Samacheer Kalvi Class 9 Science Solutions Chapter 3 Fluids

I. Choose The Correct Answer:

 

Question 1. The size of an air bubble rising up in the water
(a) decreases
(b) increases
(c) remains the same
(d) may increase or decrease
Answer: (b) increases
In simple words: As an air bubble rises in water, the pressure around it decreases. This lower pressure allows the bubble to expand and become larger.

🎯 Exam Tip: Remember that gas volume changes with pressure; rising bubbles experience less pressure and expand, while sinking bubbles experience more pressure and shrink.

 

Question 2. Clouds float in the atmosphere because of their low
(a) density
(b) pressure
(c) velocity
(d) mass
Answer: (a) density
In simple words: Clouds are made of tiny water droplets or ice crystals. Because they are spread out, their overall density is less than the surrounding air, allowing them to float.

🎯 Exam Tip: Understand that objects float when their density is less than the fluid they are in, a key concept for buoyancy.

 

Question 3. In a pressure cooker, the food is cooked faster because
(a) increased pressure lowers the boiling point.
(b) increased pressure raises the boiling point.
(c) decreased pressure raises the boiling point.
(d) increased pressure lowers the melting point.
Answer: (b) increased pressure raises the boiling point
In simple words: When you cook in a pressure cooker, the steam builds up and creates more pressure. This higher pressure makes water boil at a temperature higher than \( 100^\circ \text{C} \), so food cooks faster.

🎯 Exam Tip: Remember the direct relationship between pressure and boiling point: higher pressure leads to a higher boiling point, which speeds up cooking.

 

Question 4. An empty plastic bottle closed with an airtight stopper is pushed down into a bucket filled with water. As the bottle is pushed down, there is an increasing force on the bottom. This is because
(a) more volume of liquid is displaced.
(b) more weight of liquid is displaced.
(c) pressure increases with depth.
(d) All of the options
Answer: (c) pressure increases with depth
In simple words: When you push the bottle deeper into the water, the water above it presses down harder. This means the pressure at the bottom of the bottle gets bigger.

🎯 Exam Tip: This is a fundamental concept of fluid pressure: pressure exerted by a fluid increases directly with the depth below the surface.

II. Fill In The Blanks:

 

Question 1. The weight of the body immersed in a liquid appears to be ............... than its actual weight
Answer: less
In simple words: When an object is in a liquid, it feels lighter. This is because the liquid pushes it upwards, reducing its apparent weight.

🎯 Exam Tip: This feeling of lightness is due to the buoyant force, a key part of Archimedes' principle.

 

Question 2. The instrument used to measure atmospheric pressure is ...............
Answer: Barometer
In simple words: A barometer is a tool that tells us how much air is pressing down around us. It measures the weight of the air above a certain point.

🎯 Exam Tip: Know that barometers are crucial for weather forecasting as changes in atmospheric pressure often indicate changing weather conditions.

 

Question 3. The magnitude of buoyant force acting on an object immersed in a liquid depends on ................ of the liquid.
Answer: density
In simple words: How much a liquid pushes an object upwards depends on how heavy that liquid is for its size. Denser liquids push more forcefully.

🎯 Exam Tip: A higher liquid density means a greater buoyant force, explaining why objects float more easily in salt water than fresh water.

 

Question 4. A drinking straw works on the existence of ....................
Answer: atmospheric pressure
In simple words: When you suck on a straw, you reduce the air pressure inside it. The higher air pressure outside then pushes the liquid up into the straw and into your mouth.

🎯 Exam Tip: Many everyday actions, like using a plunger or a syringe, also rely on atmospheric pressure differences.

III. State Whether True Or False. If False, Correct The Statement:

 

Question 1. The weight of fluid displaced determines the buoyant force on an object.
Answer: True.
In simple words: When an object is put in a liquid, the amount of liquid it pushes aside has a certain weight. This weight is exactly how much the liquid pushes the object upwards.

🎯 Exam Tip: This statement is a direct definition of Archimedes' principle, which is fundamental to understanding buoyancy.

 

Question 2. The shape of an object helps to determine whether the object will float or not.
Answer: False.
Correct statement: The density of an object helps to determine whether the object will float or sink.
In simple words: It's not the shape of an object that makes it float or sink, but rather how much "stuff" is packed into its size compared to the liquid. This is called density.

🎯 Exam Tip: While shape influences how an object displaces water and thus its effective density, the fundamental floating/sinking criterion is the object's overall density compared to the fluid.

 

Question 3. The foundations of high-rise buildings are kept wide so that they may exert more pressure on the ground.
Answer: False.
Correct statement: They may exert less pressure on the ground.
In simple words: Big buildings have wide bases so that their heavy weight is spread out over a larger area. This makes the force pressing on any one spot on the ground smaller.

🎯 Exam Tip: Remember that pressure is force divided by area; to reduce pressure (e.g., on the ground or a surface), increase the contact area.

 

Question 4. Archimedes' principle can also be applied to gases.
Answer: True.
In simple words: Archimedes' principle isn't just for liquids; it also works for gases. For example, a hot air balloon floats because it displaces a weight of air greater than its own weight.

🎯 Exam Tip: Realize that a fluid can be either a liquid or a gas, and the principles of buoyancy apply to both.

 

Question 5. Hydraulic press is used in the extraction of oil from oilseeds.
Answer: True.
In simple words: A hydraulic press uses liquid pressure to create a very strong squeezing force. This force is powerful enough to extract oil from seeds.

🎯 Exam Tip: Hydraulic systems are powerful because they efficiently transmit force and can multiply it by applying pressure over different piston areas.

IV. Match The Following:

 

Question. Match the following:

Answer:In simple words: This table links important terms in physics with their definitions, units, or related concepts. For example, density is how much mass is in a certain volume, and a lactometer is used to check milk.

🎯 Exam Tip: Memorize these fundamental definitions and relationships, as they are essential for solving problems and understanding fluid mechanics.

V. Answer In Brief:

 

Question 1. On what factors the pressure exerted by the liquid depends on?
Answer: The pressure exerted by a liquid depends on these factors:

• Depth
• Density of the liquid
• Acceleration due to gravity

These three factors together determine the pressure at any point within a liquid.
In simple words: Liquid pressure depends on how deep you go, how heavy the liquid is, and how strong gravity is.

🎯 Exam Tip: The formula \( P = \rho h g \) (Pressure = density x height x acceleration due to gravity) summarizes these factors, so understanding it helps you recall them easily.

 

Question 2. Why does a helium balloon float in the air?
Answer: A helium balloon floats in the air because helium gas is much lighter than the normal air around it. This lower density of helium creates an upward pushing force, called buoyancy, which is strong enough to make the balloon rise. The balloon displaces a weight of air greater than its own weight, allowing it to float.
In simple words: Helium balloons float because helium is lighter than the air around it, so the air pushes the balloon upwards.

🎯 Exam Tip: Remember that flotation in air (or any fluid) occurs when the object's overall density is less than the fluid's density, making the buoyant force greater than the object's weight.

 

Question 3. Why it is easy to swim in river water than in seawater?
Answer: It is actually easier to swim in seawater than in river water. This is because seawater has:
(i) greater density, meaning it is heavier for the same amount.
(ii) a larger buoyant force compared to river water. The dissolved salts in seawater make it denser than freshwater, providing a greater upward push on a swimmer.
In simple words: Swimming is easier in salty ocean water than in fresh river water because salty water is denser and pushes you up more easily.

🎯 Exam Tip: To avoid losing marks, always clarify if a premise in the question is incorrect before providing the correct explanation, as was done here.

 

Question 4. What is meant by atmospheric pressure?
Answer: Atmospheric pressure is the force per unit area exerted by the weight of the air above a surface. This pressure is felt by everything on Earth's surface and is caused by the atmospheric gases pressing on their surroundings. The air around us is constantly pushing down on everything.
In simple words: Atmospheric pressure is simply the weight of all the air above us, pushing down on everything.

🎯 Exam Tip: Understand that atmospheric pressure acts in all directions, not just downwards, and decreases as you go higher in altitude.

 

Question 5. State Pascal's law.
Answer: Pascal's law states that if external pressure is applied to an incompressible liquid that is enclosed in a container, this pressure will be transmitted equally and without any reduction to every part of the liquid. This means the pressure will reach all points in the fluid, and the walls of the container, with the same intensity.
In simple words: Pascal's law says that if you press on a liquid that cannot be squished, that pressure will spread out evenly everywhere in the liquid.

🎯 Exam Tip: Pascal's law is the foundation for hydraulic systems like car brakes and hydraulic lifts, where a small force can generate a large force.

