Exercise Questions and Answers
Q1:
(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 βHβ. To get water with more pressure on the ground floor, one has to
(a) increase the height βHβ at which the tank is placed.
(b) decrease the height βHβ 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. 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.
(a) PA = PB, FA = FB
(b) PA = PB, FA < FB
(c) PA < PB, FA = FB
(d) PA > PB, FA > FB
A1: The correct statements are:
(i) (d) equal in all three vessels
Reason: The vessels are interconnected, so water will settle at the same level in P, Q, and R due to the principle of communicating vessels.
(ii) (c) M will stick but N will not stick
Reason: A rubber sucker works only on a smooth surface where an airtight seal can form; on a rough surface, air leaks in, preventing suction.
(iii) The correct answer is: (a) increase the height βHβ at which the tank is placed
Reason: Water pressure at the outlet depends on the height of the water column above it (hydrostatic pressure). The greater the height HH, the greater the pressure on the ground floor.
(iv) The correct answer is: (b) PA=PB,β βFA<FB
Reason:
Force (FF) is pressure Γ area of the bottom surface. Vessel B has a larger base area, so FB>FA
Pressure (PP) at the bottom depends only on the height of the water column, not the shape or width of the container. Since both have the same water level, PA=PB.
Q2: 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. [ ]
A2: (i) True (T) β
β Air naturally moves from high-pressure regions to low-pressure regions.
(ii) False (F) β β Liquids exert pressure in all directions, not just at the bottom.
(iii) False (F) β β The eye of a cyclone is calm; the stormy weather is around the eye wall.
(iv) True (T) β
β A car acts as a Faraday cage, protecting you from lightning during a thunderstorm.
Q3:
Fig. 6.23 a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on
loose sand bed. In which case does the boy sink more in sand? Give reasons.
A3: Conclusion:
The boy sinks more in the sand when standing vertically (Fig. 6.23b). This is because the pressure is higher due to the smaller contact area, causing the sand to yield more under his weight compared to when he is lying horizontally, where the pressure is lower due to the larger contact area.
Reason:
The sinking depends on the pressure exerted on the sand. Higher pressure (from standing) compresses the loose sand more, allowing the boy to sink deeper, while lower pressure (from lying down) distributes the weight, reducing the depth of sinking.
Q4: 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 20000N.
A4: Letβs calculate step-by-step:
Given:
- Area of one foot = 0.25βm^2
- Number of feet = 4
- Total area in contact with ground = 4Γ0.25=1.0βm^2
- Weight (Force) F = 20000N
Formula: Pressure=Force/Area
Substitute values: P=20000/1.0=20000βPa
Answer: The pressure exerted = 20000βPa
Q5: There are two boats, A and B. Boat A has a base area of 7 mΒ², and 5 persons are seated in it. Boat B has a base area of 3.5 mΒ², 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?
A5:
- Boat A: Total force = 5 persons Γ 700 N = 3500 N. Base area = 7 mΒ².
Pressure = Force / Area = 3500 N / 7 mΒ² = 500 N/mΒ². - Boat B: Total force = 3 persons Γ 700 N = 2100 N. Base area = 3.5 mΒ².
Pressure = Force / Area = 2100 N / 3.5 mΒ² = 600 N/mΒ². - Comparison: Boat B experiences more pressure (600 N/mΒ²) than Boat A (500 N/mΒ²).
- Difference: 600 N/mΒ² – 500 N/mΒ² = 100 N/mΒ².
Boat B experiences 100 N/mΒ² more pressure than Boat A.
Q6: Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
A6: No, lightning would not occur if air and clouds were good conductors of electricity. Lightning occurs because air is an insulator, allowing a buildup of opposite charges in clouds and between clouds and the ground. When the charge difference becomes large, the airβs insulating property breaks down, causing a sudden flow of charges (lightning). If air and clouds were good conductors, charges would flow continuously, preventing the buildup needed for lightning.
Q7: 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.
A7: Both balloons will bulge, and they will bulge equally. The pressure exerted by the water depends on the height of the water column, not the volume or diameter of the container. Since the water is filled to the same height in both bottles, the pressure at the bottom is the same, causing both balloons to bulge to the same extent.
Q8: Explain how a storm becomes a cyclone.
A8: A storm becomes a cyclone when it forms over warm ocean waters. Warm, moist air rises, creating a low-pressure area. As the air rises, water vapor condenses, releasing heat that further lowers the pressure. Surrounding air rushes in, begins to spin due to Earthβs rotation (Coriolis effect), and forms a large, rotating system of clouds, winds, and rain. This system, with a calm center (eye) and high-speed winds, is a cyclone.
Q9: Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land – A or B. Explain your answer.
A9: Side A is land. In a summer afternoon, the land heats up faster than the sea, creating a low-pressure area over the land. Air moves from the high-pressure sea (side B) to the low-pressure land (side A), causing a sea breeze. The trees lean toward side A due to the wind blowing from the sea (side B) to the land (side A).
Q10: Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
A10:
- Take two balloons and a straw. Inflate one balloon and secure one end of the straw inside it with a rubber band. Attach the other end of the straw to an uninflated balloon.
- Observe that air flows from the inflated balloon (high-pressure region) to the uninflated balloon (low-pressure region), causing the uninflated balloon to inflate and the inflated balloon to shrink until the pressures equalize.
This demonstrates that air moves from a high-pressure region to a low-pressure region.
Q11: What is a thunderstorm? Explain the process of its formation.
A11: A thunderstorm is a storm with lightning and thunder, often accompanied by heavy rain or hail.
Formation: Warm, moist air rises due to heating, creating a low-pressure area. Cooler air flows in, rises, and cools, condensing water vapor into clouds. Strong upward and downward winds cause water droplets and ice particles to rub, generating static electric charges. Positive charges accumulate in the upper cloud, and negative charges in the lower cloud or ground. When the charge difference overcomes airβs insulating property, a sudden charge flow produces lightning, and the rapid air expansion causes thunder.
Q12: Explain the process that causes lightning.
A12: Lightning is caused by a buildup of opposite electric charges in clouds or between clouds and the ground. Warm, moist air rises, cools, and forms clouds. Strong winds cause water droplets and ice particles to rub, generating static chargesβpositive charges in the upper cloud and negative charges in the lower cloud or ground. Air, normally an insulator, prevents charge flow until the charge difference becomes large, breaking down airβs insulation. This results in a sudden flow of charges, producing a bright flash called lightning.
Q13: Explain why holes are made in banners and hoardings.
A13: Holes are made in banners and hoardings to allow wind to pass through, reducing wind pressure on the surface. Without holes, high-speed winds create a low-pressure area on one side, increasing the pressure difference across the banner, which could tear it or cause it to collapse. Holes minimize this pressure difference, making the banner more stable.