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 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: Comparative pressure analysis, multi-step reasoning

c) 5 times
[Solution Description]
Calculate pressures individually.
Flat brick:
$P_{flat} = \frac{10}{100 \times 10^{-4}} = 1000 \text{ Pa}$
Vertical brick:
$P_{vertical} = \frac{10}{20 \times 10^{-4}} = 5000 \text{ Pa}$
The ratio of pressures is:
$\frac{P_{vertical}}{P_{flat}} = \frac{5000}{1000} = 5$
Thus, the vertical brick exerts 5 times more pressure.

Key Concept: SI unit of pressure

c) Pascal (\$Pa\$)
[Solution Description]
The SI unit of pressure is newton per square metre (N/m$^2$), which is also called pascal (Pa). This is derived from the definition of pressure as force per unit area.

Key Concept: Multi-concept: Pressure calculation with unit conversion

b) 0.637 MPa
[Solution Description]
Force = 5 kN = 5000 N.
Radius of piston = $\frac{10\,cm}{2} = 5\,cm = 0.05\,m$.
Area = $\pi r^2 = \pi (0.05)^2 = 0.00785\,m^2$.
Pressure = $\frac{Force}{Area} = \frac{5000}{0.00785} ≈ 636,942\,Pa$.
Convert to MPa: $636,942\,Pa = 0.637\,MPa$.

Key Concept: Practical Unit of Air Pressure

b) Millibar (mb)
[Solution Description]
While the SI unit of pressure is pascal (Pa), the practical unit of air pressure is millibar (mb), where 1 mb = 100 Pa.

Key Concept: Pressure Calculation

c) 100 N
[Solution Description]
Rearrange the pressure formula to solve for force:
$Force = Pressure \times Area$
Substitute the given values:
$Force = 25\,Pa \times 4\,m^2 = 100\,N$
The force applied is 100 N.

Key Concept: Pressure and Area

d) 40 Pa
[Solution Description]
To find the pressure, use the formula:
$Pressure = \frac{Force}{Area}$
Substituting the given values:
$Pressure = \frac{200\,N}{5\,m^2} = 40\,Pa$
The pressure exerted is 40 Pa.

Key Concept: Pressure and Area

c) The sharp knife has a smaller edge area, increasing pressure
[Solution Description]
A sharp knife has a smaller edge area compared to a blunt knife. Pressure is calculated as $Pressure = \frac{Force}{Area}$. For the same applied force, a smaller area results in higher pressure, making it easier to cut through objects. The higher pressure allows the sharp knife to penetrate materials with less effort.

Key Concept: Effect of Sharpness on Cutting

b) A sharp knife has a smaller edge area, increasing pressure.
[Solution Description]
A sharp knife has a smaller edge area, which increases the pressure applied when cutting, making it easier to slice through materials. A blunt knife has a larger edge area, reducing the pressure and making cutting more difficult. $Pressure = \frac{Force}{Area}$ explains this relationship.

Key Concept: Pressure Unit

c) Pascal ($Pa$)
[Solution Description]
The SI unit of pressure is newton per square meter ($N/m^2$), also called pascal ($Pa$). Other units like millibar and hectopascal are derived from this fundamental unit.

Key Concept: Pressure Directionality, Balloon Inflation

c) Air pressure acts equally in all directions.
[Solution Description]
Air inside the balloon exerts pressure uniformly in all directions. This is because gas molecules move randomly and collide with the inner walls of the balloon in every direction equally. As a result, the balloon expands symmetrically, demonstrating that air pressure is isotropic—it acts equally in all directions.

Key Concept: Atmospheric pressure, force calculation

d) 2,250 N
[Solution Description]
First, convert the area from \(\mathrm{cm}^2 \rightarrow \mathrm{m}^2\) : $15 \text{ cm} \times 15 \text{ cm} = 225 \text{ cm}^2 = 0.0225 \text{ m}^2$.
Atmospheric pressure is given as $100 \text{ kPa} = 100,000 \text{ Pa} = 100,000 \text{ N/m}^2$.
Force required to overcome atmospheric pressure is calculated using $F = P \times A$:
$F = 100,000 \text{ N/m}^2 \times 0.0225 \text{ m}^2 = 2,250 \text{ N}$.

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: Atmospheric Pressure, Pressure-Force Relationship

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that atmospheric pressure exerts a force of 2250 N over an area of 225 cm$^2$. From the given information, we know that the force exerted by the atmospheric air column over an area of 15 cm $\times$ 15 cm (225 cm$^2$) is nearly equal to 2250 N, which matches the Assertion. The Reason explains that the internal pressure in our bodies balances the external atmospheric pressure, preventing us from being crushed, which is also correct. However, the Reason does not explain why the Assertion is true (i.e., it does not justify the force calculation). Thus, both are true, but the Reason is not the correct explanation of the Assertion.