Descriptive Questions:

 

Question 1. An appropriate illustration proves that the force acting on a smaller area exerts a greater pressure.
Answer: Imagine standing on loose sand. Your feet will sink deeply into the sand. Now, if you lie down on the same sand, you will notice that your body does not sink as deep. In both situations, the force pushing down on the sand is the same, which is the weight of your body. This force, acting straight down on the surface, is called thrust. When you stand, your weight is focused on a small area (your feet). When you lie down, your weight is spread over a much larger area (your whole body). Because the same force is applied over a smaller area when standing, it creates much greater pressure, causing your feet to sink more. This shows how pressure depends on the area over which a force is distributed.
In simple words: Standing on sand makes your feet sink deep because your weight is on a small area, creating high pressure. Lying down spreads your weight over a big area, making the pressure low, so you don't sink as much.

🎯 Exam Tip: To illustrate pressure concepts effectively, always use clear, relatable examples and explicitly link the force, area, and resulting pressure.

 

Question 2. Describe the construction and working of the mercury barometer.
Answer:
A mercury barometer is an instrument used to measure atmospheric pressure.
Construction:
1. It was first designed by Torricelli.
2. A mercury barometer is made up of a long glass tube, which is sealed at one end and open at the other.
3. The tube is completely filled with mercury. Then, the open end is carefully covered with a thumb and inverted into a trough also filled with mercury. The thumb is then removed, allowing the mercury to settle.
Working:
4. The barometer works by balancing the weight of the mercury column inside the glass tube against the pressure of the outside air.
5. If the atmospheric air pressure increases, it pushes down harder on the mercury in the trough, forcing more mercury up into the tube.
6. Conversely, if the air pressure decreases, less force pushes on the mercury in the trough, causing some mercury to drain out of the tube.
7. The space above the mercury in the sealed tube contains a vacuum (Torricellian vacuum), which does not exert pressure. Therefore, the height of the mercury column directly measures the atmospheric pressure.
8. Mercury barometers are commonly used in laboratories and weather stations to get precise measurements of air pressure.
In simple words: A mercury barometer is a glass tube with mercury that measures air pressure. Air pushes on the mercury in a dish, which pushes mercury up the tube. The higher the mercury goes, the higher the air pressure.

🎯 Exam Tip: When describing the working of a barometer, clearly explain the role of atmospheric pressure in pushing the mercury column and the significance of the vacuum above the mercury.

 

Question 3. How does an object's density determine whether the object will sink or float in water?
Answer: An object's density is the key factor in determining if it will sink or float in a liquid. If an object's density is less than the density of the liquid it is placed in, the object will float. For instance, a piece of wood floats in water because wood is less dense than water. On the other hand, if an object's density is greater than the density of the liquid, it will sink. For example, a stone sinks in water because it is denser than water. The liquid applies an upward buoyant force, but if the object's weight is greater than this force, it sinks.
In simple words: If an object is lighter than the same amount of water, it floats. If it's heavier, it sinks. This lightness or heaviness for its size is called density.

🎯 Exam Tip: Always compare the density of the object to the density of the fluid; it's a direct comparison. Remember that the density of water is approximately \( 1 \text{ g/cm}^3 \) or \( 1000 \text{ kg/m}^3 \).

 

Question 4. Explain the construction and working of a hydrometer with a diagram.
Answer:
Purpose: A hydrometer is used to measure the relative density of a liquid.
Principle: It works on the principle of flotation. The weight of the liquid displaced by the submerged part of the hydrometer is equal to the total weight of the hydrometer itself.
Construction:
The hydrometer consists of a long, thin cylindrical stem with markings on it, and a spherical bulb at its lower end. The bulb is partially filled with lead shots or mercury. These lead shots or mercury help the hydrometer to float upright and remain stable in liquids.
Working:
1. To use it, the liquid to be tested is carefully poured into a measuring jar or beaker.
2. The hydrometer is then gently lowered into the liquid until it floats freely. It will sink deeper into less dense liquids and float higher in denser liquids.
3. The reading on the graduated stem, which is level with the surface of the liquid, directly indicates the relative density of that liquid. This reading shows how dense the liquid is compared to water.
In simple words: A hydrometer is a tool shaped like a floating stick that measures how thick or heavy a liquid is. It sinks to a certain level, and that level tells you the liquid's density.

🎯 Exam Tip: Clearly distinguish between the construction features (stem, bulb, lead shots) and the working principle (flotation and reading the scale at the liquid level).

 

Question 5. State the laws of flotation.
Answer: The laws of flotation describe the conditions under which an object will float in a fluid:
1. The weight of a floating body in a fluid is always equal to the weight of the fluid that it displaces. This means the upward buoyant force precisely balances the object's weight.
2. For a floating body to be stable, the center of gravity of the floating body and the center of buoyancy (the center of gravity of the displaced fluid) must lie along the same vertical line. If they are not aligned, the object might tilt or capsize. This ensures rotational stability.
In simple words: An object floats if it pushes aside a weight of liquid equal to its own weight. Also, for it to float steadily, its balance point and the liquid's upward push point must be directly one above the other.

🎯 Exam Tip: When stating the laws, emphasize "equal weight of displaced fluid" for the first law and "vertical alignment of centers of gravity and buoyancy" for the second law to ensure full marks.

VII. Assertion And Reason:

 

Question 1. Assertion (A): To float, body must displace liquid whose weight is equal to the actual weight.
Reason (R): The body will experience no net downward force in that case.
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
In simple words: The assertion is correct because an object floats when the upward push from the liquid exactly equals its own weight. The reason explains this well, because when these forces are balanced, there is no net force pulling the object down, so it stays afloat.

🎯 Exam Tip: For assertion-reason questions, first determine if both statements are individually true, then check if the reason logically explains the assertion using "because".

 

Question 2. Assertion (A): Pascal's law is the working principle of a hydraulic lift.
Reason (R): Pressure is thrust per unit area.
Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.
In simple words: Both statements are true individually. Pascal's law indeed makes hydraulic lifts work by evenly spreading pressure through a liquid. The reason also correctly defines pressure as force per area. However, the reason does not directly explain *why* Pascal's law applies to hydraulic lifts; it's a general definition of pressure.

🎯 Exam Tip: Even if both statements are true, the reason must *directly* explain the assertion to be considered the correct explanation. A general definition might be true but not a direct cause for the assertion.

VIII. Numerical Problems:

 

Question 1. A block of wood of weight 200 g floats on the surface of water. If the volume of the block is 300 cm³, calculate the upthrust due to water.
Answer:
Given:
Weight of woodblock \( = 200 \text{ g} \)
Volume of the woodblock \( V = 300 \text{ cm}^3 \)
Since the woodblock floats, the upthrust (buoyant force) due to water is equal to the weight of the fluid displaced, which in turn is equal to the weight of the floating object.
Upthrust \( = \) Weight of the fluid displaced
Upthrust \( = \) Weight of the woodblock
Upthrust \( = 200 \text{ g} \)
Also, according to Archimedes' principle, the weight of the fluid displaced is equal to the weight of the floating body. For an object to float, the weight of the water it pushes aside must be equal to its own weight.
If the question implies that the entire volume \( 300 \text{ cm}^3 \) is submerged, then the upthrust would be equal to the weight of \( 300 \text{ cm}^3 \) of water, which is \( 300 \text{ g} \) (since density of water is \( 1 \text{ g/cm}^3 \)). However, the object's weight is \( 200 \text{ g} \), so only \( 200 \text{ cm}^3 \) of water must be displaced for it to float. Therefore, the upthrust is \( 200 \text{ g} \).
In simple words: Because the wood floats, the upward push from the water (upthrust) is exactly the same as the wood's weight. So, the upthrust is 200 g.

🎯 Exam Tip: For floating objects, the buoyant force (upthrust) is always equal to the object's weight. This is a crucial concept for solving such problems.

 

Question 2. Density of mercury is \( 13600 \text{ kg m}^{-3} \). Calculate the relative density
Answer:
Given:
Density of Mercury \( = 13600 \text{ kg m}^{-3} \)
Density of water at \( 4^\circ \text{C} = 1000 \text{ kg m}^{-3} \) (This is a standard value, important to remember.)
Relative density is calculated as the ratio of the density of the substance to the density of water at \( 4^\circ \text{C} \).
Relative density \( = \frac{\text{Density of mercury}}{\text{Density of water at } 4^\circ \text{C}} \)
\( = \frac{13600 \text{ kg m}^{-3}}{1000 \text{ kg m}^{-3}} \)
Relative Density \( = 13.6 \)
Relative density has no units, as it is a ratio of two similar quantities.
In simple words: To find the relative density of mercury, we divide its density by the density of water. This gives us a value of 13.6, which tells us mercury is 13.6 times denser than water.

🎯 Exam Tip: Always remember that relative density is a dimensionless quantity (has no units) and that the density of water at \( 4^\circ \text{C} \) is the standard reference for calculation.

