Key Concept: Pressure and Area

d) 40 $N/m^2$
[Solution Description]
Pressure is calculated using the formula:
$Pressure = \frac{Force}{Area} = \frac{200\,N}{5\,m^2} = 40\,N/m^2$

Key Concept: Pressure and Force Distribution

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 is more comfortable to carry, which is true because it reduces the pressure on the shoulders. The reason correctly explains that broad straps increase the area over which the force (weight of the bag) is distributed, thereby decreasing the pressure according to the formula $Pressure = \frac{Force}{Area}$. Since both are true and the reason explains the assertion, the correct choice is option (a).

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: Practical Application

b) Sharp knife reduces area, increasing pressure
[Solution Description]
A sharp knife has a smaller edge area compared to a blunt knife. Since $P = \frac{F}{A}$, for the same applied force, the pressure is higher when the area is smaller. Thus, the sharp knife cuts more easily due to higher pressure.

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: Pressure conversion, real-world application

b) 102,000 Pa, higher than 1 atm
[Solution Description]
1 hectopascal (hPa) = 100 Pa.
Given pressure = 1020 hPa = 1020 × 100 Pa = 102,000 Pa.
Standard atmospheric pressure = 101,325 Pa.
Comparison: 102,000 Pa is slightly higher than standard atmospheric pressure.

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: Practical Applications

c) It concentrates force over a smaller area increasing pressure
[Solution Description]
A sharp knife has a smaller edge area, so the same force produces higher pressure, making it easier to cut.

Key Concept: Practical Example of Pressure

c) To increase the area of contact, reducing pressure
[Solution Description]
Placing a round piece of cloth under a heavy load increases the contact area between the load and the head. Pressure is given by $Pressure = \frac{Force}{Area}$, so increasing the area reduces the pressure experienced by the head, making it more comfortable to carry the load.

Key Concept: Pressure, Cutting Tools

b) Sharp edge exerts 10 times more pressure
[Solution Description]
Pressure is calculated as $P = \frac{F}{A}$. For the sharp edge ($A_1 = 0.01\,mm^2$), $P_1 = \frac{F}{0.01}$ = $100F$ (in $N/mm^2$). For the blunt edge ($A_2 = 0.1\,mm^2$), $P_2 = \frac{F}{0.1}$ = $10F$. Thus, $P_1 = 10P_2$, meaning the sharp edge exerts 10 times more pressure.

Key Concept: Pressure Exerted by Air, Atmospheric Pressure, Force and Area Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the activity, when the paper plate is covered with an unfolded chart paper, the area exposed to atmospheric pressure increases compared to when it is folded. Since pressure ($P$) is defined as force ($F$) per unit area ($A$), i.e., $P = \frac{F}{A}$, an increase in area for the same atmospheric pressure results in a greater total force exerted on the plate. The Assertion correctly states that more effort is needed to lift the unfolded sheet due to increased area. The Reason correctly explains this using the relationship between pressure, force, and area. Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Atmospheric Pressure, Force-Area Relationship

c) Air exerts more pressure on the larger surface area of the unfolded sheet.
[Solution Description]
When the unfolded sheet covers the paper plate, it has a larger surface area exposed to the air. Atmospheric pressure exerts force on all surfaces in contact with the air. Since pressure is force per unit area ($P = F/A$), a larger area results in a greater total force acting on the sheet, making it harder to lift. Conversely, the folded sheet has a smaller exposed area, meaning less force from atmospheric pressure acts on it, requiring less effort to lift.

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: Atmospheric Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The rubber sucker sticks to a smooth surface because when it is pressed against the surface, air inside it is expelled, reducing the internal pressure. The higher external atmospheric pressure then pushes the sucker firmly onto the surface. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

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: Pressure units, conversion

c) 1,020 mb
[Solution Description]
Both hectopascal (hPa) and millibar (mb) are equal in magnitude: $1 \text{ hPa} = 1 \text{ mb}$.
Therefore, $1,020 \text{ hPa} = 1,020 \text{ mb}$.