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: Air Pressure Experiment

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that more effort is needed to lift the paper plate with an unfolded chart paper than a folded one, which matches observations from Activity 6.3. The reason explains this phenomenon by stating that air exerts pressure on the sheet's surface area, and since $\text{Pressure} = \frac{\text{Force}}{\text{Area}}$, a larger area increases the force exerted by air pressure. Therefore, both statements are true, and the reason correctly explains 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: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lifting the paper plate covered with folded chart paper requires less effort compared to unfolded chart paper. This is true because in the experiment, it was observed that the unfolded chart paper has a larger surface area exposed to air pressure, making it harder to lift. The reason explains that air exerts more pressure on a larger surface area, which is correct according to the observation and concept of atmospheric pressure. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

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: 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: 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: Effect of air pressure on balloons

c) The balloon inflates because air exerts pressure on its walls.
[Solution Description]
When air is blown into a balloon, it gets inflated because the air exerts pressure on the inner walls of the balloon. This pressure acts in all directions, causing the balloon to expand uniformly.

Key Concept: Effect of air pressure, Pressure equilibrium

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the given scenario, the inflated balloon has higher internal pressure compared to the uninflated one. According to the concept of pressure equilibrium, air will flow from the high-pressure region (inflated balloon) to the low-pressure region (uninflated balloon) through the straw. The flow continues until the pressures in both balloons become equal, resulting in both balloons attaining similar sizes. The reason correctly explains why the air moves, as it aligns with the principle that air moves from high to low pressure regions. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of 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, Suction cup

d) The air inside is at higher pressure than the surrounding atmosphere.
[Solution Description]
The air inside the balloon is at higher pressure than the atmospheric pressure outside. Air always moves from high-pressure regions to low-pressure regions. Hence, when the balloon's mouth is left open, the high-pressure air inside escapes to the lower-pressure surroundings until equilibrium is achieved.

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: Effect of high-speed winds on pressure

d) A low-pressure area forms above the roof
[Solution Description]
High-speed winds create a low-pressure area above the roof. The higher air pressure inside the house pushes the roof upwards due to the pressure difference, which can cause the roof to be blown off if it's weak.

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 and wind direction

c) Wind is generated
[Solution Description]
Air moves from a high-pressure region to a low-pressure region due to the pressure gradient force. This movement of air is what we perceive as wind. The greater the pressure difference, the stronger the wind speed.

Key Concept: Pressure Difference and Wind Formation

d) The flow of air increases
[Solution Description]
The speed of wind is directly proportional to the pressure difference between two regions. A higher pressure difference results in faster wind movement as air flows from high-pressure to low-pressure regions until equilibrium is reached.

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: 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: 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 winds form due to the movement of air from high-pressure to low-pressure regions, which is correct as seen in activities like balloon experiments and sea/land breezes. The Reason states that wind speed increases with greater pressure differences, which is also true and explains why stronger winds occur in cases of larger pressure differentials, such as during storms or cyclones. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Formation of Wind, 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, sea breeze occurs due to warm air rising over the land, creating a low-pressure area, which causes cooler air from the sea (high-pressure area) to move toward the land. This is correct as per the concept of pressure differences in wind formation. The reason explains the general principle that air moves from high-pressure regions to low-pressure regions until equilibrium is reached, which directly explains why the sea breeze occurs. Both are true, and the reason correctly explains the assertion.

Key Concept: Pressure difference, Wind speed

c) The wind speed doubles.
[Solution Description]
The wind speed is directly proportional to the pressure difference:
$\text{Wind speed} \propto \text{Pressure difference}$.
Initially, let the wind speed be $v_1$ when pressure difference is 12 hPa.
When pressure difference doubles to 24 hPa, the new wind speed $v_2$ can be calculated as:
$\frac{v_2}{v_1} = \frac{24 \text{ hPa}}{12 \text{ hPa}} = 2$
Therefore, $v_2 = 2v_1$. The wind speed doubles.

Key Concept: Sea breeze and land breeze

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

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: 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: Pressure Differences

c) Wind moves from high-pressure areas to low-pressure areas.
[Solution Description]
Wind is caused by the movement of air from a region of high pressure to a region of low pressure. This principle explains phenomena like sea and land breezes.