 

Question 3. The density of water is \( 1 \text{ g cm}^{-3} \). What is its density in S.I. units?
Answer:
Given:
Density of water \( = 1 \text{ g cm}^{-3} \)
To convert this to S.I. units (kilogram per cubic meter, \( \text{kg/m}^3 \)):
We know that \( 1 \text{ g} = 10^{-3} \text{ kg} \)
And \( 1 \text{ cm} = 10^{-2} \text{ m} \), so \( 1 \text{ cm}^3 = (10^{-2} \text{ m})^3 = 10^{-6} \text{ m}^3 \)
So, \( 1 \text{ g cm}^{-3} = \frac{1 \text{ g}}{1 \text{ cm}^3} = \frac{10^{-3} \text{ kg}}{10^{-6} \text{ m}^3} \)
Density of water in S.I. units \( = 10^3 \text{ kg m}^{-3} \) or \( 1000 \text{ kg/m}^3 \).
In simple words: Water's density is 1 gram per cubic centimeter. In bigger scientific units (S.I.), this is 1000 kilograms per cubic meter.

🎯 Exam Tip: Be careful with unit conversions, especially when dealing with cubic units, as powers of 10 add up quickly (e.g., \( (10^{-2})^3 = 10^{-6} \)).

 

Question 4. Calculate the apparent weight of wood floating on water if it weighs 100g in air.
Answer:
When an object floats on water, it means it is completely supported by the buoyant force from the water.
For a floating object, the buoyant force is equal to its actual weight.
The apparent weight of a floating object is defined as its actual weight minus the buoyant force.
Apparent weight \( = \) Actual weight \( - \) Buoyant force
Since the object is floating, Buoyant force \( = \) Actual weight.
Apparent weight \( = \) Actual weight \( - \) Actual weight \( = 0 \text{ g} \)
Therefore, the apparent weight of the wood floating on water is \( 0 \text{ g} \). This is why a floating object feels weightless when fully submerged and floating.
In simple words: When wood floats, the water pushes it up with the exact same force as its weight. So, it feels like it has no weight at all when it's floating.

🎯 Exam Tip: A floating object always has an apparent weight of zero because the buoyant force perfectly counteracts its actual weight.

IX. Higher Order Thinking Skills:

 

Question 1. How high does the mercury barometer stand on a day when atmospheric pressure is 98.6 kPa?
Answer:
Given:
Atmospheric Pressure \( P_{\text{atm}} = 98.6 \text{ kPa} = 98.6 \times 10^3 \text{ Pa} \)
Density of Mercury \( \rho_{\text{Hg}} = 13.6 \times 10^3 \text{ kg m}^{-3} \)
Acceleration due to gravity \( g = 9.8 \text{ m/s}^2 \)
The pressure exerted by a liquid column is given by the formula:
\( P = \rho h g \)
We need to find the height \( h \). Rearranging the formula:
\( h = \frac{P_{\text{atm}}}{\rho_{\text{Hg}} \times g} \)
\( h = \frac{98.6 \times 10^3 \text{ Pa}}{(13.6 \times 10^3 \text{ kg m}^{-3}) \times (9.8 \text{ m s}^{-2})} \)
\( h = \frac{98.6 \times 10^3}{13.6 \times 9.8 \times 10^3} \)
\( h = \frac{98.6}{13.6 \times 9.8} \)
\( h = \frac{98.6}{133.28} \)
\( h \approx 0.7397 \text{ m} \)
To convert meters to millimeters, multiply by 1000:
\( h = 0.7397 \times 1000 \text{ mm} \)
\( h = 739.7 \text{ mm} \)
So, the mercury barometer stands at a height of approximately \( 739.7 \text{ mm} \). This calculation shows the direct relationship between atmospheric pressure and the height of a mercury column.
In simple words: We use a formula that links pressure, density of mercury, and gravity to find the height. When the air pressure is 98.6 kPa, the mercury in the barometer will stand at about 739.7 millimeters high.

🎯 Exam Tip: Always ensure consistent units throughout your calculation (S.I. units are best), and clearly state the given values and the formula used.

 

Question 2. How does a fish manage to rise up and move down in the water?
Answer: Fish use a special organ called a swim bladder to control their buoyancy and easily move up or down in the water.

• To rise up: A fish fills its swim bladder with oxygen, which it gets from the water through its gills. Adding more gas to the bladder increases the fish's overall volume without significantly increasing its mass, thereby decreasing its average density. This makes the fish lighter than the surrounding water, and the buoyant force pushes it upwards.
• To move down: To descend, the fish releases oxygen from its swim bladder. This reduces the bladder's volume, making the fish's overall average density greater than the surrounding water. As the fish becomes denser, the buoyant force is no longer enough to support its weight, and it sinks.

This process is similar to how a submarine uses ballast tanks to control its depth.
In simple words: Fish go up by putting more air (oxygen) into a special bag inside them, making themselves lighter. They go down by letting air out of the bag, making themselves heavier.

🎯 Exam Tip: Focus on the concept of changing density by adjusting volume (via the swim bladder) as the core mechanism for buoyancy control in fish.

 

Question 3. If you put one ice cube in a glass of water and another in a glass of alcohol, what would you observe? Explain your observations.
Answer:
When an ice cube is placed in water, it floats.
When an ice cube is placed in alcohol, it sinks.
This observation can be explained by comparing the densities of ice, water, and alcohol.
Ice cube in water: The density of ice is approximately \( 0.917 \text{ g/cm}^3 \), which is less than the density of water (\( 1.00 \text{ g/cm}^3 \)). Since ice is less dense than water, the ice cube experiences an upward buoyant force greater than its weight, causing it to float. This is a common property that allows icebergs to exist.
Ice cube in alcohol: The density of alcohol is approximately \( 0.78 \text{ g/cm}^3 \). The density of ice (\( 0.917 \text{ g/cm}^3 \)) is greater than the density of alcohol. Because ice is denser than alcohol, the buoyant force from the alcohol is not enough to support the ice cube's weight, so the ice cube sinks.
The densities are: Water = \( 1.00 \text{ g/cm}^3 \), Ice cube = \( 0.917 \text{ g/cm}^3 \), Alcohol = \( 0.78 \text{ g/cm}^3 \).
In simple words: Ice floats in water because ice is lighter than water. But ice sinks in alcohol because ice is heavier than alcohol. It all depends on how dense each substance is.

🎯 Exam Tip: Always clearly state the densities of the substances involved and directly compare them to explain floating or sinking phenomena.

 

Question 4. Why does a boat with a hole in the bottom would eventually sink?
Answer: A boat with a hole in the bottom will eventually sink for two main reasons:

• Increased weight: When there is a hole in the bottom, water will flow into the boat. This incoming water adds significantly to the boat's total weight.
• Reduced buoyancy: As the boat fills with water, its overall density increases. At some point, the boat, along with the water inside it, becomes heavier than the amount of water it can displace. When the boat's total weight exceeds the maximum buoyant force that the water can provide, it will no longer be able to float and will sink.

This is a straightforward application of Archimedes' principle and the concept of density.
In simple words: A hole lets water into the boat, making it much heavier. Once the boat becomes too heavy to be pushed up by the water, it sinks.

🎯 Exam Tip: Explain both aspects: the increase in the boat's total weight and the resulting increase in its average density, which causes it to overcome the buoyant force.

Intex Activities

Activity - 1

 

Question. Stand on the loose sand. Your feet go deep into the sand. Now, lie down on the sand. What happens? You will find that your body will not go that deep into the sand. Why?
Answer:
Aim: To understand the effect of thrust and pressure.
Materials Required: Sand
Procedure:
1. First, stand on the sand on your feet.
2. Then, lie down on the sand with your whole body.
Observation:
1. While standing on your feet on the sand, your feet will sink deep into the sand.
2. While lying down with your body on the sand, your body will not sink as deep into the sand.
Conclusion:
1. This happens because pressure depends on the area over which a force acts.
2. The effect of thrust (your body weight) on the sand is much larger when standing than when lying down, because your weight is spread over a smaller area when standing. This demonstrates that pressure is inversely proportional to the area for a given force.
This activity illustrates how a constant force (your body weight) can produce different pressures depending on the contact area.
In simple words: Standing on sand pushes your weight onto a small spot, making it sink more. Lying down spreads your weight over a bigger area, so the push on any one spot is less, and you don't sink as much.

🎯 Exam Tip: When explaining experiments, clearly state the aim, procedure, observation, and conclusion, linking observations back to the underlying physical principle (pressure, force, and area).

Activity - 2

 

Activity - 3

 

Activity - 4

 

Activity - 5

 

9th Science Guide Fluids Additional Important Questions And Answers

 

I. Choose The Correct Answer:

 

Question 1. Intermolecular forces are stronger in
(a) gases
(b) liquids
(c) solids
(d) all the above
Answer: (c) solids
In simple words: Solids have the strongest forces between their particles, which keeps them in a fixed shape. This is why solids are generally hard and rigid.

🎯 Exam Tip: Remember the order of intermolecular forces: solids > liquids > gases. This determines their physical properties.

 

Question 2. Water (or) liquids exert pressure on
(a) Upward direction
(b) Downward direction
(c) Lateral direction
(d) All of the options
Answer: (d) All of the options
In simple words: Liquids push outwards in every direction. This is why water in a container presses on the bottom, sides, and even upwards if you push something into it.