Key Concept: Pressure Exerted by Air, Surface Area Effect

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the given experiment, the assertion states that more effort is needed to lift the paper plate when covered with an unfolded sheet than a folded one, which is true as observed in Activity 6.3. The reason claims this is due to higher atmospheric pressure on the unfolded sheet because of its larger surface area, which is also true. Moreover, the reason correctly explains the assertion since atmospheric pressure acts on all surfaces, and its effect increases with the surface area exposed to air. Hence, both the assertion and reason are true, and the reason is the correct explanation for the assertion.

Key Concept: Force calculation, Pressure-area relationship

c) 6 N
[Solution Description]
1) Given: Force $F_1 = 12$ N for area $A_1 = 0.392$ m$^2$.
2) New area $A_2 = A_1/2 = 0.196$ m$^2$.
3) Since pressure $P = F/A$ is constant (atmospheric pressure), $P = \frac{F_1}{A_1} = \frac{F_2}{A_2}$.
4) Thus, $F_2 = \frac{F_1 × A_2}{A_1} = \frac{12 × 0.196}{0.392} = 6$ N.

Key Concept: Air Pressure Experiment

c) The unfolded chart paper has a larger surface area exposed to atmospheric pressure
[Solution Description]
When the unfolded chart paper covers the paper plate, it increases the surface area exposed to atmospheric pressure. Air exerts force per unit area (pressure) on all objects. The larger area of the unfolded sheet means more air pressure acts downward, making it harder to lift. The folded sheet has less area, so less air pressure acts on it, requiring less effort.

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: Air Pressure Equilibrium

c) The inflated balloon decreases in size while the uninflated one increases until pressures equalize.
[Solution Description]
Initially, the inflated balloon has higher air pressure than the uninflated one. Air flows from the high-pressure region (inflated balloon) to the low-pressure region (uninflated balloon) through the straw. This transfer continues until the air pressure in both balloons becomes equal, resulting in both balloons attaining similar sizes.

Key Concept: Air pressure behavior, Balloon inflation dynamics

c) Air escapes until internal pressure equals external atmospheric pressure.
[Solution Description]
When the balloon's mouth is open, the high-pressure air inside escapes to the lower-pressure environment outside. The air continues to flow out until the pressure inside equals the atmospheric pressure outside, causing complete deflation. Without a closed system, equilibrium is achieved only when internal pressure matches external pressure.

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 equilibrium

d) The air pressure in both balloons becomes equal.
[Solution Description]
The air flow stops when the pressure inside both balloons becomes equal. Initially, air flows from the inflated (high-pressure) balloon to the uninflated (low-pressure) balloon. Once the pressures balance, there is no further net movement of air, and both balloons reach nearly the same size.

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: Pressure and Area

b) 10 $N/m^2$
[Solution Description]
The pressure exerted can be calculated using the formula: $P = \frac{F}{A}$. Here, $F = 50\;N$ and $A = 5\;m^2$. Substituting these values into the formula gives:
$P = \frac{50}{5} = 10\;N/m^2$
Therefore, the pressure exerted is 10 $N/m^2$.

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: Air Pressure's Role in Daily Life

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a rubber sucker sticks to a smooth surface because of the difference in air pressure, which is correct. When the sucker is pressed against a smooth surface, most of the air inside is pushed out, reducing the internal pressure. The external atmospheric pressure, being higher, exerts a force that keeps the sucker firmly attached to the surface. The reason correctly explains why this happens: it describes the process of expelling air from inside the sucker, creating a low-pressure region, and how the higher external pressure maintains the attachment. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Pressure difference and wind formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that wind moves from high pressure to low pressure, which is correct as per Activity 6.5 where air moved from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The reason is also true because air flows to equalize the pressure difference, as observed when both balloons attained the same size and air flow stopped. Moreover, the reason correctly explains the assertion since the movement of wind due to pressure differences is a fundamental concept in wind formation.

Key Concept: Formation of Wind, Pressure Difference, Land and Sea Breeze

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
During the day, land gets heated faster than water, causing the air above it to warm up and rise, creating a low-pressure area. The sea remains cooler, maintaining higher pressure. According to the principle of pressure difference, air flows from the high-pressure region (sea) to the low-pressure region (land), resulting in a sea breeze. The reason correctly explains the assertion because it highlights the fundamental principle that governs the movement of air, which is the pressure gradient.

