor, one has to

Answer: (a) increase the height ‘H’ at which the tank is placed.

Pressure of a liquid increases with the height of the liquid column above that point.

(iv) Two vessels A and B (B is wider) contain water up to the same level. PA, PB are the pressures and FA, FB the forces at the bottom.

Answer: (b) PA = PB, FA < FB

The height of water is the same, so the pressure at the bottom is the same. Vessel B has a larger base area. Force = pressure × area, so the force on B is greater.

2State 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. — T

(ii) Liquids exert pressure only at the bottom of a container. — F

Liquids exert pressure at the bottom and also on the walls of the container.

(iii) Weather is stormy at the eye of a cyclone. — F

The wind is calm at the eye of a cyclone.

(iv) During a thunderstorm, it is safer to be in a car. — T

3A boy lies horizontally (Fig. 6.23a) and stands vertically (Fig. 6.23b) on a loose sand bed. In which case does he sink more? Give reasons.

The boy sinks more when he is standing.

His weight (force) is the same in both cases. When standing, his weight acts on the small area of his feet. Pressure = force ÷ area. Smaller area means more pressure. So he sinks more.

When lying down, his weight spreads over a large area. The pressure is less, so he sinks less.

4An elephant stands on four feet. 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.
Total area of four feet = 4 × 0.25 m2 = 1 m2
Pressure = Force ÷ Area
= 20000 N ÷ 1 m2
= 20000 N/m2 (20000 Pa)

The elephant exerts a pressure of 20000 Pa on the ground.

5Boat A has a base area of 7 m2 with 5 persons. Boat B has a base area of 3.5 m2 with 3 persons. Each person weighs 700 N. Which boat experiences more pressure on its base, and by how much?
Boat A: Force = 5 × 700 N = 3500 N
Pressure = 3500 N ÷ 7 m2 = 500 Pa
Boat B: Force = 3 × 700 N = 2100 N
Pressure = 2100 N ÷ 3.5 m2 = 600 Pa
Difference = 600 Pa − 500 Pa = 100 Pa

Boat B experiences more pressure, by 100 Pa (100 N/m2).

6Would lightning occur if air and clouds were good conductors of electricity? Give reasons.

No, lightning would not occur.

Lightning occurs because air is an insulator. It does not let opposite charges meet. So a very large amount of charge builds up. Then the insulation breaks down suddenly, giving a flash.

If air and clouds were good conductors, the charges would keep flowing away slowly as soon as they formed. They would never build up enough to produce lightning.

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7What will happen to the two identical balloons A and B (Fig. 6.24) when water is filled into the bottle up to a certain height? Will both bulge? Will they bulge equally?

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

Water exerts pressure on the walls of the bottle in all directions. Both balloons are fixed at the same depth (same height from the bottom). The pressure of water at the same depth is the same. So both bulge equally.

8Explain how a storm becomes a cyclone.
Over warm ocean water, the warm and moist air rises.
Water vapour in the rising air condenses into raindrops. This releases heat into the air.
The air gets warmer and rises even higher. A region of very low pressure forms.
Air from surrounding high-pressure regions rushes in. It also gets heated and rises.
The rotation of the Earth makes the moving air spin.
The cycle repeats. A spinning system of clouds, high-speed winds and rain forms around a very low-pressure centre. This is a cyclone.
9Fig. 6.25 shows trees along the sea coast on a summer afternoon. Identify which side is land — A or B. Explain.

Side A is land.

The trees are bending towards side A. So the wind is blowing from B to A.

On a summer afternoon, land heats faster than water. The air over the land becomes warm and rises, creating low pressure. Cooler air from the sea (high pressure) blows towards the land. This is the sea breeze. So the wind blows from the sea (B) to the land (A).

10Describe an activity to show that air flows from a region of high pressure to a region of low pressure.

Materials: two similar balloons, a drinking straw, rubber bands.

  1. Fix one end of the straw into an uninflated balloon with a rubber band.
  2. Inflate the second balloon. Hold its mouth so that air does not escape.
  3. Fix the free end of the straw into the neck of the inflated balloon without leaking air.
  4. Release your fingers and observe.

Observation: the inflated balloon becomes smaller and the uninflated balloon becomes bigger. After some time, both become almost the same size and the air flow stops.

Conclusion: air pressure is higher in the inflated balloon. Air moves from it to the low-pressure balloon till the pressure becomes equal. So air flows from high pressure to low pressure.

11What is a thunderstorm? Explain the process of its formation.

A storm accompanied by lightning and thunder is called a thunderstorm.

Land gets heated. Warm, moist air rises and creates a low-pressure area.
Rising air expands and cools. Its moisture condenses to form water droplets and clouds.
Sometimes warm air rises so high that the water droplets turn into ice particles.
Strong winds blowing up and down rub ice particles and water droplets. Electric charges develop in the clouds.
When the charges become very large, lightning occurs. Lightning heats the air suddenly. The air expands and produces the sound of thunder.
12Explain the process that causes lightning.
  1. Strong winds blowing up and down rub ice particles and water droplets in the clouds. Static electric charges develop.
  2. The lighter, positively charged ice particles move to the upper part of the cloud. The heavier, negatively charged water droplets stay in the lower part.
  3. The negatively charged lower part of the cloud makes the ground and tall objects below it positively charged.
  4. Air normally acts as an insulator. But when the charges become very large, its insulation breaks down.
  5. A sudden flow of charges takes place, producing a bright flash of light. This is lightning.

Lightning can occur within a cloud, between two clouds, or between a cloud and the ground.

13Explain why holes are made in banners and hoardings.

High-speed wind creates a large pressure difference between the front and the back of a banner. This can tear the banner or blow away the hoarding.

Holes let the wind pass through the banner. This reduces the pressure difference. So the banner does not tear and the hoarding stays safe.

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