🎯 Exam Tip: This omnidirectional pressure is a key characteristic of fluids (liquids and gases) and is fundamental to understanding concepts like buoyancy and hydraulics.

 

Question 3. The pressure does not depend upon
(a) Depth
(b) Area
(c) Density
(d) Acceleration due to gravity
Answer: (b) Area
In simple words: The pressure a liquid puts on a point depends on how deep that point is, the liquid's thickness (density), and gravity. It doesn't depend on how wide the container is.

🎯 Exam Tip: Pressure in a liquid at a certain depth is given by \( P = h \rho g \), where \( h \) is depth, \( \rho \) is density, and \( g \) is acceleration due to gravity. Area is not a factor for pressure at a specific point.

 

Question 4. Fluids in general are
(a) Gases
(b) liquids
(c) Gases or Liquids
(d) Both (a) and (b)
Answer: (c) Gases or Liquids
In simple words: Anything that can flow, like water or air, is called a fluid. This means both liquids and gases are types of fluids.

🎯 Exam Tip: The main characteristic of a fluid is its ability to flow and take the shape of its container.

 

Question 5. Scuba divers wear special suits to withstand
(a) Low pressure
(b) High pressure
(c) Low temperature
(d) High temperature
Answer: (b) High pressure
In simple words: As divers go deeper into the water, the pressure around them becomes very high. Their special suits help their bodies handle this strong pressure.

🎯 Exam Tip: Pressure increases significantly with depth in water, so deep-sea divers need specialized gear to protect their bodies from the immense force.

 

Question 6. To find out relative density of the substance, with respect to density of water...............C is taken.
(a) 4°
(b) 0°
(c) 100°
(d) 60°
Answer: (a) 4°
In simple words: When we compare how dense something is to water, we use water at 4 degrees Celsius. This is because water is at its most dense at this temperature.

🎯 Exam Tip: Water's maximum density at 4°C is an important property used as a standard for relative density measurements.

 

Question 7. Density Bottle is also called as
(a) Saccharometer
(b) Lactometer
(c) Pycnometer
(d) Hydrometer
Answer: (c) Pycnometer
In simple words: A pycnometer is another name for a density bottle. It is a special bottle used to measure how dense a liquid is.

🎯 Exam Tip: Pycnometers are preferred for precise density measurements due to their fixed volume and precise stopper.

 

Question 8. An object completely immersed in fluid displaces its own volume of fluid.
(a) Floatation principle
(b) Principle of buoyancy
(c) Pascal's law
(d) Archimedes principle
Answer: (d) Archimedes principle
In simple words: Archimedes' principle says that when an object is fully underwater, it pushes aside a volume of water exactly equal to its own volume. This displaced water creates an upward push on the object.

🎯 Exam Tip: Archimedes' principle also states that the buoyant force on an object is equal to the weight of the fluid it displaces.

 

Question 9. A solid floats in liquid with a portion of it being submerged. Then
(a) The liquid exerts an upthrust equal to weight of the solid
(b) The weight of the dispersed liquid is equal to the weight of solid
(c) Solid exerts a force equal to its weight on liquid
(d) All of the options
Answer: (A) a & b
In simple words: When an object floats, the upward push from the liquid (upthrust) is exactly the same as the object's weight. Also, the weight of the liquid pushed away by the floating object is equal to the object's weight.

🎯 Exam Tip: For an object to float, the buoyant force must balance the object's weight. This is the principle of flotation.

 

Question 10. The principle of "Hydrostatic balance” was devised by
(a) Torricelli
(b) Pascal
(c) Archimedes
(d) Newton
Answer: (c) Archimedes
In simple words: The idea behind a hydrostatic balance, which helps measure density by weighing things in and out of water, was developed by Archimedes. This device applies his principle of buoyancy.

🎯 Exam Tip: Archimedes' principle is foundational to understanding buoyancy and specific gravity, which are measured using hydrostatic balances.

 

Question 11. Saccharometer is used to measure the density of ................in a liquid.
(a) Milk
(b) Sugar
(c) Alcohol
(d) Ether
Answer: (b) Sugar
In simple words: A saccharometer is a tool that tells us how much sugar is in a liquid. It measures the liquid's density, which changes with the amount of sugar.

🎯 Exam Tip: Each type of hydrometer is designed for specific liquids or substances, such as lactometers for milk and alcoholometers for alcohol.

 

Question 12. Most buoyant objects are those with relatively
(a) high volume
(b) higher mass
(c) low density
(d) less viscosity
Answer: (B) a & c
In simple words: Objects that float best usually have a big size (high volume) and are not very heavy for their size (low density). This combination helps them get a good upward push from the liquid.

🎯 Exam Tip: Buoyancy is directly related to the volume of fluid displaced and inversely related to the object's density compared to the fluid.

 

Question 13. If there were no gravity, which of the following will not be there for fluid? (HOTS)
(a) Viscosity
(b) Surface Tension
(c) Pressure
(d) upthrust
Answer: (d) upthrust
In simple words: Upthrust, which is the upward push from a fluid, happens because of gravity. If there was no gravity, there would be no weight to measure, and thus no upthrust.

🎯 Exam Tip: Upthrust (buoyancy) is a direct consequence of the pressure difference in a fluid due to gravity acting on the fluid column.

 

Question 14. Human lung is well adopted to breath at a pressure of .............kPa.
(a) 106.7
(b) 101.3
(c) 98.4
(d) 33.7
Answer: (b) 101.3
In simple words: Our lungs are designed to breathe easily at normal atmospheric pressure, which is about 101.3 kilopascals (kPa). This is the standard pressure at sea level.

🎯 Exam Tip: 101.3 kPa is approximately 1 atmosphere (atm) of pressure, the average atmospheric pressure at sea level.

 

Question 15. Petroleum-based products float on the surface of the water. This is due to their low ................
(a) volume
(b) density
(c) specific gravity
(d) viscosity
Answer: (D) b & c
In simple words: Petroleum products float on water because they are less dense than water, meaning they are lighter for the same amount of space. Specific gravity is just a way to compare their density to water's density.

🎯 Exam Tip: Objects float if their density is less than the fluid they are in. Specific gravity is a dimensionless quantity that compares a substance's density to a reference fluid's density.

 

II. Fill In The Blanks:

 

Question 1. The weight of the body immersed in a liquid appears to be ..................... than its actual weight
Answer: less
In simple words: When you put something in a liquid, it feels lighter. This is because the liquid pushes it up with an upward force.

🎯 Exam Tip: This apparent loss of weight is due to the buoyant force acting on the object, which opposes gravity.

 

Question 2. The instrument used to measure atmospheric pressure is .....................
Answer: Barometer
In simple words: A barometer is a tool that tells us how much air pressure there is around us. It helps predict weather changes.

🎯 Exam Tip: A mercury barometer is a classic example, where atmospheric pressure supports a column of mercury. Aneroid barometers use a sealed metal chamber.

 

Question 3. All flowing substances, both liquids, and gases are called .....................
Answer: Fluids
In simple words: Any substance that can pour or move freely, like water or air, is known as a fluid. They don't have a fixed shape.

🎯 Exam Tip: Fluids are characterized by their ability to deform continuously under applied shear stress, meaning they can flow.

 

Question 4. The air pressure at sea level is referred as .....................
Answer: Atmospheric pressure
In simple words: The weight of the air all around us at sea level creates a force called atmospheric pressure. This is the normal air pressure we experience.

🎯 Exam Tip: Standard atmospheric pressure at sea level is approximately 101.3 kPa, often referred to as 1 atmosphere (atm).

 

Question 5. The pressure in mines is .....................than sea level.
Answer: Greater
In simple words: In mines, which are deep underground, the pressure is higher than at the surface. This is because there is more air and rock pushing down from above.

🎯 Exam Tip: Pressure increases with depth in both liquids and gases because the weight of the overlying fluid column increases.

 

Question 6. .....................is the instrument used to measure the atmospheric pressure.
Answer: Barometer
In simple words: A barometer is the device used to measure the force of the air pressing down on us. It helps in weather forecasting.

🎯 Exam Tip: Knowing the atmospheric pressure is crucial for meteorologists as it indicates weather conditions; high pressure usually means clear weather, low pressure indicates storms.

 

Question 7. On each 1m² of surface, the force acting is .....................
Answer: 1.013 kN
In simple words: For every square meter of surface at sea level, the air pushes down with a force of about 1.013 kilonewtons. This is a very strong push!

🎯 Exam Tip: This value represents the force due to standard atmospheric pressure. Remember that 1 kN = 1000 N.

 

Question 8. .....................is a device for measuring atmospheric pressure without the use of liquids.
Answer: Aneroid Barometer
In simple words: An aneroid barometer is a type of tool that measures air pressure without using any liquid, unlike a mercury barometer. It uses a sealed metal box that expands or shrinks with pressure changes.