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: 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: 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: Bernoulli's principle, Wind speed

c) 57.7 m/s
[Solution Description]
Using Bernoulli's equation, the pressure difference $\Delta P = \frac{1}{2} \rho v^2$. Convert $\Delta P$ to pascals: $1000 \text{ mb} - 980 \text{ mb} = 20 \text{ mb} = 2000 \text{ Pa}$. Rearranging for $v$: $v = \sqrt{\frac{2 \Delta P}{\rho}} = \sqrt{\frac{4000}{1.2}} \approx 57.7 \text{ m/s}$.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that wind moves from a high-pressure region to a low-pressure region, which is true as explained in the activity where air flows from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The Reason correctly explains this phenomenon by stating that the difference in air pressure causes the movement of air. Therefore, both Assertion and Reason are true, and Reason provides the correct explanation for the Assertion.

Key Concept: Land and Sea Breeze

d) Low pressure develops over the land
[Solution Description]
During the day, land heats up faster than water, creating a low pressure area over the land. The cooler air from the high pressure region over the sea moves towards the land, resulting in a sea breeze.

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: Sea breeze, Land breeze

c) Due to the differential heating and cooling rates of land and sea.
[Solution Description]
During the day, the land heats up faster than the sea, causing the air above the land to rise (creating low pressure). Cooler air from the sea moves towards the land (sea breeze). At night, the land cools faster than the sea, so the warmer air over the sea rises (low pressure), and cooler air from the land moves towards the sea (land breeze).

Key Concept: Pressure difference and wind speed

d) Higher pressure difference increases wind speed.
[Solution Description]
Wind speed depends on the pressure gradient (difference in pressure) between two regions. A larger pressure difference ($\Delta P$) results in a stronger force pushing the air from the high-pressure region to the low-pressure region, which increases the wind speed. Conversely, a smaller pressure difference leads to slower wind speeds.

Key Concept: Land breeze formation

d) Cooler air sinks over the land, creating a high-pressure zone that pushes air towards the sea.
[Solution Description]
At night, the land cools down faster than the sea. As a result, the air above the land becomes cooler and denser, leading to a high-pressure area over the land. Meanwhile, the sea retains heat longer, keeping its surface warmer. The cooler air from the high-pressure land area moves towards the lower pressure over the sea, creating a land breeze.

Key Concept: Land Breeze, Pressure Differences

c) Low pressure over sea, high pressure over land
[Solution Description]
At night, land cools faster than the sea, creating high pressure over the land and low pressure over the sea. The pressure difference drives the land breeze. Since wind speed $\propto$ pressure difference, the larger the temperature difference, the greater the pressure gradient. Given the land is colder ($15^\circ C$) and the sea is warmer ($20^\circ C$), the pressure gradient will cause air to flow from land (high pressure) to sea (low pressure).

Key Concept: Land Breeze

c) Because the sea retains heat longer and develops lower pressure.
[Solution Description]
At night, the land cools down faster than the sea. The air above the sea remains warmer, creating a low-pressure area over the sea. Consequently, the cooler air from the high-pressure region over the land moves towards the sea, forming a land breeze.

Key Concept: Sea Breeze and Pressure Differences

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during the day, the wind blows from the sea to the land because the land is warmer than the sea. This is true as per the syllabus, which explains that during the day, the land becomes warmer and lighter, creating a low-pressure area. The reason given is that the land heats up faster than the sea during the day, creating a low-pressure region over the land. This is also true and correctly explains why the wind blows from the high-pressure sea to the low-pressure land, resulting in a sea breeze. Therefore, both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Land Breeze

c) The land cools down faster than the sea, causing air to move from the land to the sea.
[Solution Description]
At night, the land cools down faster than the sea. The air above the sea becomes warmer and rises, creating a low-pressure area. The cooler, high-pressure air from the land moves towards the sea, resulting in a land breeze.

Key Concept: High-speed winds, Pressure difference, Structural safety

b) 100365 Pa
[Solution Description]
According to Bernoulli's principle, the total energy per unit volume remains constant. The pressure difference can be calculated using the formula $\Delta P = \frac{1}{2}\rho v^2$. Given $\rho = 1.2$ kg/m$^3$ and $v = 40$ m/s, we have:
Step 1: Compute $\frac{1}{2}\rho v^2$
$\frac{1}{2} \times 1.2 \times (40)^2 = 0.6 \times 1600 = 960$ Pa.
Step 2: Subtract this from the internal pressure
Pressure above roof $= P_{\text{inside}} - \Delta P = 101325 - 960 = 100365$ Pa.
Thus, the pressure above the roof is approximately $100365$ Pa.

