Keep the curiosity alive (Page 146-152)
1. Define the following
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a. Pressure
Pressure is defined as the force acting perpendicularly per unit area.
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b. Pascal
Pascal (Pa) is the SI unit of pressure, which is equivalent to one newton per metre square ($N/m^{2}$).
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c. Atmospheric pressure
The pressure exerted by the envelope of air (atmosphere) surrounding the Earth on all objects is known as atmospheric pressure.
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d. Lightning
Lightning is a sudden flow of electric charges through the air accompanied by a bright flash of light, occurring when the insulating property of air breaks down due to a very large build-up of static charges.
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e. Lightning conductor
A lightning conductor is a metallic rod installed along the outer walls of buildings with a pointed top higher than the building and the lower end buried deep in the ground to provide an easy path for transferring electric charges safely into the ground.
2. Pressure is best described as
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A. The total force acting on an object
B. The force acting per unit area
C. The area over which force is applied
D. The weight of an objectAnswer: B. The force acting per unit area
3. What will happen to the two identical balloons A and B as shown in Fig. 4.22 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.
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Yes, both balloons will bulge because liquids exert pressure on the walls and bottom of a container. They will bulge equally because the height of the water column above both balloons is the same, and liquid pressure depends on the height (depth) of the water column, not on the shape or width of the container.
4. Fig. 4.23 shows trees along the sea coast in a summer afternoon. Identify which side is land - A or B. Explain your answer.
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Side B is land. During a summer afternoon, land gets heated faster than water, causing the air above the land to become warmer, lighter, and rise to create a low-pressure area. Cooler air from the sea blows towards the land as a sea breeze, causing the trees to bend towards side B (the land).
Keep the curiosity alive (continued)
5. Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
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Take two similar thin rubber balloons and a drinking straw. Secure one end of the straw inside an uninflated balloon using a thread or rubber band. Inflate the second balloon, pinch its mouth to prevent air from escaping, attach it to the other end of the straw, and secure it tightly. When the grip is released, air flows from the inflated balloon (high pressure region) through the straw into the uninflated balloon (low pressure region) until the pressure in both balloons becomes equal.
6. What is a thunderstorm? Explain the process of its formation.
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A storm accompanied by lightning and thunder is called a thunderstorm.
Formation process: When land gets heated, warm and moist air rises to create a low-pressure area, drawing in cooler air. As the warm air rises, it expands, cools, and water vapour condenses to form clouds and heavy rain. Strong updrafts and downdrafts cause ice particles and water droplets to rub against each other, building up static electric charges. When opposite charges meet, they produce lightning and thunder, resulting in a thunderstorm.
7. Explain the process that causes lightning.
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During a storm, strong upward and downward winds cause water droplets and ice particles to rub against each other, creating static electric charges inside clouds. Lighter positive charges move to the top of the cloud, while heavier negative charges occupy the lower part, inducing a positive charge on the ground below. When the accumulation of charges becomes very large, the insulating property of air breaks down, resulting in a sudden flow of electric charges between clouds or between a cloud and the ground, producing a bright flash of light called lightning.
8. Explain why holes are made in banners and hoardings.
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Holes are made in banners and hoardings to allow high-speed wind to pass through them freely. This reduces the wind pressure acting on the surface of the banner, preventing it from tearing or getting blown away by strong winds.
9. A person finds it easier to carry a schoolbag with wide straps than narrow straps because:
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A) Wide straps reduce the weight of the bag
B) Wide straps reduce pressure on the shoulders
C) Wide straps increase force on the shoulders
D) Narrow straps increase surface areaAnswer: B) Wide straps reduce pressure on the shoulders
10. Choose the correct statement.
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i) Look at Fig. 4.24 carefully. Vessel R is filled with water. When pouring the 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 vesselsAnswer: D) equal in all three vessels
11. A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker (N) is pressed on a rough surface:
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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.Answer: C) M will stick but N will not stick.
Keep the curiosity alive (continued)
12. 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
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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.Answer: A) increase the height 'H' at which the tank is placed.
13. Two vessels, A and B contain water up to the same level as shown in Fig. 4.25. PA and PB is the pressure at the bottom of the vessels. FA and FB is the force exerted by the water at the bottom of the vessels A and B.
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A) $PA=PB$, $FA=FB$
B) $PA=PB$, $FAAnswer: A) $PA=PB$, $FA=FB$ (Note: Depending on base areas/vessel shapes shown in standard textbook problem options, pressure depends on height so $PA=PB$; if base areas differ, force $F = P \times A$ varies accordingly, matching option A when heights are equal).
14. An elephant stands on four feet. If the area covered by one foot is $0.25~m^{2}$, calculate the pressure exerted by the elephant on the ground if its weight is 20000 N.
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Given:
Force (Weight of the elephant) = $20000~N$
Area covered by one foot = $0.25~m^{2}$
Total area for four feet ($A$) = $4 \times 0.25~m^{2} = 1~m^{2}$
$\text{Pressure} = \frac{\text{Force}}{\text{Area}} = \frac{20000~N}{1~m^{2}} = 20000~N/m^{2}~(or~20000~Pa)$
15. There are two boats, A and B. Boat A has a base area of $7~m^{2}$ and 5 persons are seated in it. Boat B has a base area of $3.5~m^{2}$, 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?
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For Boat A:
Total Force ($F_A$) = $5 \times 700~N = 3500~N$
Base Area ($A_A$) = $7~m^{2}$
Pressure ($P_A$) = $\frac{3500}{7} = 500~N/m^{2}$
For Boat B:
Total Force ($F_B$) = $3 \times 700~N = 2100~N$
Base Area ($A_B$) = $3.5~m^{2}$
Pressure ($P_B$) = $\frac{2100}{3.5} = 600~N/m^{2}$
Difference in pressure = $600~N/m^{2} - 500~N/m^{2} = 100~N/m^{2}$
Therefore, Boat B will experience more pressure by $100~N/m^{2}$.
16. Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
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No, lightning would not occur if air and clouds were good conductors of electricity. Lightning occurs because air acts as an insulator that prevents opposite static charges from meeting, allowing charges to build up to a very high magnitude until the insulating property breaks down suddenly. If they were good conductors, charges would continuously and smoothly neutralize as soon as they formed, preventing any massive buildup and subsequent sudden discharge (lightning).
Keep the curiosity alive (continued)
17. State whether the following statements are True [T] or False [F].
| Statement | Ans |
|---|---|
| A) Air flows from a region of higher pressure to a region of lower pressure. | True |
| B) Liquids exert pressure only at the bottom of a container. | False |
| C) Weather is stormy at the eye of a cyclone. | False |
| D) During a thunderstorm, it is safer to be in a car. | True |
18. Fig. 4.26(a) shows a boy lying horizontally, and Fig. 4.26(b) shows the boy standing vertically on a loose sand bed. In which case does the boy sink more in sand? Give reasons.
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The boy sinks more in sand when standing vertically (Fig. 4.26(b)).
Reason: When the boy stands vertically, his entire weight acts on a much smaller surface area (area of his feet), resulting in higher pressure on the sand. When lying horizontally (Fig. 4.26(a)), the same weight is distributed over a much larger surface area of the body, creating lesser pressure and less sinking.
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