🎯 Exam Tip: Aneroid barometers are portable and commonly used in homes and aircraft because they are less fragile and do not contain toxic mercury.

 

Question 9. Absolute Pressure is zero-referenced against a .....................
Answer: Perfect Vacuum
In simple words: Absolute pressure is measured from a starting point where there is no pressure at all, like in outer space. This starting point is called a perfect vacuum.

🎯 Exam Tip: Gauge pressure is measured relative to atmospheric pressure, while absolute pressure is measured relative to a perfect vacuum.

 

Question 10. Psi stands for .....................
Answer: Pascal per inch
In simple words: Psi is a unit of pressure, often used for things like tire pressure. It actually stands for 'pounds per square inch' in English units. The question provides 'Pascal per inch' as the answer which is incorrect but following instructions.

🎯 Exam Tip: While the source indicates 'Pascal per inch', in standard physics, PSI stands for 'Pounds per Square Inch' and is an imperial unit of pressure.

 

Question 11. A tyre pressure of 30psi is almost .....................the atmospheric pressure.
Answer: Twice
In simple words: If your tire is inflated to 30 psi, that pressure is roughly double the normal air pressure outside. This high pressure inside helps the tire support the car.

🎯 Exam Tip: Standard atmospheric pressure is about 14.7 psi, so 30 psi is indeed approximately twice that value.

 

Question 12. The density of the substance is the .....................of a given substance.
Answer: mass per unit volume
In simple words: Density tells us how much 'stuff' (mass) is packed into a certain space (volume). If something has a lot of mass in a small volume, it's very dense.

🎯 Exam Tip: The formula for density is \( \rho = \frac{m}{V} \), where \( m \) is mass and \( V \) is volume. Its SI unit is kg/m³.

 

Question 13. Hydrometer is based on the principle of .....................
Answer: Flotation
In simple words: A hydrometer works because things float differently in liquids of different densities. It sinks more in less dense liquids and less in denser liquids.

🎯 Exam Tip: Hydrometers use Archimedes' principle, specifically the principle of flotation, where a floating object displaces a weight of fluid equal to its own weight.

 

Question 14. The upward force that is caused due to the pressure difference in liquid (or fluid) is called .....................
Answer: Buoyancy
In simple words: When an object is in water, the water pushes it upwards. This upward push, caused by the difference in pressure, is called buoyancy.

🎯 Exam Tip: Buoyancy is stronger at greater depths because the pressure difference between the top and bottom of the immersed object increases.

 

Question 15. Hot air is .....................dense than ordinary air.
Answer: less
In simple words: Hot air is lighter than the cooler air around it. This is why hot air balloons can fly – the warm, less dense air inside makes them rise.

🎯 Exam Tip: When air is heated, its molecules move faster and spread out, increasing its volume and decreasing its density, which leads to a buoyant force from the cooler, denser surrounding air.

 

Question 16. The Lactometer works on the principle of ......................of milk.
Answer: gravity
In simple words: A lactometer checks how pure milk is by seeing how it floats. It uses the principle of gravity, as denser milk (more fat, less water) will cause it to float differently.

🎯 Exam Tip: The lactometer is a type of hydrometer specifically calibrated to measure the density of milk, which indicates its fat content and purity.

 

Question 17. Icebergs and ships stay afloat due to .....................
Answer: Buoyancy
In simple words: Icebergs and ships float because the water pushes them up with a force called buoyancy. This upward push is strong enough to hold them up.

🎯 Exam Tip: For an object to float, the buoyant force must be equal to or greater than its weight. Ships are designed to displace enough water to achieve this.

 

Question 18. Archimedes principle is the consequence of .....................
Answer: Pascal's law
In simple words: Archimedes' principle, which explains why things float or sink, comes from Pascal's law. Pascal's law says that pressure changes in a fluid spread everywhere, creating the upward buoyant force.

🎯 Exam Tip: While related, it's more accurate to say Archimedes' principle describes buoyancy, and Pascal's law describes how pressure is transmitted in an enclosed fluid.

 

Question 19. The point in which the force of buoyancy is supposed to act is known as .....................
Answer: Centre of buoyancy
In simple words: The upward push from a liquid, called buoyancy, acts through a special point. This point is called the center of buoyancy, which is the center of gravity of the displaced fluid.

🎯 Exam Tip: For a floating object to be stable, its center of gravity must be below its metacenter, which is related to the center of buoyancy.

 

Question 20. The centre of gravity of the floating body and the centre of buoyance are in the same ......................line.
Answer: Vertical
In simple words: For a body to float steadily, its center of gravity (where its weight acts) and its center of buoyancy (where the upward push acts) must be lined up vertically. This ensures balance.

🎯 Exam Tip: When these two centers are vertically aligned, there is no rotational force (torque) acting on the object, contributing to its stability.

III. Match the Following:

Answer:
1. (d) Milk
2. (c) Sugar
3. (b) Alcohol
4. (a) Relative density
In simple words: Matching helps us connect measuring tools with what they are used for. A lactometer checks milk, a saccharometer checks sugar, an alcoholometer checks alcohol, and a pycnometer checks the relative density of liquids.

🎯 Exam Tip: Remember the specific purpose of each measuring instrument; knowing their applications is key for matching questions.

IV. State Whether True or False. If False, Correct the Statement:

 

Question 1. The shape and size of the solids do not easily change.
Answer: True. Solids have a fixed shape and size, which means they do not change easily. This is because their particles are held tightly together in a fixed pattern.
In simple words: Solids usually keep their shape and size and do not change easily.

🎯 Exam Tip: Focus on the fixed nature of solids compared to liquids and gases when answering questions about their shape and volume.

 

Question 2. Liquid exerts pressure in the upward direction.
Answer: False.
Correct statement: Liquid exerts pressure in all directions. Liquid pressure acts equally in all directions within the fluid at a given depth, not just upwards.
In simple words: This statement is false because liquids push pressure in all directions, not just upwards.

🎯 Exam Tip: Understand that fluid pressure is isotropic, meaning it acts equally in all directions at a certain depth, which is a key concept in fluid mechanics.

 

Question 3. The barometer works by balancing the Mercury in the glass tube along with the outside air pressure.
Answer: False.
Correct statement: The barometer works by balancing the mercury column in the glass tube against the outside air pressure. It measures atmospheric pressure.
In simple words: This is false. A barometer works by balancing the mercury inside the tube with the air pressure outside, showing how strong the air is pushing.

🎯 Exam Tip: When correcting false statements, ensure your corrected version is scientifically accurate and clearly addresses the error in the original statement.

 

Question 4. The absolute pressure is zero-referenced against atmospheric pressure.
Answer: False.
Correct statement: The absolute pressure is zero-referenced against a perfect vacuum. This means it is measured relative to a complete absence of pressure.
In simple words: This is false. Absolute pressure is measured from a perfect vacuum (no pressure at all), not from the air pressure around us.

🎯 Exam Tip: Distinguish between absolute pressure (referenced to vacuum) and gauge pressure (referenced to atmospheric pressure) to avoid common errors.

 

Question 5. The external pressure applied on an incompressible liquid is transmitted uniformly throughout the liquid.
Answer: True. This principle is known as Pascal's Law, and it applies to enclosed fluids. It's crucial for hydraulic systems.
In simple words: This statement is true. If you push on an enclosed liquid that cannot be squished, that push goes equally to every part of the liquid.

🎯 Exam Tip: Recognize Pascal's Law as the foundation for hydraulic systems, which transmit force effectively using this uniform pressure distribution.

 

Question 6. The correct lactometer reading is only obtained at a temperature of 60° C.
Answer: True. A lactometer is calibrated for a specific temperature to ensure accurate density measurements for milk. Temperature affects the density of liquids.
In simple words: This is true. For a lactometer to give a correct reading, it must be used at a specific temperature of 60°C.

🎯 Exam Tip: Mention the importance of consistent temperature in scientific measurements, especially for density, as it influences accuracy.

 

Question 7. If the buoyant force is less, the object will float.
Answer: False.
Correct statement: If the buoyant force is less than the weight of the object, the object will sink. An object floats only when the buoyant force is equal to or greater than its weight.
In simple words: This is false. If the push-up force (buoyant force) is smaller than the object's weight, the object will go down, not float.

🎯 Exam Tip: Remember the relationship between buoyant force and an object's weight: buoyant force must be greater than or equal to the object's weight for it to float.

 

Question 8. If the volume of object is above the water surface, then the object is less densed.
Answer: True. If an object floats with part of its volume above the water, it indicates that the object is less dense than the water it is floating in. The less dense an object is, the higher it floats.
In simple words: This is true. If a part of an object stays above the water, it means the object is lighter (less dense) than the water.

🎯 Exam Tip: Connect the concept of an object floating with part of its volume exposed to the idea that its overall density is less than the fluid's density.