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: Pressure difference in storms

c) Low pressure outside the roof causes it to be pushed upwards by the higher internal pressure.
[Solution Description]
During a storm, high-speed winds reduce the air pressure above the roof. The higher air pressure inside the house exerts an upward force on the roof, causing it to be lifted or blown away. Balancing internal and external pressure by keeping windows and doors open can prevent this.

Key Concept: Effect of high-speed winds

c) The pressure difference created by high-speed winds lifts the leaves.
[Solution Description]
High-speed winds create a low-pressure area above the leaves. The higher air pressure below the leaves pushes them upwards, lifting them into the air.

Key Concept: High-speed winds, Bernoulli's principle, Low-pressure areas

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during a storm, keeping windows and doors closed increases the likelihood of the roof blowing off due to pressure difference. This is correct because high-speed winds reduce pressure above the roof as per Bernoulli's principle ($P + \frac{1}{2} \rho v^2 = \text{constant}$). The reason explains this phenomenon correctly by mentioning the low-pressure region created by high-speed winds and the resulting pressure imbalance. Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is option a).

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: High-speed winds, pressure difference

c) It reduces the pressure difference between the inside and outside of the house
[Solution Description]
High-speed winds create a low-pressure area above the roof due to Bernoulli's principle. If windows and doors are kept closed, the higher air pressure inside the house pushes upwards on the roof while the lower pressure outside pulls the roof upwards. This large pressure difference can cause the roof to blow off. Keeping windows and doors open allows wind to pass through the house, equalizing the pressure inside and outside, thus reducing the risk of roof damage.

Key Concept: Wind Speed and Pressure Relationship

b) Air pressure decreases due to fewer molecular collisions.
[Solution Description]
According to Bernoulli's principle, an increase in wind speed leads to a decrease in air pressure. When wind blows faster over a surface, the kinetic energy of the air molecules increases, causing fewer collisions per unit area, resulting in lower pressure in that region.

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: Bernoulli's Principle, Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Step 1: Analyzing Assertion (A).
- The assertion states that when air is blown between two balloons, they move towards each other because of decreased pressure between them.
- From Activity 6.6, blowing air between two inflated balloons creates a region of lower pressure between them compared to the surrounding air pressure.
- The higher external pressure pushes the balloons together, confirming the assertion is true.
Step 2: Analyzing Reason (R).
- The reason refers to Bernoulli's principle, which states that an increase in fluid velocity decreases its pressure.
- In this experiment, blowing air increases its speed between the balloons, leading to lower pressure as per Bernoulli's principle.
- This directly explains why the balloons move towards each other, making the reason both true and the correct explanation of the assertion.
Conclusion: Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Effect of Blowing Harder on Balloons

c) They move towards each other faster.
[Solution Description]
Blowing harder increases the speed of the air between the balloons, further reducing the air pressure in that region. The greater difference in pressure compared to the surrounding air causes the balloons to move towards each other at a faster rate.

Key Concept: Effect of High-Speed Wind on Air Pressure

c) The air pressure decreases.
[Solution Description]
When air is blown between the two balloons, it creates a region of high-speed wind, which reduces the air pressure in that area. The surrounding higher air pressure pushes the balloons towards each other. Thus, the air pressure between the balloons decreases.

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: 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: Impact of pressure differences on structures

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, high-speed winds create low pressure over the roof while the air pressure inside the house remains higher when windows/doors are closed. This pressure difference exerts an upward force on the roof (Pressure = Force/Area). If the roof is weak, it may blow away. The Reason correctly explains why keeping windows/doors closed increases this pressure difference. Hence, both Assertion and Reason are true, and Reason is the correct explanation.

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: Lightning

c) Lower part of the cloud (heavier water droplets)
[Solution Description]
In a thundercloud, heavier water droplets collect in the lower part of the cloud, accumulating negative charges. Lighter ice particles with positive charges remain suspended in the upper part of the cloud.

