Key Concept: Pressure and Area Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a bag with broad straps exerts less pressure on the shoulders than a bag with narrow straps when the weights are equal. This is correct because pressure is calculated as $Pressure = \frac{Force}{Area}$. For the same force (weight of the bag), increasing the area (broad straps) reduces the pressure. Therefore, the assertion is true.
The reason states that pressure is inversely proportional to the area over which the force is applied. This is also correct because the formula for pressure shows that as area increases, pressure decreases for a constant force. Moreover, this correctly explains why broad straps reduce pressure. Hence, the reason is true and correctly explains the assertion.

Key Concept: Pressure, Force and Area

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that the pressure exerted by a liquid depends only on the height of the liquid column and not on the shape or cross-sectional area of the container. This is true because pressure at any point in a liquid is given by $P = \rho g h$, where $\rho$ is the density of the liquid, $g$ is the acceleration due to gravity, and $h$ is the height of the liquid column. The Reason states that pressure in liquids is transmitted equally in all directions due to Pascal's principle, which is also true. However, Pascal's principle explains how pressure is distributed, not why it depends only on the height. Therefore, the Reason does not correctly explain the Assertion.

Key Concept: Application of pressure

d) The pointed end decreases the area, increasing pressure
[Solution Description]
The pointed end of a nail has a smaller surface area compared to the blunt end. Since pressure is inversely proportional to area ($Pressure = \frac{Force}{Area}$), the smaller area results in higher pressure, making it easier to penetrate surfaces.

Key Concept: Definition of pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a bag with broad straps exerts less pressure on the shoulders compared to a bag with narrow straps carrying the same weight. This is true because pressure is calculated as $Pressure = \frac{Force}{Area}$. For the same force (weight of the bag), increasing the area (broad straps) decreases the pressure.
The reason defines pressure as force per unit area, which is correct and directly explains why broad straps reduce pressure. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Pressure Definition

c) Force applied per unit area
[Solution Description]
Pressure is defined as force per unit area. The formula is: $Pressure = \frac{Force}{Area}$.

Key Concept: Pressure & Area Relationship

a) Surface with 2 m$^2$
[Solution Description]
Pressure is inversely proportional to area when the force is constant. Since $P = \frac{F}{A}$, a smaller area results in higher pressure. Here, 2 m$^2$ is smaller than 4 m$^2$, so the first surface will experience greater pressure.

Key Concept: Formula for Pressure

b) \(50\ \mathrm{N/m^2}\)
[Solution Description]
Pressure tells us how much force is acting on each unit area. Here, a force of 200 N is spread over an area of 4 square metres. To find the pressure, divide the force by the area. So, each square metre receives 50 N of force. Therefore, the pressure exerted is 50 pascals (Pa).

Key Concept: Pressure and Area Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a sharp knife cuts better because it exerts more pressure. This is true since pressure ($P$) is calculated as $P = \frac{F}{A}$, where $F$ is force and $A$ is area. For the same force, reducing the area (sharp edge) increases pressure. The reason correctly explains the inverse relationship between pressure and area when force is constant. Thus, both assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Pressure, Force Distribution

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a sharp knife cuts better due to its smaller contact area, which increases pressure for the same force. The reason correctly explains this by stating the inverse relationship between pressure and area under constant force (P = F/A). Thus, both are true and the reason is the correct explanation.

Key Concept: Pressure, Area Relationship

b) Pressure doubles
[Solution Description]
The pressure is given by $P = \frac{F}{A}$. When the strap width is halved (from 5 cm to 2.5 cm), the area of contact decreases since $A$ is proportional to width for a given length. Thus, pressure doubles because $P$ is inversely proportional to $A$.

Key Concept: Atmospheric Pressure Concept

b) Atmospheric pressure
[Solution Description]
The pressure exerted by air around us is known as atmospheric pressure, as explained in the syllabus. It acts on all objects and balances the internal pressure of our bodies.

Key Concept: Atmospheric Pressure and Suction Cup

d) The higher external atmospheric pressure pushes the sucker onto the surface.
[Solution Description]
When the sucker is pressed, most of the air inside is pushed out, reducing the internal pressure. The external atmospheric pressure ($P_{atm}$) becomes higher than the pressure inside ($P_{in}$). This difference in pressure ($P_{atm} - P_{in}$) creates a net inward force that holds the sucker firmly against the surface.