 

Question 9. Upthrust = weight of the fluid displaced – apparent weight of the object.
Answer: False.
Correct statement: Upthrust = Weight of the fluid displaced = Apparent loss of weight of the object. Upthrust is directly equal to the weight of the fluid displaced, as stated by Archimedes' principle.
In simple words: This is false. The upthrust is simply equal to the weight of the liquid that is pushed away, or how much lighter the object seems when in the water.

🎯 Exam Tip: Clearly define upthrust (buoyant force) as being equal to the weight of the fluid displaced, which also represents the apparent loss of weight of the submerged object.

 

Question 10. Salt water provides less buoyant force than freshwater.
Answer: False.
Correct statement: Salt water provides more buoyant force than freshwater. This is because salt water is denser than freshwater, so it displaces more weight of fluid for the same volume.
In simple words: This is false. Salt water actually pushes up with more force than fresh water because it is heavier for the same amount.

🎯 Exam Tip: Remember that denser fluids provide greater buoyant force because they have more mass (and thus weight) in the same volume, causing more upward push.

V. Very Short Answer Questions:

 

Question 1. Differentiate Liquid from Gas.
Answer: It is easy to compress a gas because its particles are far apart and can be pushed closer together. In contrast, a liquid is incompressible, meaning its particles are already close, making it very difficult to squeeze them into a smaller space. Liquids also have a fixed volume but take the shape of their container, while gases have neither fixed volume nor fixed shape.
In simple words: Gases can be easily squished, but liquids cannot. Liquids keep their amount but change shape, while gases change both their amount and shape.

🎯 Exam Tip: Focus on the spacing and arrangement of particles when differentiating between states of matter; gases have much larger intermolecular spaces than liquids.

 

Question 2. What is the SI unit of pressure?
Answer: The SI unit of pressure is the Pascal (Pa), which is equivalent to Newton per square meter \( (N m^{-2}) \). One Pascal means a force of one Newton spread over one square meter.
In simple words: The standard unit for pressure is the Pascal, which is the same as one Newton of force spread over one square meter of area.

🎯 Exam Tip: Remember that pressure is force per unit area, and its SI unit (Pascal) directly reflects this definition.

 

Question 3. What are factors determining liquid pressure?
Answer: The pressure exerted by a liquid depends on three main factors:
(i) Depth (h) of the liquid column.
(ii) Density (\( \rho \)) of the liquid.
(iii) Acceleration due to gravity (g).
All these factors contribute to the formula \( P = \rho g h \).
In simple words: Liquid pressure depends on how deep the liquid is, how heavy the liquid is (its density), and the force of gravity.

🎯 Exam Tip: Always list these three factors (depth, density, gravity) and relate them to the formula \( P = \rho g h \) for liquid pressure.

 

Question 4. Write the equation for pressure due to liquid column.
Answer: The equation for pressure due to a liquid column is \( P = \rho g h \), where:
\( P \) represents Pressure.
\( h \) represents depth of the liquid column.
\( \rho \) represents density of the liquid.
\( g \) represents acceleration due to gravity. This formula helps calculate the pressure at any depth in a fluid.
In simple words: The formula is \( P = \rho g h \). This means pressure equals density times gravity times depth.

🎯 Exam Tip: Clearly define all variables when writing out a formula, as it shows a complete understanding of the concept.

 

Question 5. What is referred to as atmospheric pressure?
Answer: Atmospheric pressure refers to the pressure exerted by the column of air above the Earth's surface at sea level. This pressure is caused by the weight of the air in the atmosphere pushing down on everything below it.
In simple words: Atmospheric pressure is simply the force that the air all around us pushes down with, especially at sea level.

🎯 Exam Tip: Remember that atmospheric pressure is due to the weight of the air column above us and decreases as altitude increases.

 

Question 6. Expand the abbreviation 'psi'.
Answer: The abbreviation 'psi' stands for pounds per square inch. It is a unit of pressure commonly used in various applications, such as measuring tire pressure or water pressure.
In simple words: 'psi' means 'pounds per square inch'. It is a way to measure pressure.

🎯 Exam Tip: Be familiar with common units of pressure like Pascal (Pa) and pounds per square inch (psi), and know what they represent.

 

Question 7. What are Force multipliers?
Answer: Force multipliers are systems or devices that can increase the magnitude of an applied force. Hydraulic systems are well-known examples of force multipliers because they use Pascal's principle to generate a larger output force from a smaller input force.
In simple words: Force multipliers are tools or systems that make a small push turn into a big push. Hydraulic systems are a good example.

🎯 Exam Tip: When defining force multipliers, always link them to the concept of applying a small force to produce a larger output force, often using principles like Pascal's law.

 

Question 8. Write the SI unit & symbol for density?
Answer: The SI unit for density is kilograms per cubic meter \( (kg / m^3) \). The symbol commonly used for density is the Greek letter rho \( (\rho) \). Density helps describe how much mass is packed into a certain volume.
In simple words: The standard unit for density is kilograms per cubic meter \( (kg / m^3) \), and its symbol is \( \rho \).

🎯 Exam Tip: Remember the SI unit for density and its symbol, as they are fundamental in calculations involving mass and volume.

 

Question 9. Where do we use lactometers?
Answer: Lactometers are primarily used in milk processing units and dairies. Their main function is to measure the purity and density of milk, which helps determine its quality and fat content. They operate based on the principle of buoyancy.
In simple words: Lactometers are used in places that handle milk, like dairies, to check how pure and dense the milk is.

🎯 Exam Tip: Be precise about the application of specific instruments; a lactometer's unique use is for milk density.

VI. Answer in Brief:

 

Question 1. What happens when pressure is increased in solids?
Answer: When pressure is increased in solids:

• The solid experiences tension, which is a pulling force that tries to stretch it.
• If the pressure becomes too high, the solid will ultimately deform or break. Solids have limits to how much stress they can withstand.

In simple words: When you put more pressure on solids, they feel a pulling force, and if you push too hard, they can change shape or break.

🎯 Exam Tip: For solids, relate increased pressure to concepts of stress and strain, which can lead to deformation or fracture if limits are exceeded.

 

Question 2. How will you calculate fluid pressure?
Answer: Fluid pressure is calculated by dividing the total force exerted by the fluid by the area over which that force is applied. Mathematically, it is expressed as:
Fluid pressure \( (P) = \frac{\text{Total Force exerted by the fluid} (F)}{\text{Area over which the force is exerted} (A)} \)
\( \implies P = \frac{F}{A} \)
This fundamental formula applies to both liquids and gases.
In simple words: To find fluid pressure, you divide the total push (force) by the size of the area where the push is happening. So, pressure equals force divided by area.

🎯 Exam Tip: Always remember the basic definition of pressure as force per unit area and its corresponding formula \( P = F/A \).

 

Question 3. How will you find the absolute pressure?
Answer: Absolute pressure is calculated differently depending on whether the pressure is higher or lower than atmospheric pressure:
1. For pressures higher than atmospheric pressure:
Absolute pressure = Atmospheric pressure + Gauge pressure.
2. For pressures lower than atmospheric pressure:
Absolute pressure = Atmospheric pressure – Gauge pressure (or Vacuum pressure).
Absolute pressure is always positive and measured relative to a perfect vacuum.
In simple words: To find absolute pressure, if it's stronger than air pressure, you add the extra pressure to the air pressure. If it's weaker, you subtract the missing pressure from the air pressure.

🎯 Exam Tip: Clearly differentiate between absolute, atmospheric, and gauge/vacuum pressures in your understanding and calculations.

 

Question 4. Why do Scuba divers wear special suits and equipment?
Answer: Scuba divers wear special suits and equipment for essential protection in deep water due to two main reasons:
1. The pressure deep in the sea is much greater than atmospheric pressure. For every 10 meters of depth, pressure increases by one atmosphere.
2. At these high pressures, parts of the human body, including blood vessels and soft tissues, cannot withstand the immense force. Without protection, divers would suffer severe injuries or even death.
Therefore, divers use special suits and equipment to protect themselves from the high-pressure environment.
In simple words: Scuba divers wear special gear to protect themselves from the very high pressure deep in the ocean. Our bodies cannot handle such strong pushes without help.

🎯 Exam Tip: Emphasize the direct relationship between depth and pressure in fluids, and how specialized gear is necessary to equalize pressure on the human body.

 

Question 5. Define Relative Density.
Answer: Relative density of a substance is defined as the ratio of the density of the substance to the density of water at \( 4^\circ C \). Since it is a ratio of two densities, it is a pure number and has no units. The density of water at \( 4^\circ C \) is chosen as a standard because water has its maximum density at this temperature.
\( \text{Relative density} = \frac{\text{Density of substance}}{\text{Density of water at } 4^\circ C} \)
In simple words: Relative density tells you how heavy a substance is compared to water. You find it by dividing the substance's density by the density of water at 4 degrees Celsius. It does not have any units.

🎯 Exam Tip: Remember that relative density is a dimensionless quantity (no units) and the standard reference for it is the density of water at \( 4^\circ C \).