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 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 high-speed winds blow over a house, the roof may get blown away due to the pressure difference. This is true because high-speed winds reduce the air pressure above the roof (as per Bernoulli's principle), while the air pressure inside the house remains relatively higher. If this pressure difference is significant and the roof is not strong enough, it can be blown away. The reason correctly explains why the roof may get blown away: high-speed winds indeed create a low-pressure area above the roof, while the air pressure inside the house remains higher. Thus, the reason is the correct explanation for the assertion.

Key Concept: Bernoulli's principle, Airflow dynamics

c) Low pressure forms between them due to fast airflow
[Solution Description]
When air is blown between the balloons, the speed of airflow increases, creating a low-pressure region between the balloons (as per Bernoulli's principle). The surrounding higher-pressure air exerts a force on the outer surfaces of the balloons, pushing them towards each other. Hence, the movement occurs due to the pressure difference caused by high-speed airflow.

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: Bernoulli's Principle, Air Pressure

c) They move towards each other due to the low pressure created between them
[Solution Description]
According to Bernoulli's principle, when air is blown between the two balloons, the speed of air increases in that space, creating a low-pressure region between them. The surrounding higher air pressure pushes the balloons toward each other. This demonstrates that high-speed winds result in reduced air pressure.

Key Concept: High-speed winds and air pressure

b) Air pressure decreases
[Solution Description]
When high-speed winds blow over a surface, they create a region of lower air pressure because the fast-moving air reduces the pressure in that area. This is due to Bernoulli's principle, which states that increasing the speed of a fluid decreases its pressure.

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: 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: 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: Air Pressure, Speed-Pressure Relationship

c) The distance will decrease less in the second case.
[Solution Description]
According to Bernoulli's principle, lower airspeed results in higher pressure between the balloons. Reduced blowing speed means less pressure difference is created, so the balloons will move towards each other slower.
For 20 m/s: Distance decreases significantly as balloons accelerate faster due to high pressure difference.
For 10 m/s: Distance decreases at half the rate since pressure difference is smaller.
After 2 seconds, the distance covered in the second case will be less than in the first case, meaning the balloons will be farther apart compared to the first scenario.

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: 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: High-speed winds, pressure difference, structural impact

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
High-speed winds create low pressure over the roof while the air pressure inside the house remains higher if the roof is sealed. This pressure difference can cause the roof to lift or blow off. When doors and windows are open, air flows through the house, reducing the pressure difference between the inside and outside. Thus, both Assertion and Reason are true, and the Reason correctly explains why open doors and windows prevent roofs from being blown off.

Key Concept: Bernoulli's Principle Application

c) The balloons move closer together due to reduced pressure between them
[Solution Description]
Blowing air between the balloons creates a low-pressure region in between. The higher pressure on the outer sides of the balloons pushes them toward each other. This demonstrates Bernoulli's principle, where increased air speed results in decreased pressure.

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: 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: Lightning Conductor

b) To provide a safe path for electric charges to reach the ground
[Solution Description]
A lightning conductor provides an easy path for the transfer of electric charges from a lightning strike into the ground, preventing damage to the building. It consists of a metallic rod with one end pointed higher than the building's highest point and the other end buried deep in the ground.

Key Concept: Lightning formation, Charge separation

d) Strong winds rubbing against each other
[Solution Description]
The charge separation in thunderclouds is caused by strong winds rubbing against each other, creating static electric charges. Positively charged ice particles move upwards while negatively charged water droplets remain in the lower part of the cloud. This separation builds up until a discharge (lightning) occurs.

Key Concept: Formation of Storms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during a storm, warm air rises and creates a low-pressure area, which is correct as per the syllabus. The reason explains that cooler air rushes in to replace the rising warm air, leading to continuous wind circulation, which is also correct and directly explains the assertion. Therefore, both are true, and the reason correctly explains the assertion.

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: Thunderstorms and Lightning

d) Positive charges in the upper part and negative charges in the lower part
[Solution Description]
Before lightning, positive charges accumulate in the upper part of the cloud (ice particles), while negative charges (water droplets) collect in the lower part of the cloud.

Key Concept: Formation of Storms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that high-speed winds create a low-pressure area over houses, which is true as per the syllabus. The reason explains that this happens because high-speed winds are accompanied by reduced air pressure, which is also true and correctly explains why low pressure is created. Hence, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Lightning conductors

c) To provide a safe path for electric discharge into the ground
[Solution Description]
A lightning conductor provides an easy path for electric charges to travel safely into the ground, preventing damage to the building by discharging the accumulated static electricity from lightning strikes.

Key Concept: Thunderstorm formation, Charge separation in clouds

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion claims that lightning occurs exclusively between a cloud and the ground due to maximum charge separation there, which is false. Lightning can occur within a cloud, between clouds, or between a cloud and the ground. The Reason correctly describes the charge distribution in a thundercloud but does not justify the Assertion, as it explains general charge separation, not ground-specific phenomena.