Key Concept: Activity-Based Question

d) Air exerts greater pressure on the larger area of the unfolded sheet
[Solution Description]
Air exerts force on the surface area in contact. Since the unfolded sheet has a larger area than the folded one, the total force (and thus pressure) acting on it increases, requiring more effort to lift.

Key Concept: Magnitude of Atmospheric Pressure

c) The pressure inside our body balances the atmospheric pressure
[Solution Description]
Atmospheric pressure exerts a large force, but our body’s internal pressure balances it, preventing us from being crushed. This equilibrium ensures no net force acts on our bodies.

Key Concept: Units of Pressure Conversion

c) $1013 \hspace{0.1em} \text{mb}$
[Solution Description]
1 hectopascal \(1\ \mathrm{hPa} = 1\ \mathrm{millibar} = 1\ \mathrm{mb}\). Thus, the conversion is direct:
$1013 \hspace{0.1em} \text{hPa} = 1013 \hspace{0.1em} \text{mb}$

Key Concept: Atmospheric Pressure and Area

c) The force doubles because force is directly proportional to area under constant pressure.
[Solution Description]
Atmospheric pressure is defined as force per unit area ($P = F/A$). If the area ($A$) doubles while pressure ($P$) remains constant (since it's atmospheric pressure), the force ($F$) must also double to maintain the equation $F = P \times A$. Thus, the force increases with increased area.

Key Concept: Air pressure, Surface area comparison

c) 1:4
[Solution Description]
1) When the chart paper is folded twice, its area reduces to 1/4th of the original (since folding halves the length and width: (70/2) × (56/2) = 35 × 28 cm).
2) The unfolded sheet maintains full area (70 × 56 cm).
3) Air pressure $P = F/A$, where F remains constant (same weight of chart paper), but A changes.
4) Pressure ratio = $\frac{P_{\text{unfolded}}}{P_{\text{folded}}} = \frac{F/A_{\text{unfolded}}}{F/A_{\text{folded}}} = \frac{A_{\text{folded}}}{A_{\text{unfolded}}}$
5) Calculation: $\frac{35×28}{70×56} = \frac{980}{3920} = \frac{1}{4}$
Thus, the pressure ratio is 1:4.

Key Concept: Direction of Air Pressure

c) Air pressure acts uniformly in all directions
[Solution Description]
Air inside the balloon exerts pressure uniformly in all directions (a property of fluids/gases). This isotropic pressure causes the balloon's walls to stretch outward equally, leading to expansion in all directions. The explanation aligns with the concept that air pressure is not directional but acts omnidirectionally.

Key Concept: Pressure and Area Relationship

b) Atmospheric pressure exerts a force proportional to the area exposed
[Solution Description]
Since the weight of the covering sheets remains constant but the effort varies with area, the increase in effort must be due to air pressure. Pressure ($P$) is force ($F$) per unit area ($A$), given by $P = F/A$. Larger area means more total force from atmospheric pressure acting downward, hence greater lifting effort.

Key Concept: Atmospheric Pressure Directionality

b) Air pressure inside the balloon acts uniformly in all directions
[Solution Description]
Air inside the balloon exerts pressure uniformly in all directions. This property of gases ensures that the pressure is not limited to one direction, causing the balloon walls to expand outward equally in all directions until internal pressure balances external forces.

Key Concept: Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] When air is blown into a balloon, it fills the space inside and exerts pressure on the inner walls of the balloon. This pressure acts equally in all directions, causing the balloon to expand uniformly. The reason correctly explains why the balloon expands in all directions because air indeed exerts pressure uniformly in all directions.

Key Concept: Air movement due to pressure difference

c) Air moves from high-pressure regions to low-pressure regions.
[Solution Description]
In Activity 6.5, air moves from the inflated balloon to the uninflated one because the air pressure inside the inflated balloon is higher than that in the uninflated balloon. Air flows from a region of high pressure to a region of low pressure until the pressures equalize.

Key Concept: Pressure Exerted by Air

b) The balloon inflates because the air inside exerts pressure on its walls.
[Solution Description]
When air is blown into an uninflated balloon, the air exerts pressure on the inner walls of the balloon in all directions. This causes the balloon to expand uniformly as the rubber stretches to accommodate the increased pressure inside. The expansion continues until the internal air pressure balances with the elastic forces of the rubber.