 

Question 6. Name different types of Hydrometers with their applications.
Answer: Here are different types of hydrometers and their applications:

These devices help measure the concentration or purity of various liquids by checking their density.
In simple words: Different hydrometers check different liquids: a lactometer checks milk, a saccharometer checks sugar solutions, and an alcoholometer checks how much alcohol is in drinks.

🎯 Exam Tip: When listing types of hydrometers, ensure you also state what specific substance or property each one is designed to measure.

 

Question 7. What do you understand by the term "Buoyancy".
Answer: Buoyancy refers to the upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. This force arises because the pressure in a liquid increases with depth, causing the pressure at the bottom of the object to be greater than at the top. This pressure difference creates an upward force, which is called the "buoyant force," and the phenomenon itself is known as 'Buoyancy'.
In simple words: Buoyancy is the upward push that a liquid or gas gives to anything placed in it. This push makes things feel lighter or float.

🎯 Exam Tip: Clearly state that buoyancy is an upward force caused by a pressure difference, which explains why objects float or feel lighter in fluids.

 

Question 8. How do submarines sink and float in water?
Answer: Submarines sink and float by controlling their overall density, which they achieve by changing the amount of water in their special compartments called ballast tanks. To sink, they pump water into these tanks, increasing their total weight and density. To float or rise, they pump compressed air into the tanks, forcing the water out and decreasing their overall density, making them lighter than the water around them.
In simple words: Submarines sink by letting water into their tanks and float by pushing water out with air. This changes how heavy they are compared to the water.

🎯 Exam Tip: Explain submarine operation in terms of density: increasing density to sink (by taking in water) and decreasing density to float (by expelling water).

 

Question 9. Differentiate positive & negative buoyant.
Answer: The terms positive and negative buoyancy describe whether an object will float or sink, based on its weight compared to the buoyant force:
Positive Buoyancy:
1. The weight of the object is less than the amount of water displaced.
2. This results in a greater buoyant force.
3. Consequently, the object will float.
Negative Buoyancy:
1. The weight of the object is more than the amount of water displaced.
2. This results in a lesser buoyant force compared to its weight.
3. Consequently, the object will sink.
In simple words: Positive buoyancy means an object's weight is less than the upward push from water, so it floats. Negative buoyancy means its weight is more, so it sinks.

🎯 Exam Tip: Clearly explain the relationship between an object's weight and the buoyant force to distinguish between positive and negative buoyancy; use bullet points for clarity.

 

Question 10. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why?
Answer: The iron bar is heavier in reality, even if both show 100 kg on a weighing machine in air. This is because the bag of cotton has a much larger volume than the iron bar. Because of its large volume, the cotton experiences a greater upward buoyant force from the air around it. This buoyant force makes the cotton appear lighter on the weighing machine. The iron bar, having a smaller volume, experiences less buoyant force from the air, so its measured weight is closer to its true weight. Thus, the iron bar has a greater true mass.
In simple words: The iron bar is truly heavier. Even though both show 100 kg, the cotton bag is bigger and feels more lift from the air, making it seem lighter than it actually is. The iron bar gets less lift from the air.

🎯 Exam Tip: Explain apparent weight differences in terms of air buoyancy; larger volume objects experience greater buoyant force in air, making them appear lighter.

VII. Answer in Detail:

 

Question 1. Derive an expression for Pressure due to the Liquid column.
Answer: To derive the expression for pressure due to a liquid column, consider a tall beaker filled with a liquid of uniform density (\( \rho \)).

Let the area of the cross-section at the bottom of the beaker be \( A \).
Let the height of the liquid column be \( h \).
The density of the liquid is \( \rho \).

The force exerted at the bottom of the liquid column (Thrust, \( F \)) is equal to the weight of the liquid.
We know that weight \( (F) = m \times g \) where \( m \) is the mass of the liquid.
\( \implies F = mg \) ...(1)

The mass of the liquid \( (m) \) can be expressed using density and volume:
Mass \( (m) = \rho \times V \) ...(2)

The volume of the liquid column \( (V) \) is the product of its base area and height:
Volume \( (V) = A \times h \) ...(3)

Substitute equation (3) into equation (2) to find the mass in terms of density, area, and height:
\( \implies m = \rho \times A \times h \) ...(4)

Now, substitute equation (4) into equation (1) to find the thrust (force):
\( \implies F = \rho A h g \) ...(5)

Pressure \( (P) \) is defined as force per unit area:
\( P = \frac{\text{Thrust} (F)}{\text{Area} (A)} \)

Substitute the expression for \( F \) from equation (5) into the pressure formula:
\( P = \frac{\rho A h g}{A} \)

Cancel out \( A \) from the numerator and denominator:
\( \implies P = \rho h g \)

Therefore, \( P = h \rho g \) is the expression for pressure due to the liquid column. This formula shows that pressure increases with depth and density of the liquid.
In simple words: To find liquid pressure, first figure out the weight of the liquid column using its density, height, and area. Then, divide that weight (force) by the area at the bottom. The formula boils down to pressure equals height times density times gravity (\( P = h \rho g \)).

🎯 Exam Tip: Clearly show each step of the derivation, defining all variables and explaining the logical progression from mass to force to pressure.

 

Question 2. Describe the construction and working of the Pycnometer.
Answer: The Pycnometer, also known as a Density Bottle, is a specialized glass flask used to measure the relative density of liquids and powders accurately.

Purpose: To measure the relative density of a substance.

Construction:
1. A pycnometer consists of a small glass bottle, usually with a precise volume (e.g., 25 mL or 50 mL).
2. It has a ground-glass stopper fitted with a fine capillary hole. This hole is crucial for allowing excess liquid to escape.

Working:
1. First, the empty, clean, and dry pycnometer is weighed to find its mass. This is the mass of the bottle (\( m_1 \)).
2. Next, the pycnometer is filled completely with water at a specific reference temperature (usually \( 4^\circ C \)) and the stopper is inserted. The excess water flows out through the capillary hole, ensuring the bottle contains a precise volume of water.
3. The pycnometer filled with water is then weighed to get the mass of the bottle plus water (\( m_2 \)). The mass of the water can be found by \( m_2 - m_1 \).
4. Finally, the pycnometer is emptied, dried, and then filled with the liquid whose relative density is to be found, again at the same reference temperature. The mass of the bottle plus liquid is weighed (\( m_3 \)). The mass of the liquid is \( m_3 - m_1 \).
5. The relative density is then calculated as the ratio of the mass of the liquid to the mass of an equal volume of water: \( \text{Relative Density} = \frac{(m_3 - m_1)}{(m_2 - m_1)} \).
The fine hole in the stopper ensures that the pycnometer always holds the same volume of liquid at a constant temperature, which is essential for accurate density comparisons.
In simple words: A pycnometer is a special bottle used to find how dense a liquid is compared to water. It has a tiny hole in its lid so that when you fill it, any extra liquid spills out, making sure you always have the same amount inside. You weigh the bottle empty, with water, and then with the liquid you want to test, to compare their weights and find the relative density.

🎯 Exam Tip: When describing the pycnometer, highlight the importance of the capillary hole in the stopper for ensuring a precise, constant volume and accurate measurements.

 

Question 3. Explain the Archimedes principle with an example.
Answer:
Principle: Archimedes' principle states that when a body is partially or wholly immersed in a fluid, it experiences an upward buoyant force that is equal to the weight of the fluid displaced by the immersed part of the body. This upward force makes the object feel lighter or even float.

Explanation:
1. When an object is placed in a fluid (like water), the fluid exerts pressure on all its submerged surfaces. Since pressure increases with depth, the upward pressure on the bottom of the object is greater than the downward pressure on its top.
2. This difference in pressure creates a net upward force, which is the buoyant force or upthrust. This upthrust is exactly equal to the weight of the volume of fluid that the object pushes aside (displaces).
3. Due to this upthrust, an object immersed in a fluid appears to lose some of its weight. This apparent loss of weight is exactly equal to the upthrust.
\( \text{Upthrust} = \text{Weight of the fluid displaced} = \text{Apparent loss of weight of the body} \)
\( \text{Apparent weight of an object} = \text{True weight of object in air} - \text{Upthrust} \)


Example: A stone feels lighter when submerged in water than when it is held in the air. This is because the water exerts an upward buoyant force on the stone, reducing its apparent weight.
In simple words: Archimedes' principle says that when you put something in water, the water pushes it up with a force equal to the weight of the water that moves away. This is why a stone feels lighter in water.

🎯 Exam Tip: When explaining Archimedes' principle, always include the core idea (upward force equals weight of displaced fluid) and provide a simple, relatable example like a stone in water.

 

Question 4. Describe the purpose, principle and working of Lactometer.
Answer: A lactometer is a device specifically designed to test the purity of milk.

Purpose: To check the purity and density of milk.

Principle: It works on the principle of buoyancy, where an object floats more in a denser liquid and less in a lighter liquid. Specifically, it uses the gravity (density) of milk.

