Chapter 6: Pressure, Winds, Storms, and Cyclones - Solutions

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 ____________________.

[ Vessels P, Q, and R connected, with water being poured into R ]

• (a) the highest in vessel P
• (b) the highest in vessel Q
• (c) the highest in vessel R
• (d) equal in all three vessels

Ans: (d) equal in all three vessels.

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

• (a) Both M and N will stick to their surfaces.
• (b) Both M and N will not stick to their surfaces.
• (c) M will stick but N will not stick.
• (d) M will not stick but N will stick.

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

(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

• (a) increase the height ‘H’ at which the tank is placed.
• (b) decrease the height ‘H’ at which the tank is placed.
• (c) replace the tank with another tank of the same height that can hold more water.
• (d) replace the tank with another tank of the same height that can hold less water.

Ans: (a) increase the height ‘H’.

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

[Figure 6.22: Two vessels A and B with water at the same level ]

• (a) P_A = P_B, F_A = F_B
• (b) P_A = P_B, F_A < F_B
• (c) P_A < P_B, F_A = F_B
• (d) P_A > P_B, F_A > F_B

Ans: (a) P_A = P_B, F_A = F_B.

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. [ T ]
• (ii) Liquids exert pressure only at the bottom of a container. [ F ]
• (iii) Weather is stormy at the eye of a cyclone. [ F ]
• (iv) During a thunderstorm, it is safer to be in a car. [ T ]

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 does the boy sink more in sand? Give reasons.

[ Figure 6.23: (a) Boy lying horizontally on sand, (b) Boy standing vertically on sand ]

Ans: The boy sinks more when he is standing vertically on the loose sand.

Reason: His weight (force) is the same in both cases, but when he stands the contact area with sand is much smaller (only the soles of his feet). Since a smaller area gives greater pressure on the sand, so he sinks deeper. When he lies down, the area in contact with sand becomes large, the pressure becomes smaller, and he sinks less.

Pressure = Force / Area

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.

Ans:

Total area = 4 × 0.25 m² = 1 m²
Pressure = Force / Total Area
→ Pressure = 20000 N / 1 m²
= 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 seating 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?

Ans:

Force on Boat A = 5 persons × 700 N = 3500 N
Pressure on Boat A = Force / Area = 3500 N / 7 m² = 500 Pa

Force on Boat B = 3 persons × 700 N = 2100 N
Pressure on Boat B = Force / Area = 2100 N / 3.5 m² = 600 Pa

→ Boat B experiences more pressure by (600 Pa - 500 Pa) = 100 Pa.

Question 6

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

Ans: No, lightning would not occur if air and clouds were good conductors of electricity.

Reason: Lightning is caused by the accumulation of opposite charges in different parts of the cloud or between clouds and the ground. Air normally acts as an insulator, preventing these charges from meeting immediately. When the charge builds up to a very high level, the insulating property of air breaks down suddenly, causing a flash of lightning. If air and clouds were good conductors, the charges would flow and neutralize continuously without building up. Hence, no sudden discharge (lightning) would take place.

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? Explain your answer.

[Figure 6.24: Bottle filled with water with two identical balloons attached to its sides ]

Ans: Yes—both balloons will bulge, and they will bulge equally.

Why: The pressure a liquid exerts at a point depends only on the height (depth) of the water column above that point, not on the volume or shape of the container. Points at the same depth experience the same pressure, so both balloons receive equal outward push and expand the same amount.

Question 8

Explain how a storm becomes a cyclone.

Ans:

• A storm becomes a cyclone when it draws energy from warm ocean water.
• Warm, moist air over the sea rises, creating a low-pressure area.
• As the air rises, water vapour condenses into clouds and raindrops. This releases latent heat, which further warms the surrounding air.
• The warmed air rises even more strongly, lowering the pressure further.
• Surrounding air rushes in to fill the low-pressure region and also rises.
• Due to the Earth’s rotation (Coriolis effect), the winds begin to spin around the low-pressure centre.
• This cycle repeats, strengthening the system into a cyclone with very strong winds, heavy rain, and a calm central eye.

Question 9

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

[Figure 6.25: Trees bending towards side A, away from side B ]

Ans: Side A is land and side B is sea.

Reason: On a summer afternoon, the land heats up faster than the sea. The warm air above the land rises, creating a low-pressure area. Cooler air from the sea (high-pressure area) moves towards the land, forming a sea breeze. The trees shown bending towards side A indicate that the wind is blowing from side B (sea) to side A (land).

Question 10

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

Ans: An activity to show that air flows from a region of high pressure to a region of low pressure is the balloon experiment:

• Take two balloons and a straw. Fix one balloon (inflated) at one end of the straw and connect the other end of the straw to an empty balloon.
• When the setup is sealed, air starts moving from the inflated balloon (where pressure is high) into the empty balloon (where pressure is low).
• Gradually, the size of the inflated balloon decreases while the empty balloon expands until both become almost equal in size.
• This shows that air always moves from a region of high pressure to a region of low pressure.

Question 11

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

Ans: A thunderstorm is a storm accompanied by strong winds, rain, lightning, and thunder.

Formation process:

• During hot, humid conditions, warm moist air rises rapidly, creating a low-pressure area.
• As this air rises higher, it cools and condenses to form clouds.
• Strong upward and downward winds within the cloud cause water droplets and ice particles to rub against each other.
• This rubbing produces static electric charges – the lighter, positively charged ice particles move upwards, and the heavier, negatively charged water droplets remain at the lower part of the cloud.
• When the build-up of charges becomes very large, a sudden discharge takes place in the form of lightning.
• The rapid heating of air due to lightning causes it to expand suddenly, producing the loud sound called thunder.
• Thus, a thunderstorm is formed when moisture, rising warm air, and charge separation in clouds combine to produce lightning, thunder, and strong winds.

Question 12

Explain the process that causes lightning.

Ans: Lightning is caused by the separation and sudden discharge of electric charges in clouds.

Process:

• In a thundercloud, strong upward and downward winds make water droplets and ice particles collide and rub against each other.
• This rubbing produces static charges. The lighter, positively charged ice particles move to the upper part of the cloud, while the heavier, negatively charged water droplets remain in the lower part.
• This leads to a charge separation within the cloud. The negative charge at the cloud bottom also induces a positive charge on the ground below.
• Normally, air acts as an insulator, but when the difference in charges becomes very large, the insulating property of air breaks down.
• A sudden flow of charges takes place between the cloud and the ground (or between clouds), producing a bright flash of light called lightning.
• The rapid heating and expansion of air around the lightning channel produces the sound of thunder.

Question 13

Explain why holes are made in banners and hoardings.

Ans: Holes are made in banners and hoardings to reduce the wind pressure acting on them.

When strong winds blow, a banner or hoarding without holes experiences a large pressure difference on its surface. This can cause it to tear or get blown away. The holes allow air to pass through, which reduces the overall pressure on the surface and makes the banner or hoarding more stable and durable even in high-speed winds.