Key Concept: Pressure Exerted by Air

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that when air is blown between two inflated balloons, they move towards each other. This is true because, as demonstrated in Activity 6.6, blowing air between the balloons creates a low-pressure area between them. The higher air pressure outside the balloons then pushes them towards the low-pressure region, causing them to move closer.
The Reason correctly explains the Assertion, as it describes the phenomenon of high air pressure moving towards low air pressure, which causes the balloons to move towards each other. Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Atmospheric Pressure

b) Narrow straps concentrate pressure on a smaller area
[Solution Description]
Broad straps increase the area over which the weight of the bag is distributed. Since pressure is defined as force per unit area ($P = \frac{F}{A}$), increasing the area reduces the pressure exerted on the shoulders. Therefore, carrying a bag with broad straps feels more comfortable because the pressure is spread out over a larger area.

Key Concept: Units of pressure, Real-world application

d) 100,000 Pa
[Solution Description]
First, convert mass to force: $F = mg = 225 \, \text{kg} \times 10 \, \text{m/s}^2 = 2250 \, \text{N}$. Then, convert area to square meters: $225 \, \text{cm}^2 = 0.0225 \, \text{m}^2$. Pressure $P = F/A = 2250 / 0.0225 = 100,\!000 \, \text{Pa}$ (or $1000 \, \text{hPa}$).

Key Concept: Atmospheric Pressure

b) Air exerts pressure equally in all directions inside the balloon.
[Solution Description]
When air is blown into a balloon, it exerts pressure on the inner walls of the balloon. Since air exerts pressure equally in all directions (isotropic nature of pressure), the balloon expands uniformly in all directions. This is due to the balanced force distribution caused by the air molecules colliding with the balloon's surface.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When a sucker is pressed against a smooth surface, most of the air inside is expelled, creating a low-pressure region inside the sucker. The atmospheric pressure outside is much higher than the pressure inside the sucker, resulting in a net force pressing the sucker onto the surface. Thus, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: High-speed winds, Air pressure

d) Low pressure above the roof creates an upward force.
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure below the roof (inside the house) remains higher. This pressure difference exerts an upward force on the roof. If the roof is not strong enough to withstand this force, it gets blown away.

Key Concept: Pressure difference, Wind formation

a) Wind is formed with speed proportional to the pressure difference.
[Solution Description]
When air moves from a high-pressure region to a low-pressure region, it results in the formation of wind. The greater the pressure difference, the faster the wind speed. This is because nature tends to balance pressure differences by moving air from areas of higher pressure to areas of lower pressure.

Key Concept: Sea breeze and land breeze

c) Because the air above the land rises, creating a low-pressure region
[Solution Description]
During the day, land heats up faster than water, causing the air above the land to become warmer and rise, creating a low-pressure region. Cooler air from the sea (high-pressure region) moves towards the land to fill this gap, resulting in a sea breeze.

Key Concept: Pressure difference and wind direction

c) Difference in air pressure between regions
[Solution Description]
Air moves from a region of high pressure to a region of low pressure due to the pressure difference between the two regions. This movement of air is what we perceive as wind.

Key Concept: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that air moves from a region of high pressure to a region of low pressure, which is true as demonstrated in Activity 6.5 where air flows from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The reason explains that this pressure difference causes the movement of air, which is also true and correctly explains the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Units of Pressure

c) Newton per cubic meter ($N/m^3$)
[Solution Description]
Air pressure can be measured in pascals ($Pa$), millibars ($mb$), or hectopascals ($hPa$). Newtons per cubic meter ($N/m^3$) measures density, not pressure.

Key Concept: Land and Sea Breeze

c) Because the sea is at a lower pressure than the land
[Solution Description]
At night, the land cools faster than the sea, resulting in higher pressure over the land compared to the sea. Air moves from the high-pressure land region to the low-pressure sea region, forming a land breeze.

Key Concept: Basic Concept

b) Difference in air pressure
[Solution Description]
Wind is caused by the movement of air from a high pressure region to a low pressure region. This happens because air naturally moves to balance the pressure differences between two regions.

Key Concept: Pressure difference and wind speed

c) Wind speed increases
[Solution Description]
The speed of the wind is directly proportional to the pressure difference between two regions. A higher pressure difference results in a higher wind speed as air moves more rapidly from the high-pressure region to the low-pressure region.

Key Concept: Formation of Wind - High and Low Pressure Regions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion correctly states that air moves from a high-pressure region to a low-pressure region, which is the fundamental principle behind wind formation. The Reason explains why this happens: warm air rising creates low pressure, and cool air sinking creates high pressure, leading to the movement of air from high to low pressure. Thus, the Reason correctly explains the Assertion.