Key Concept: Formation of thunderstorms and lightning

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that lightning occurs due to the sudden flow of charges when the insulating property of air breaks down, which is correct as per the syllabus. The reason mentions that rapid heating of air around lightning causes expansion, resulting in thunder, which is also true as per the syllabus. However, the reason does not explain why lightning occurs but instead explains the phenomenon of thunder, which is a consequence of lightning. Therefore, both are true, but the reason is not the correct explanation of the assertion.

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: Local thunderstorms, Climate phenomena

c) Bordoisila
[Solution Description]
Bordoisila is a local thunderstorm specific to Assam, characterized by strong winds and heavy rainfall. It occurs during pre-monsoon months.

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: 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: Cyclone formation, Pressure systems, Eye wall dynamics

c) Descending air in the eye inhibits cloud formation and wind activity.
[Solution Description]
The eye forms due to descending air, which suppresses cloud formation and reduces wind speed, creating a low-pressure center. Surrounding the eye is the eyewall, where rising warm moist air leads to intense thunderstorms and high-speed winds.

Key Concept: Pressure difference, Wind speed, Roof stability

b) It reduces the pressure difference between the inside and outside of the house.
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the air pressure below remains higher. This difference can cause roofs to blow off. Keeping doors and windows open allows wind to flow through the house, balancing the pressure inside and outside, thus reducing the net force on the roof.

Key Concept: Charge Separation in Clouds

c) Friction between water droplets and ice particles creates opposite charges
[Solution Description]
Strong upward and downward winds cause friction between water droplets and ice particles, leading to static electric charges. Positively charged lighter ice particles move to the upper part of the cloud, while negatively charged heavier water droplets occupy the lower part. This results in charge separation.

Key Concept: Dangers and Precautions, Thunderstorms and Lightning

c) It reduces contact area with the ground and avoids being a tall target
[Solution Description]
Lying flat increases the body's surface area in contact with the ground, making it more susceptible to a ground current if lightning strikes nearby. Crouching reduces this contact area, minimizing risk. Additionally, staying low avoids being the tallest object, which is more likely to be struck by 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: Assertion and Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that thunderstorms are more frequent in hot, humid, and tropical regions like India, which is true because such conditions provide the necessary moisture and temperature contrasts for thunderstorm development. The Reason explains that warm air rises to great heights, forming ice particles and causing charge separation, which is also correct and directly explains why thunderstorms occur more frequently in these regions. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Static charge development, Lightning conductor

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The lightning conductor is designed to protect buildings by providing a low-resistance path for lightning to follow, safely directing the charge into the ground. The pointed end of the conductor helps ionize the air, reducing the insulating property of air and allowing easier discharge. Both the assertion and reason are true, and the reason correctly explains why the lightning conductor works as described in the assertion.

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: Safety during thunderstorms

c) Staying inside a car with windows closed
[Solution Description]
During a thunderstorm, staying inside a car or bus is safer because their metal bodies act as a shield against lightning, provided you do not touch any metallic parts.

Key Concept: Lightning formation

b) When static charges accumulate sufficiently to break down the insulating property of air
[Solution Description]
Lightning occurs when the build-up of opposite charges becomes large enough to overcome the insulating property of air, causing a sudden discharge of electricity.

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: 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: 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: Thunder Formation

b) Rapid heating and expansion of air caused by lightning
[Solution Description]
Thunder is produced when lightning rapidly heats the surrounding air, causing it to expand explosively. This expansion creates a shockwave that manifests as a loud sound known as thunder.

Key Concept: Cyclone formation and Earth's rotation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Coriolis effect caused by Earth's rotation deflects moving air, creating the spinning motion characteristic of cyclones. If Earth stopped rotating, this deflection would not occur, preventing the development of the cyclonic spin. Therefore, both Assertion and Reason are true, and Reason correctly explains Assertion.

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: Eye of the Cyclone

c) Calm winds and low pressure
[Solution Description]
The eye of the cyclone is a region of calm winds and low pressure at the center of the storm, surrounded by intense winds and heavy rainfall.

Key Concept: Cyclone Formation and Characteristics

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that the eye of a cyclone has calm winds and clear skies, which is true because it is a region of descending air that suppresses cloud formation. However, the reason claims that the eye forms due to subsidence of warm air, which is incorrect. The eye forms due to the centrifugal force balancing the pressure gradient force, resulting in minimal wind movement. Thus, the Assertion is true, but the Reason is false.

