Pressure, Winds, Storms, and CyclonesClass 8 Science NCERT Solutions
13 Solutions
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Solution 1 of 13
Q1Keep the curiosity alive
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 ______ .
(a)
the highest in vessel P
(b)
the highest in vessel Q
(c)
the highest in vessel R
(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.
(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 ' ' 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.
(iv)
Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. and is the pressure at the bottom of the vessels. and is the force exerted by the water at the bottom of the vessels A and B .
(a)
(b)
Solution
(i)
(d) equal in all three vessels
(ii)
(c) M will stick but N will not stick.
(iii)
(a) increase the height 'H' at which the tank is placed.
(iv)
(b)
Q2Keep the curiosity alive
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.[ ]
Solution
(i)
True
(ii)
False
(iii)
False
(iv)
True
Q3Keep the curiosity alive
Fig. 6.23a 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.
Solution
The boy will sink more in sand when he is standing vertically (Fig 6.23b). This is because when the boy is standing, the force (weight) is acting on a smaller area, thus creating higher pressure. Pressure is equal to force divided by area. When the area is smaller, the pressure is higher, causing him to sink more.
Q4Keep the curiosity alive
An elephant stands on four feet. If the area covered by one foot is , calculate the pressure exerted by the elephant on the ground if its weight is 20000 N .
Solution
The total area covered by the elephant's four feet is 4 * 0.25 m² = 1 m².
The pressure exerted by the elephant is calculated as:
Pressure = Force / Area = 20000 N / 1 m² = 20000 N/m² or 20000 Pa.
Q5Keep the curiosity alive
There are two boats, A and B . Boat A has a base area of , and 5 persons are seated in it. Boat B has a base area of , 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?
Solution
Total weight on boat A = 5 persons * 700 N/person = 3500 N
Pressure on boat A = 3500 N / 7 m² = 500 N/m²
Total weight on boat B = 3 persons * 700 N/person = 2100 N
Pressure on boat B = 2100 N / 3.5 m² = 600 N/m²
Boat B experiences more pressure. The difference in pressure is 600 N/m² - 500 N/m² = 100 N/m².
Q6Keep the curiosity alive
Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
Solution
If air and clouds were good conductors of electricity, lightning would not occur. Lightning happens because air normally acts as an insulator, preventing the flow of electric charges. When a large build-up of charges occurs in clouds, the insulating property of air breaks down, causing a sudden flow of charges, which is lightning. If air were a good conductor, charges would flow continuously, preventing the build-up necessary for lightning.
Q7Keep the curiosity alive
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.
Solution
Yes, both the balloons will bulge when water is filled into the bottle. They will bulge equally because the pressure exerted by the liquid at the same height is the same. The water exerts pressure on the walls of the container, including the area where the balloons are attached. Since the balloons are at the same height, they will experience the same pressure and bulge equally.
Q8Keep the curiosity alive
Explain how a storm becomes a cyclone.
Solution
A storm can develop into a cyclone under certain weather conditions. Initially, warm and moist air rises over warm ocean waters, creating a low-pressure area. As the moist air rises, water vapor condenses, releasing heat into the atmosphere. This heat causes more air to rise, further lowering the pressure. Air from surrounding regions rushes in to replace the rising air, and this air also begins to rise. The Earth's rotation causes this moving air to spin. The cycle repeats itself, creating a very low-pressure area with high-speed winds revolving around it, forming a cyclone.
Q9Keep the curiosity alive
Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land - A or B. Explain your answer.
Solution
Side A is land. The trees on side A are bending towards the sea (side B). This indicates that the wind is blowing from the land (side A) towards the sea (side B). This is a characteristic of land breeze, which occurs in the afternoon when the land heats up faster than the sea, creating a pressure difference that causes wind to blow from the land to the sea.
Q10Keep the curiosity alive
Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
Solution
To demonstrate that air flows from high to low pressure, take two balloons of thin rubber and a drinking straw. Insert one end of the straw into one balloon and secure it. Inflate the second balloon and hold its mouth. Insert the free end of the straw into the inflated balloon and secure it, ensuring no air leaks. One end of the straw is now inside the inflated balloon, and the other end is inside the uninflated balloon. The air pressure in the inflated balloon is higher than in the uninflated balloon. As a result, air will move from the inflated balloon to the uninflated balloon, causing a change in the size of both balloons. This demonstrates that air moves from a region of high air pressure to a region of low air pressure.
Q11Keep the curiosity alive
What is a thunderstorm? Explain the process of its formation.
Solution
A thunderstorm is a storm accompanied by lightning and thunder. Thunderstorms form when warm, humid air rises rapidly into cooler air. As the warm air rises, it cools, and the moisture in it condenses to form water droplets, creating clouds. Strong updrafts and downdrafts facilitate the rubbing between water droplets and ice particles, causing static electric charges to develop within the clouds. The positive charges accumulate at the top of the cloud, and negative charges accumulate at the bottom. When the electrical potential between the cloud and the ground becomes great enough, a discharge of electricity occurs, resulting in lightning. The rapid heating of the air around the lightning creates a shockwave that we hear as thunder.
Q12Keep the curiosity alive
Explain the process that causes lightning.
Solution
Lightning is caused by the build-up and discharge of electrical charges in clouds. Strong winds cause water droplets and ice particles within the clouds to collide and rub against each other. This process causes a separation of charges, with positive charges accumulating in the upper part of the cloud and negative charges in the lower part. When the negative charge at the bottom of the cloud becomes large enough, it induces a positive charge on the ground below. The air normally acts as an insulator, but when the charge difference between the cloud and the ground becomes too great, the insulating property of air breaks down. A sudden flow of electric charge then occurs between the cloud and the ground (or within the cloud), creating a bright flash of light called lightning.
Q13Keep the curiosity alive
Explain why holes are made in banners and hoardings.
Solution
Holes are made in banners and hoardings to reduce the pressure exerted by the wind. When strong winds blow against a solid surface like a banner, a high-pressure area is created on the windward side. If the banner is solid, the force exerted by the wind can be very high, potentially tearing the banner or damaging the structure holding it. By making holes in the banner, the wind can pass through, reducing the pressure difference between the front and back of the banner. This minimizes the force exerted on the banner, making it less likely to be damaged by strong winds.