Key Concept: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
During the day, the land heats up faster than the sea due to differences in specific heat capacity. This causes the air above the land to warm up and rise, creating a low-pressure region. Cooler air from the higher-pressure region over the sea moves toward the low-pressure region over the land, resulting in a sea breeze. Thus, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Thunderstorms, Pressure difference

d) The pressure difference between outside and inside the house lifts the roof.
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure inside the house remains higher, creating a pressure difference. If this difference is large enough and the roof is weak, it can be blown away. Keeping windows open equalizes the pressure.

Key Concept: Pressure Differences, Sea Breeze

b) 1007 hPa
[Solution Description]
During the day, land heats up faster than the sea, creating a low-pressure area over the land. The cooler air over the sea has higher pressure, causing a sea breeze. Since the land pressure is given as 1005 hPa (low pressure), the sea pressure must be higher. Typically, pressure differences for sea breezes range between 2-4 hPa. Thus, the sea pressure is approximately 1007 hPa to 1009 hPa.

Key Concept: Sea Breeze, Land Breeze, Pressure Differences

d) Assertion is false, but Reason is true.
[Solution Description]
Let's analyze both Assertion (A) and Reason (R):
1. **Assertion (A):** "During the day, wind blows from the sea to the land because the land is cooler than the sea."
- This is false. During the day, the land gets heated faster than the sea, making the air above the land warmer and lighter. This creates a low-pressure area over the land, causing wind (sea breeze) to blow from the high-pressure sea region to the low-pressure land region.
2. **Reason (R):** "At night, the land cools faster than the sea, creating a high-pressure area over the land."
- This is true. At night, the land loses heat faster than the sea, making the air above it cooler and denser, which leads to high pressure over the land. As a result, wind (land breeze) blows from the land to the sea where the pressure is lower.
Since Assertion (A) is false but Reason (R) is true, the correct option is the one stating this relationship.

Key Concept: Sea and Land Breezes

b) The land heats up faster than the sea, causing air to move from the sea to the land.
[Solution Description]
The land heats up faster than the sea during the day, causing the air above it to warm and rise, creating a low-pressure area. The cooler, high-pressure air over the sea moves towards the land, resulting in a sea breeze.

Key Concept: Sea and Land Breezes

a) Sea breeze occurs because the sea is cooler and has higher pressure compared to the land.
[Solution Description]
During the day, land heats up faster than the sea. The air above the land becomes warm and rises, creating a low-pressure area. As a result, cooler air from the high-pressure region over the sea moves towards the land, forming a sea breeze.

Key Concept: Pressure equalization, Wind effects on structures

b) To reduce the pressure difference between inside and outside
[Solution Description]
When high-speed winds blow over a house, they create a low-pressure region above the roof. If the windows are tightly shut, the pressure inside the house remains higher compared to the outside, creating a significant pressure difference. This can cause the roof to lift or blow away. By keeping windows and doors slightly open, the wind flows through the house, reducing the pressure difference between the inside and outside, thereby minimizing structural damage.

Key Concept: High-Speed Winds Result in Lowering of Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, high-speed winds blowing over a surface create a low-pressure area because the fast-moving air reduces the number of air molecules applying force per unit area. This explains why roofs may be blown off during storms when high-speed winds lower the pressure outside while higher pressure inside pushes upward. Therefore, the Assertion is true because high-speed winds indeed reduce air pressure, and the Reason correctly explains this phenomenon by describing how fast-moving air leads to fewer air molecules exerting pressure.

Key Concept: High-speed winds and air pressure

c) The balloons move towards each other due to reduced pressure between them.
[Solution Description]
When high-speed wind is blown between the two balloons, it creates a low-pressure region between them. The higher air pressure on the outer sides of the balloons pushes them towards each other. This demonstrates that high-speed winds lower air pressure.

Key Concept: High-Speed Winds Result in Lowering of Air Pressure, Bernoulli's Principle

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that keeping doors and windows closed during a storm increases the likelihood of roofs blowing off. This is true because when high-speed winds pass over the roof, they create a low-pressure area above it (Bernoulli's principle). If the house remains airtight (doors/windows closed), the higher air pressure inside the house exerts an upward force on the roof, potentially causing it to blow off. The reason correctly explains this phenomenon using Bernoulli's principle, which relates high-speed winds ($v$) and reduced pressure ($P$): $P + \frac{1}{2} \rho v^2 = \text{constant}$. Thus, both assertion and reason are true, and the reason is the correct explanation for the assertion.