Key Concept: Energy release, Cyclone formation

c) It provides energy to sustain and intensify the cyclone.
[Solution Description]
When water vapor condenses into raindrops, latent heat is released into the atmosphere. This additional heat warms the surrounding air, causing it to rise further, reducing surface pressure, and intensifying the cyclone by drawing more air inward, thus sustaining the storm's energy.

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 form over warm ocean waters, which is true as per the syllabus. The reason explains that warm ocean waters cause the air above them to rise, creating low pressure and leading to cyclone formation, 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: Cyclone formation, Eye of the cyclone

b) The eye contains descending air that suppresses wind activity.
[Solution Description]
The eye of the cyclone is a region characterized by descending air. As warm, moist air rises in the surrounding eyewall, it cools and condenses, releasing latent heat, which further drives the upward motion. At the center (eye), the descending air creates a low-pressure area but suppresses cloud formation and wind activity, leading to calm conditions.

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: Formation of Cyclones

b) Condensation of water vapor releasing heat
[Solution Description]
Cyclones form when warm, moist air rises from the ocean surface, causing water vapor to condense into raindrops and release latent heat. This further warms the surrounding air, creating a low-pressure system and initiating cyclonic circulation due to Earth's rotation.

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: 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: Structure of a cyclone: Eye of the cyclone

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the eye of a cyclone has calm weather conditions, which is true as the eye is known for its calm and clear skies. The reason explains that the eye has the lowest pressure, causing air to descend and suppress wind formation, which is also true. Furthermore, the reason correctly explains why the eye has calm weather conditions. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Impact of Cyclones on Agriculture

c) Salt from seawater makes the soil less fertile, reducing crop yields.
[Solution Description]
When seawater floods agricultural land due to a cyclone, the high salt content in the water can make the soil less fertile. This reduces crop productivity for months or even years until the salt is washed away naturally or through human intervention such as soil treatment.

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: Impact of cyclones on land

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that cyclones cause extensive flooding in coastal areas due to storm surges, which is true because cyclones generate strong winds that push seawater inland, leading to flooding. The reason explains that storm surges occur when strong winds push seawater towards the shore, forming a wall of water that floods coastal areas. Both the assertion and reason are correct, and the reason accurately explains why cyclones lead to flooding through storm surges.

Key Concept: Impact of Cyclones on Agriculture

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that cyclones cause long-term agricultural damage due to seawater contamination, which is true as cyclones often bring storm surges that flood coastal regions. The reason states that salt from seawater reduces soil fertility, which is also true and directly explains why the assertion is correct since saline water adversely affects crops and soil quality.

Key Concept: Emergency preparedness, IMD alerts

c) Stay updated with IMD alerts, prepare emergency kits, and move to shelters when advised.
[Solution Description]
The first step is to stay updated with IMD warnings. Next, the community should ensure emergency kits are ready with essentials like food, water, and medicines. Finally, during the cyclone, they must move to designated shelters for safety. These steps align with the syllabus guidelines for cyclone preparedness.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the IMD issues alerts and warnings for cyclones, which is true as per the syllabus. The Reason states that weather monitoring satellites help track cyclones and predict their path, which is also true as mentioned in the syllabus. The Reason correctly explains why the IMD can issue alerts and warnings because satellite data is crucial for cyclone prediction and tracking. Hence, both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Cyclone monitoring, Prediction measures

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that IMD uses weather satellites to predict cyclone paths and intensity accurately. This is true as satellites provide critical data for monitoring cyclones. The Reason explains that satellites supply real-time data on atmospheric conditions necessary for tracking cyclones, which is also true. Moreover, the Reason correctly explains why satellites are used for cyclone prediction, as they provide essential parameters like wind speed and pressure. Hence, both statements are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Cyclone Monitoring

b) Weather monitoring satellites
[Solution Description]
Weather monitoring satellites provide real-time data on cloud patterns, wind speeds, and pressure systems, enabling accurate tracking and prediction of cyclone paths. Doppler radar and weather stations also contribute, but satellites offer the most comprehensive coverage.