SCIENCE CLASS- 8

CHAPTER-6 (Pressure, Winds, Storms, and Cyclones)

CBSEChapter 6 Keep the curiosity alive

Question 1. Choose the correct statement.

(i) Look at Fig. 6.21 carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be __________.

Answer: (d) Equal in all three vessels

Explanation:

The three vessels are connected at the bottom. According to the principle of communicating vessels, a liquid always settles at the same level in all connected containers when it comes to rest.

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

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

Explanation:

A rubber sucker sticks because air is pushed out and atmospheric pressure presses it against the surface. On a smooth surface, an airtight seal is formed. On a rough surface, air enters through gaps and prevents sticking.

(iii) A water tank is placed on the roof of a building at a height 'H'. 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.

Explanation:

Pressure exerted by a liquid increases with depth. Increasing the height of the tank increases the water column above the ground floor and therefore increases water pressure.

(iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. P_A and P_B are pressures at the bottom and F_A and F_B are forces exerted by water at the bottom.

Answer: (b) P_A = P_B, F_A < F_B

Explanation:

Pressure at the same depth depends only on the height of the liquid column and not on the shape or width of the vessel.

Since both vessels contain water to the same height:

P_A = P_B

However, force = Pressure × Area

Vessel B has a larger base area than vessel A.

Therefore:

F_B > F_A

Question 2. State 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.

Answer: True

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

Answer: False

Explanation: Liquids exert pressure in all directions.

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

Answer: False

Explanation: The eye of a cyclone is relatively calm and has light winds.

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

Answer: True

Explanation: The metal body of the car conducts electric charge around the passengers and offers protection from lightning.

Question 3. Fig. 6.23(a) shows a boy lying horizontally, and Fig. 6.23(b) shows the boy standing vertically on a loose sand bed. In which case will the boy sink more in sand? Give reasons.

Answer:

The boy will sink more when standing vertically as shown in Fig. 6.23(b).

Reason:

Pressure is given by:

Pressure = Force ÷ Area

When standing, the same body weight acts on a much smaller area (feet).

This produces greater pressure on the sand.

When lying down, the body weight is spread over a larger area, producing less pressure.

Therefore, the boy sinks deeper when standing.

Question 4. An elephant stands on four feet. If 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.

Given:

Weight of elephant = 20000 N

Area of one foot = 0.25 m²

Total area of four feet = 4 × 0.25

= 1 m²

Formula:

Pressure = Force ÷ Area

= 20000 ÷ 1

= 20000 N/m²

Answer: 20000 Pa

Question 5. There are two boats, A and B. Boat A has a base area of 7 m² and 5 persons are seated in it. Boat B has a base area of 3.5 m² and 3 persons are seated in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much.

For Boat A:

Total force = 5 × 700

= 3500 N

Pressure = 3500 ÷ 7

= 500 Pa

For Boat B:

Total force = 3 × 700

= 2100 N

Pressure = 2100 ÷ 3.5

= 600 Pa

Comparison:

Boat B pressure = 600 Pa

Boat A pressure = 500 Pa

Difference = 600 − 500

= 100 Pa

Answer: Boat B experiences more pressure by 100 Pa.

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

Answer:

No, lightning would not occur.

Reason:

Lightning occurs because electric charges accumulate in clouds due to poor conductivity of air.

If air and clouds were good conductors, the charges would flow away continuously and would not accumulate.

Therefore, no sudden discharge of electricity (lightning) would occur.

Question 7. What will happen to the two identical balloons A and B as shown in Fig. 6.24 when water is filled into the bottle up to a certain height? Will both the balloons bulge? If yes, will they bulge equally?

Answer:

Yes, both balloons will bulge.

Reason:

Liquids exert pressure in all directions.

As water is poured into the bottle, pressure is exerted on both balloons.

Since the balloons are at the same depth, the pressure on them is equal.

Therefore, both balloons will bulge equally.

Question 8. How does a storm become a cyclone?

Answer:

A cyclone forms over warm ocean waters.