Key Concept: Pressure difference and storm damage

b) It balances indoor and outdoor air pressure
[Solution Description]
During a storm, high-speed winds over the roof create low pressure outside. If windows are closed, the higher pressure inside the house can push the roof upward, causing damage. Opening windows equalizes the pressure inside and outside, reducing the risk of roof damage.

Key Concept: Low-pressure areas

c) Low pressure created by wind pulls them together
[Solution Description]
When air is blown between two balloons, it creates a low-pressure zone between them. The higher pressure outside the balloons pushes them inward, causing them to move closer. This demonstrates how high-speed winds reduce pressure in an area.

Key Concept: Pressure Difference Due to High-Speed Winds

b) The high pressure inside the house pushes the roof up as low pressure forms outside.
[Solution Description]
During storms, high-speed winds pass over the roof, reducing the air pressure above it. The higher air pressure inside the house pushes upwards on the roof due to this pressure difference. If the pressure difference is large enough, it can lift and blow the roof off. This demonstrates how high-speed winds create low-pressure areas.

Key Concept: Practical Example of Low Pressure

c) The balloons move towards each other.
[Solution Description]
Blowing air between two balloons increases the wind speed in that region, creating a low-pressure area between them. The surrounding higher-pressure air pushes the balloons inward, causing them to come closer together. This illustrates how high-speed winds reduce pressure.

Key Concept: Effect of Blowing Harder

c) The balloons move towards each other more quickly.
[Solution Description]
Blowing harder increases the speed of the air moving between the balloons, reducing the air pressure further. As a result, the difference in pressure causes the balloons to move towards each other more quickly.

Key Concept: Balloon Movement Due to Pressure Difference

c) The high-pressure air outside pushes them inward.
[Solution Description]
Blowing air between the balloons creates a low-pressure zone due to the high-speed wind. The higher atmospheric pressure outside the balloons exerts a force, pushing them toward the low-pressure region between them, causing them to move closer.

Key Concept: High-Speed Winds and Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that when air is blown between two inflated balloons, they move towards each other, which is observed in Activity 6.6. This happens because blowing air between the balloons increases wind speed in the gap, reducing air pressure (Bernoulli’s principle). The higher pressure outside then pushes the balloons together. The reason correctly explains why this occurs by linking high-speed wind to low pressure and the resulting pressure difference.

Key Concept: Pressure Gradient, Force Calculation

c) 0.4 N
[Solution Description]
The pressure difference ($\Delta P$) is proportional to the square of the airspeed ($v^2$). If $v$ doubles from 30 m/s to 60 m/s, $\Delta P$ becomes $(60/30)^2 = 4$ times larger.
Since force ($F$) is directly proportional to $\Delta P$, the new force will also be 4 times the original:
$F_{\text{new}} = 4 \times 0.1\,\text{N} = 0.4\,\text{N}$.

Key Concept: Unit of Pressure

b) Pascal ($Pa$)
[Solution Description]
The SI unit of pressure is $N/m^2$, also known as pascal ($Pa$). Other units like millibar ($mb$) and hectopascal ($hPa$) are also used, but $Pa$ is the standard SI unit.

Key Concept: High-speed winds and pressure difference

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When high-speed winds blow over a house, they create a low-pressure area above the roof. Inside the house, the air pressure remains higher compared to the outside. This pressure difference exerts an upward force on the roof. If the roof is weak or not properly secured, it can be blown off due to this force. Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Pressure reduction, Wind speed

b) 98,900 Pa
[Solution Description]
Given:
- Initial pressure ($P_{\text{initial}}$) = $1.02 \times 10^5$ Pa
- Pressure reduction = 3% of initial pressure
Step 1: Calculate pressure reduction amount
$0.03 \times 1.02 \times 10^5 = 3060$ Pa
Step 2: Calculate new pressure
$1.02 \times 10^5 - 3060 = 0.9894 \times 10^5$ Pa = $98,940$ Pa
Therefore, the new pressure is approximately 98,900 Pa

Key Concept: Pressure Difference Due to High-Speed Winds

b) The wind reduces air pressure above the roof, causing an upward force
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the pressure inside the house remains higher. The pressure difference causes an upward force on the roof. If the roof is weak or not properly secured, this force can blow it away.

Key Concept: Lightning Formation, Thunderstorms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lightning occurs when the charge buildup is sufficient to overcome air's insulating property, which is accurate as per the syllabus. The reason explains the mechanism where the negatively charged lower part of the cloud induces a positive charge on the ground, creating a potential difference that results in lightning. Since the reason correctly explains the cause of the phenomenon described in the assertion, both are true and the reason is the correct explanation.