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: Role of Winds, Moisture, and Temperature

a) Warm air has lower pressure than cold air.
[Solution Description]
Warm air rises because it is less dense than cooler air. When land gets heated, the warm and moist air becomes lighter and creates a low-pressure area, causing it to rise. This rising motion sets up the wind circulation necessary for thunderstorm development.

Key Concept: Concept: Formation of Thunderstorms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When land is heated, warm and moist air rises, creating a low-pressure area. As this air rises, it expands and cools, causing the moisture in it to condense and form clouds. This process can lead to the development of thunderstorms when strong winds and charge separation occur within the clouds. Thus, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

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: 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: Static charge separation, Lightning mechanism

c) 10$^5$ V/m
[Solution Description]
The electric field strength ($E$) is calculated using the formula $E = \frac{V}{d}$, where $V$ is the potential difference and $d$ is the distance. Given $V = 10^8$ V and $d = 1$ km = $1000$ m, substitute the values:
$E = \frac{10^8}{1000} = 10^5 \text{ V/m}$

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: 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: Lightning Formation

c) A sudden flow of charges occurs, producing lightning
[Solution Description]
When the charge separation in the cloud becomes significant, the insulating property of air breaks down, allowing a sudden flow of charges. This discharge produces a bright flash of light called lightning. Lightning can occur within a cloud, between clouds, or between a cloud and the ground.

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: Lightning Mechanism

d) When the buildup of opposite charges exceeds air's insulation capacity
[Solution Description]
Lightning occurs when the buildup of opposite charges becomes very large, causing the insulating property of air to break down. A sudden flow of charges produces a bright flash of light, known as lightning. This can happen within a cloud, between clouds, or between clouds and the ground.

Key Concept: Cyclone Formation

b) Warm ocean waters
[Solution Description]
Cyclones derive their energy from warm ocean waters. The heat and moisture from the ocean surface fuel the storm, causing air to rise and form low-pressure systems.

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, Storm surge

c) 1200 meters
[Solution Description]
First, calculate the effective rise in water level due to the storm surge: $8 \, \text{m} - 2 \, \text{m} = 6 \, \text{m}$.
The land slope is 0.5%, meaning for every 100 meters horizontally, the elevation changes by 0.5 meters. To find the horizontal distance $x$ where the elevation rises by 6 meters:
$0.005 = \frac{6}{x} \implies x = \frac{6}{0.005} = 1200 \, \text{meters}$

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: Impact

c) Strong winds, heavy rain, and storm surges
[Solution Description]
When a cyclone reaches land, it loses its source of moist air and weakens, but before that, it causes destruction through strong winds, heavy rainfall, storm surges, and flooding. The text explains: "Once the cyclone reaches land, the source of moist air is cut off and it gradually loses its strength... Cyclones can be extremely destructive..."

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: 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: 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: Eye of the Cyclone, High-Speed Winds

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that the eye of a cyclone has high-speed winds and heavy rainfall. However, this is false because the eye is actually a calm region with clear skies and no rain. The surrounding region, called the eyewall, experiences the high-speed winds and heavy rainfall.
The Reason claims that the eye is formed due to rapid rotation of air around a low-pressure center, generating centrifugal force pushing air outward. This is true because the centrifugal force balances the pressure gradient force, creating the calm eye at the center.
Since the Assertion is false but the Reason is true, the correct choice is:

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: Safety Measures, High-speed winds

b) Constructing cyclone-resistant buildings
[Solution Description]
For cyclones with high wind speeds like 250 km/h, the most effective safety measures include early warning systems for evacuation and sturdy shelters to withstand the winds. Public awareness campaigns and timely IMD alerts are also crucial. Among the options, constructing cyclone-resistant buildings provides long-term protection against such high-speed winds.

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: 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: 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: 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: Agricultural Impact

c) Makes soil less fertile
[Solution Description]
Seawater contains high levels of salt, which makes the soil less fertile when it intrudes into farmland. This reduces crop yields and negatively impacts agriculture.

Key Concept: Cyclone Formation

c) Warm ocean waters with temperatures above $26.5^{\circ}$C
[Solution Description]
A cyclone forms when warm ocean water heats the air above it, causing it to rise and create a low-pressure zone. As the moist air rises, condensation releases heat, further lowering pressure and drawing in surrounding air. The Earth's rotation causes the air to spin, forming a cyclone. Therefore, warm ocean waters are crucial for cyclone formation.

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: 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 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.