Warm air rises rapidly creating a low-pressure area.
Cool air rushes in to occupy the vacant space.
The incoming air also becomes warm and rises.
This process continues repeatedly.
The Earth's rotation causes the moving air to spin.
The rotating system grows stronger and develops into a cyclone.

Question 9. Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land—A or B. Explain.

Answer: Side B is the sea and Side A is the land.

Explanation:

During a summer afternoon, land heats up faster than sea water.

The air above the land becomes warm and rises.

Cool air from the sea moves towards the land producing a sea breeze.

The trees bend in the direction of the wind flow.

Since the trees are bent towards land, the wind is blowing from B to A.

Therefore:

A = Land
B = Sea

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

Activity:

Take an empty plastic bottle.
Insert a balloon inside the bottle and stretch its mouth over the bottle neck.
Try to inflate the balloon.
It will be difficult because air trapped inside the bottle prevents expansion.
Now make a small hole in the bottle.
Again inflate the balloon.

Observation:

The balloon inflates easily.

Hence

Air moves from the higher pressure inside the bottle to the lower pressure outside through the hole.

This shows that air flows from a region of high pressure to a region of low pressure.

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

Answer:

A thunderstorm is a weather phenomenon accompanied by lightning, thunder, strong winds and heavy rainfall.

Formation:

The ground becomes very hot.
Warm moist air rises rapidly.
As it rises, it cools and condenses to form clouds.
Large clouds continue growing vertically.
Electric charges develop inside the clouds.
The release of these charges produces lightning and thunder.

Question 12. Explain the process that causes lightning.

Answer:

During a thunderstorm, positive and negative charges accumulate in different parts of clouds.

A large potential difference develops between clouds or between clouds and the Earth.

When the charge becomes very large, a sudden discharge of electricity occurs.

This discharge produces a bright flash of light called lightning.

The rapid heating and expansion of air causes the sound known as thunder.

Question 13. Explain why holes are made in banners and hoardings.

Answer:

Strong winds exert pressure on banners and hoardings.

If there are no holes, a large force acts on them and they may tear or collapse.

Holes allow air to pass through, reducing wind pressure.

Therefore, banners and hoardings remain more stable and safer during strong winds.

Important Concepts from the Chapter

Pressure = Force ÷ Area
Liquids exert pressure in all directions.
Pressure in a liquid increases with depth.
Air moves from high pressure to low pressure.
Cyclones form due to differences in air pressure and temperature.
Lightning is a sudden discharge of electricity.
Atmospheric pressure helps rubber suckers stick to smooth surfaces.
Larger area produces less pressure for the same force.

CHAPTER-6 (Pressure, Winds, Storms, and Cyclones)

Question 1. Choose the correct statement.

(i) Look at Fig. 6.21 carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be __________.

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

(iii) A water tank is placed on the roof of a building at a height 'H'. To get water with more pressure on the ground floor, one has to:

(iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. PA and PB are pressures at the bottom and FA and FB are forces exerted by water at the bottom.

Question 2. State 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.

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

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

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

Question 3. Fig. 6.23(a) shows a boy lying horizontally, and Fig. 6.23(b) shows the boy standing vertically on a loose sand bed. In which case will the boy sink more in sand? Give reasons.

Question 4. An elephant stands on four feet. If 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.

Question 5. There are two boats, A and B. Boat A has a base area of 7 m2 and 5 persons are seated in it. Boat B has a base area of 3.5 m2 and 3 persons are seated in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much.

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

Question 7. What will happen to the two identical balloons A and B as shown in Fig. 6.24 when water is filled into the bottle up to a certain height? Will both the balloons bulge? If yes, will they bulge equally?

Question 8. How does a storm become a cyclone?

Question 9. Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land—A or B. Explain.

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

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

Question 12. Explain the process that causes lightning.

Question 13. Explain why holes are made in banners and hoardings.

Important Concepts from the Chapter

(iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. P_A and P_B are pressures at the bottom and F_A and F_B are forces exerted by water at the bottom.

Question 4. An elephant stands on four feet. If 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.

Question 5. There are two boats, A and B. Boat A has a base area of 7 m² and 5 persons are seated in it. Boat B has a base area of 3.5 m² and 3 persons are seated in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much.