Key Concept: Formation of Thunderstorms

c) Upward movement of warm, moist air leading to condensation and cloud formation
[Solution Description]
The primary cause for the formation of thunderstorms is the upward movement of warm, moist air leading to condensation and the development of clouds. This process creates charge separation in clouds due to friction between rising and descending air currents.

Key Concept: Thunderstorm conditions, Safety measures

c) Standing near a tall tree in an open field
[Solution Description]
Standing near a tall tree is unsafe because lightning tends to strike the tallest object in an area. Even if crouching in an open area is advised, proximity to tall objects increases the risk of being struck.

Key Concept: Local Names of Thunderstorms

b) Bordoisila
[Solution Description]
In Assam, thunderstorms are locally known as Bordoisila. Kalboishakhi is associated with West Bengal, Bihar, and Jharkhand, while Mango showers occur in Kerala, Karnataka, and Tamil Nadu.

Key Concept: Precautions during thunderstorms

c) Crouch down in a low-lying open area
[Solution Description]
During thunderstorms, it is safer to crouch down in a low-lying open area to minimize contact with the ground and avoid being the tallest object, reducing the risk of being struck by lightning.

Key Concept: Charge separation, Lightning formation, Cloud dynamics

c) The negatively charged lower part of the cloud induces a positive charge on the ground.
[Solution Description]
The negative charges accumulate in the lower part of the cloud due to heavier water droplets, while positive charges rise to the upper part. When the negative charge in the cloud induces a positive charge on the ground, the potential difference increases until it discharges as lightning.

Key Concept: Formation of thunderstorms

c) Rubbing between water droplets and ice particles due to upward and downward winds
[Solution Description]
Strong winds blowing upwards and downwards cause rubbing between water droplets and ice particles, leading to static electric charge separation. The positively charged lighter ice particles move to the top of the cloud, while negatively charged heavier water droplets settle at the bottom.

Key Concept: Formation of Storms

d) Low pressure above the roof and high pressure below it
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the air pressure inside the house remains higher. The large pressure difference can cause weak roofs to be blown away.

Key Concept: Formation of Storms, Thunderstorms and Lightning

a) Warm air rises → Moisture condenses → Wind circulation → Charge separation → Lightning
[Solution Description]
First, warm moist air rises, creating a low-pressure area. Cooler air moves in to replace it, causing wind circulation. As the air rises, it cools, condenses into clouds, and forms water droplets. In thunderstorms, strong updrafts lift droplets to freezing heights, converting them to ice particles. Friction between these particles creates static charges, separating positive (top) and negative (bottom) charges in the cloud. When the charge difference becomes large enough, air's insulating property breaks down, causing a sudden discharge (lightning).

Key Concept: Charge Separation in Thunderstorms

b) Due to friction between water droplets and ice particles caused by strong winds.
[Solution Description]
Strong winds rub water droplets and ice particles against each other, causing static electric charges to develop. Positively charged lighter ice particles move upwards while negatively charged heavier water droplets settle in the lower part of the cloud, leading to charge separation.

Key Concept: Lightning Formation

d) Breakdown of air insulation due to excessive charge buildup
[Solution Description]
When the buildup of opposite charges (positive on the ground/objects and negative in the cloud's lower part) becomes too large, the insulating property of air breaks down. A sudden flow of charges occurs, producing lightning.

Key Concept: Localized Thunderstorms

c) They supply necessary rain before monsoon for kharif crops.
[Solution Description]
Localized thunderstorms such as Kalboishakhi occur before the monsoon and provide essential rainfall that supports the growth of kharif crops and aids in ripening fruits like mangoes, benefiting agriculture.

Key Concept: Lightning formation

c) Discharge between the lower part of a cloud and the positively charged ground
[Solution Description]
When the negatively charged lower part of the cloud induces a positive charge on the ground, the buildup of opposite charges overcomes air's insulating property, leading to a sudden discharge called cloud-to-ground lightning.

Key Concept: Static charge development in clouds

b) Friction between ice particles and water droplets due to wind movement
[Solution Description]
The strong upward and downward winds within a cloud cause rubbing between water droplets and ice particles, leading to charge separation. Lighter positively charged ice particles move upwards while heavier negatively charged water droplets remain at the lower part of the cloud.

Key Concept: Static charge development within clouds leading to lightning

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that lightning occurs only when a cloud is negatively charged at the bottom and the ground is positively charged. This is false because lightning can also occur within a single cloud (intra-cloud lightning) or between two different clouds (inter-cloud lightning). The Reason, however, is true as it correctly describes the process of charge separation in clouds due to the rubbing of water droplets and ice particles from strong winds.

Key Concept: Safety precautions during lightning

d) Crouching in a low-lying open area
[Solution Description]
During a thunderstorm, crouching in a low-lying open area minimizes the risk of being struck by lightning as it reduces exposure and avoids tall objects that attract lightning.

Key Concept: Precautions During Thunderstorms

c) Using an umbrella with a metallic rod
[Solution Description]
The syllabus advises avoiding tall objects, staying away from open water, and not lying flat on the ground. Using an umbrella with a metallic rod is unsafe because metal conducts electricity, increasing the risk of being struck by lightning. Seeking shelter inside a car or low-lying area is safer.

Key Concept: Charge separation, Lightning and Thunder

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that thunder is formed due to the rapid heating and expansion of air from lightning, which is correct because the intense heat of lightning causes the surrounding air to expand explosively, creating the sound wave we hear as thunder. The Reason explains why lightning occurs—due to the breakdown of air's insulating property when charge buildup is excessive, which is also correct. However, the Reason does not explain why thunder is formed; it only explains lightning. Thus, both statements are true, but the Reason does not explain the Assertion.

Key Concept: Thunderstorm Formation, Charge Build-Up

c) Excessive build-up of electric charges
[Solution Description]
Normally, air acts as an insulator and prevents opposite charges from meeting. However, when the build-up of charges becomes very large, the insulating property of air breaks down, allowing a sudden flow of charges. This produces a bright flash of light known as lightning.

Key Concept: Formation of Thunder

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that thunder is produced due to rapid expansion of air caused by the intense heat from lightning. According to the syllabus, thunder indeed results from the explosive expansion of air heated by lightning. The reason explains that lightning involves a sudden flow of electric charges, which heats the air. This directly supports the assertion because the mechanism described in the reason leads to the effect mentioned in the assertion. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Cyclone Impact

c) Saltwater making soil more fertile.
[Solution Description]
When a cyclone reaches land, it loses its source of moist air and gradually weakens. However, it still causes destruction through strong winds, storm surges, heavy rainfall, flooding, landslides, and contamination of freshwater sources. Improved soil fertility is not an impact; instead, seawater intrusion makes soil less fertile due to salt deposits.

Key Concept: Eye of the Cyclone

b) Descending air in the eye suppresses wind activity.
[Solution Description]
The eye of the cyclone is the region of lowest pressure at the center where the ascending air stops rising and begins descending. This descending motion suppresses cloud formation and wind activity, resulting in calm conditions. Meanwhile, the surrounding regions experience strong winds and heavy rainfall due to the cyclonic circulation.

Key Concept: Cyclone dynamics, Coriolis effect

c) It would start rotating clockwise
[Solution Description]
Cyclones rotate counterclockwise in the Northern Hemisphere due to the Coriolis effect, which deflects moving air to the right. In the Southern Hemisphere, the Coriolis effect deflects moving air to the left, causing cyclones to rotate clockwise. Therefore, the cyclone's rotation would reverse direction upon crossing the equator.

Key Concept: Cyclone Impact

c) Earthquakes due to wind pressure
[Solution Description]
Cyclones cause destruction through strong winds, storm surges, and heavy rains, but they do not typically trigger earthquakes.

Key Concept: Cyclone Formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that cyclones lose strength over land due to the absence of warm, moist air, which is correct as per the syllabus. The Reason explains that warm ocean water provides heat and moisture to sustain cyclones, which is also accurate. Since the Reason correctly explains why cyclones weaken over land (due to the loss of this energy source), both statements are true and logically connected.

Key Concept: Formation Process

c) Over warm ocean waters
[Solution Description]
Cyclones form over warm ocean waters due to differences in temperature and pressure. The text states: "Cyclones are large storms that form over warm ocean waters."

Key Concept: Impact of Cyclones

d) Gradual weakening due to loss of moisture source
[Solution Description]
When a cyclone reaches land, its source of moist air from the ocean is cut off, causing it to weaken gradually. However, before weakening, cyclones can cause severe destruction, including storm surges flooding coastal areas, heavy rainfall leading to landslides, and contamination of freshwater sources due to saltwater intrusion.

Key Concept: Basic Concept

c) Eye of the cyclone
[Solution Description]
The eye of the cyclone is the central region where the pressure is the lowest and winds are calm. This is mentioned in the text as: "In a cyclone, the region of lowest pressure is at the centre, known as the eye of the cyclone."

Key Concept: Destruction Caused by Cyclones

c) Storm surges flooding coastal regions
[Solution Description]
Cyclones generate storm surges—walls of water pushed toward the shore by high-speed winds—which can flood coastal areas, damage infrastructure, and contaminate freshwater sources with saltwater.

Key Concept: Eye of the Cyclone

c) The area of low pressure at the center with calm winds
[Solution Description]
The eye of a cyclone is the central region where the atmospheric pressure is the lowest. It is characterized by calm winds and clear skies, while the surrounding areas experience high-speed winds and heavy rain.

Key Concept: Heavy rainfall, Destructive Effects

c) Soil becomes less fertile due to increased salinity
[Solution Description]
Cyclone-induced floods often bring saline seawater inland. A salinity level of 10 ppt is harmful to crops as it makes the soil less fertile by increasing salt concentration. This reduces the soil's ability to retain water and nutrients, negatively affecting crop yield.

Key Concept: Formation of Cyclones

c) By heating the air, causing it to rise and form a low-pressure zone
[Solution Description]
Cyclones form over warm ocean waters when the moist air above the heated water rises. As it rises, the water vapour condenses into raindrops, releasing heat, which warms the air further and causes it to rise more. This creates a low-pressure area that draws in surrounding air, forming a cyclonic system.

Key Concept: Cyclone energy, atmospheric pressure

c) 0.167 m/s²
[Solution Description]
To solve:
1. Pressure difference = 1010 - 950 = 60 hPa = 6000 Pa
2. Radius = 30 km = 30,000 m
3. Pressure gradient force per unit mass = $\frac{\Delta P}{\rho \times r}$
4. Take air density $\rho$ ≈ 1.2 kg/m³
5. Calculation: $\frac{6000}{1.2 \times 30000} = \frac{6000}{36000} ≈ 0.167 m/s²$

Key Concept: Cyclone formation, storm surge impact

c) 3.0 meters
[Solution Description]
To solve this:
1. Storm surge height = 8 m
2. Coastal elevation = 5 m
3. Effective flood height = surge - elevation = 8 - 5 = 3 m
4. Water will spread inland with decreasing depth but for approximation in low-lying areas close to shore, we can consider similar depth.
5. Therefore, approximate floodwater depth would be 3 m.

Key Concept: Impact of Cyclones on Land, Flooding

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that storm surges from cyclones can flood coastal and inland areas due to high water walls. This is true because cyclones generate powerful winds that push seawater, creating storm surges as mentioned in the syllabus. The reason explains the mechanism behind storm surges, which aligns with the assertion. Hence, both are true, and the reason correctly explains the assertion.

Key Concept: Storm Surge Damage

c) It causes devastating flooding, damaging homes and infrastructure.
[Solution Description]
A storm surge is a wall of water pushed towards the shore by strong cyclone winds. It can flood coastal areas, contaminate freshwater sources, destroy infrastructure, and cause significant loss of life and property.

Key Concept: Role of satellites, IMD warnings

d) Satellites provide continuous, wide-area coverage essential for tracking cyclones over oceans.
[Solution Description]
Satellites provide real-time data on atmospheric conditions over vast oceanic areas where ground-based weather stations cannot be installed. They monitor cloud patterns, wind speeds, and sea surface temperatures, enabling accurate prediction of cyclone formation and movement. The India Meteorological Department (IMD) uses this satellite data to issue timely warnings.

Key Concept: Cyclone Safety Measures

b) To ensure immediate access to essential supplies if services are disrupted
[Solution Description]
Cyclones can disrupt power, water supply, and communication networks. An emergency kit with essentials like food, water, medicines, and flashlights ensures survival during and immediately after the cyclone until help arrives or normalcy is restored.

Key Concept: Cyclone preparedness

a) Emergency kit with essential items
[Solution Description]
People in cyclone-prone areas must keep an emergency kit with essential items ready to quickly move to a designated cyclone shelter when needed.

Key Concept: Collaborative efforts, technology

d) Combined resources and shared data improve tracking accuracy and warning systems.
[Solution Description]
International collaborations pool resources, expertise, and global satellite data to improve cyclone tracking accuracy. For example, sharing meteorological data from multiple countries helps in better modeling cyclone paths and issuing early warnings, reducing response time and increasing preparedness.