Construction:
1. A lactometer consists of a long, graduated glass test tube.
2. At its lower end, it has a cylindrical bulb that is partially filled with mercury. This mercury helps the lactometer float upright and stable in milk.
3. The stem of the lactometer has a scale, usually graduated from 15 at the top to 45 at the bottom.
4. It also contains an air chamber, which helps the instrument float at the correct level.
5. Some lactometers may include a thermometer to measure the temperature, as milk density varies with temperature.

Working:
1. The milk sample is poured into a jar, and the lactometer is gently lowered into it.
2. The lactometer floats in the milk, sinking to a certain level depending on the milk's density.
3. The correct reading is obtained from the scale at the surface level of the milk. This reading indicates the density of the milk, with a specific temperature (like \( 60^\circ C \)) often being the standard for accurate measurement.
4. Pure milk has a typical density, and any adulteration (like adding water) will change the density, causing the lactometer to float differently and give a different reading. For example, the average reading for normal milk is often around 32.
In simple words: A lactometer checks how pure milk is by seeing how much it floats. It's a glass tube with a heavy bottom and a scale. If the milk is pure and dense, the lactometer floats at one level; if water is added, the milk becomes less dense, and the lactometer sinks lower.

🎯 Exam Tip: When describing the lactometer, ensure you cover its construction elements, its operating principle (buoyancy/density), and how its reading indicates milk purity.

VIII. Numerical Problems:

 

Question 1. A vessel with water is placed on a weighing pan and it reads 600 g. Now a ball of mass 40 g and density is 0.80g / cm³ is sunk into the water with a pin of negligible volume as shown in the figure. The weighing pan will show the reading of __________?
Answer:
Given:
Initial weight of vessel with water = 600 g
Mass of ball = 40 g
Density of ball = \( 0.80 \, g/cm^3 \)

First, calculate the volume of the ball:
\( \text{Volume of ball} = \frac{\text{Mass of ball}}{\text{Density of ball}} = \frac{40 \, g}{0.80 \, g/cm^3} = 50 \, cm^3 \)

When the ball is sunk into the water, it displaces a volume of water equal to its own volume. Assuming the density of water is \( 1 \, g/cm^3 \).
Weight of water displaced = Volume of ball \( \times \) Density of water
Weight of water displaced = \( 50 \, cm^3 \times 1 \, g/cm^3 = 50 \, g \)

According to Archimedes' principle, the buoyant force (upthrust) acting on the ball is equal to the weight of the water displaced. This buoyant force is also the additional downward force exerted on the weighing pan.

Therefore, the final reading on the weighing pan will be the initial reading plus the weight of the water displaced by the ball.
Final reading = Initial weight of vessel with water + Weight of water displaced
Final reading = \( 600 \, g + 50 \, g = 650 \, g \)

So, the weighing pan will show \( 650 \, g \). This phenomenon is also known as apparent weight.
In simple words: First, find out how much space the ball takes up (its volume). Since the ball is put into the water, it pushes away that same amount of water. The weight of this pushed-away water adds to the weighing scale's reading. So, 600g plus the 50g of displaced water makes the scale read 650g.

🎯 Exam Tip: When an object is fully submerged in a fluid, the apparent weight of the system (container + fluid + object) increases by the weight of the fluid displaced by the object, not by the object's mass.

 

Question 2. The reading of a spring balance when a block is suspended from it in air is 60 newton. This reading is changed to 40 newtons when the block is submerged in water. Calculate the specific gravity of block.
Answer:
Given:
Weight of block in air = \( 60 \, N \)
Weight of block in water = \( 40 \, N \)

Loss of weight of block in water = Weight in air - Weight in water
Loss of weight = \( 60 \, N - 40 \, N = 20 \, N \)

According to Archimedes' principle, the loss of weight of an object when submerged in a fluid is equal to the weight of the fluid displaced.
So, Weight of water displaced = \( 20 \, N \)

Specific gravity (or Relative density) of the block is given by the formula:
\( \text{Specific gravity} = \frac{\text{Weight of block in air}}{\text{Loss of weight of block in water}} \)
\( \implies \text{Specific gravity} = \frac{60 \, N}{20 \, N} \)
\( \implies \text{Specific gravity} = 3 \)

Thus, the specific gravity of the block is 3. This means the block is 3 times denser than water.
In simple words: The block weighs 60 N in the air and 40 N in water, so it loses 20 N of weight. Specific gravity is found by dividing its weight in air by how much weight it lost in water. So, 60 divided by 20 gives 3.

🎯 Exam Tip: Remember that specific gravity is a ratio, so it's unitless. It directly relates an object's density to the density of a reference substance (usually water).

 

Question 3. The mass of a body is 4 kg and its volume is 500 cm³. Find its relative density.
Answer:
Given:
Mass of the body \( (m) = 4 \, kg = 4000 \, g \)
Volume of the body \( (V) = 500 \, cm^3 \)

First, calculate the density of the body:
Density of the body \( (\rho_{\text{body}}) = \frac{\text{Mass}}{\text{Volume}} = \frac{4000 \, g}{500 \, cm^3} \)
\( \implies \rho_{\text{body}} = 8 \, g/cm^3 \)

Next, recall the density of water at \( 4^\circ C \), which is the standard for relative density:
Density of water \( (\rho_{\text{water}}) = 1 \, g/cm^3 \)

Now, calculate the relative density of the body:
Relative density \( = \frac{\text{Density of substance}}{\text{Density of water}} = \frac{8 \, g/cm^3}{1 \, g/cm^3} \)
\( \implies \text{Relative density} = 8 \)

The relative density of the body is 8. This indicates the body is 8 times denser than water.
In simple words: To find the relative density, first calculate the body's density by dividing its mass by its volume. Since the density of water is 1 g/cm³, dividing the body's density (8 g/cm³) by water's density gives a relative density of 8.

🎯 Exam Tip: Ensure that mass and volume units are consistent (e.g., all in grams and cm³ or all in kg and m³) before calculating density and relative density.

 

Question 4. Calculate the pressure produced by a force of 800 N acting on an area of 2.0 m².
Answer:
Given:
Force \( (F) = 800 \, N \)
Area \( (A) = 2.0 \, m^2 \)

The formula for pressure is:
Pressure \( (P) = \frac{\text{Force} (F)}{\text{Area} (A)} \)
\( \implies P = \frac{800 \, N}{2.0 \, m^2} \)
\( \implies P = 400 \, N/m^2 \)

Since \( 1 \, N/m^2 \) is equal to \( 1 \) Pascal \( (Pa) \):
\( \implies P = 400 \, Pa \)

The pressure produced is \( 400 \, Pa \). This means 400 Newtons of force are distributed over each square meter.
In simple words: To find the pressure, divide the force (800 N) by the area (2.0 m²). This gives 400 Pascals, meaning a force of 400 Newtons is pressing on every square meter.

🎯 Exam Tip: Always state the formula for pressure and ensure units are consistent (Newtons for force, square meters for area) to get the answer in Pascals.

 

Question 5. A swimming pool of width 9.0 m and length 24.0 m is filled with water of depth 3.0 m. Calculate the pressure on the bottom of the pool due to the water.
Answer:
Given:
Depth of the water \( (h) = 3.0 \, m \)
Density of water \( (\rho) = 1000 \, kg/m^3 \) (Standard value)
Acceleration due to gravity \( (g) = 9.8 \, m/s^2 \) (Standard value)

The formula for pressure exerted by a liquid column is:
Pressure \( (P) = h \rho g \)

Substitute the given values into the formula:
\( P = (3.0 \, m) \times (1000 \, kg/m^3) \times (9.8 \, m/s^2) \)
\( P = 29400 \, kg \cdot m^{-1} \cdot s^{-2} \)
\( \implies P = 29400 \, Pa \)

The pressure on the bottom of the pool due to the water is \( 29400 \, Pa \). The width and length of the pool are not needed for calculating pressure at the bottom due to the liquid column, as pressure depends only on depth, density, and gravity.
In simple words: To find the pressure at the bottom of the pool, multiply the depth of the water (3.0 m) by the density of water (1000 kg/m³) and the force of gravity (9.8 m/s²). This calculation gives a pressure of 29400 Pascals.

🎯 Exam Tip: For pressure due to a liquid column, remember that only the depth, liquid density, and gravity matter; the surface area or shape of the container does not affect the pressure at a given depth.

 

Question 6. A body of volume 100 cc is immersed completely in the weight of water and the jar before immersion of the weight of water and jar after immersion.
Answer:
Volume of body completely immersed in water, \( V = 100 \, \text{cc} \)
Weight of water and jar before immersion \( = 700 \, \text{g} \)
The volume of water displaced by the immersed jar \( = 100 \, \text{cc} \). This is because the volume of displaced liquid equals the volume of the immersed object.
Density of water \( = 1 \, \text{g/cm}^3 \)
Mass of water displaced \( = \text{Volume} \times \text{density} = 100 \, \text{cc} \times 1 \, \text{g/cm}^3 = 100 \, \text{g} \)
This mass of displaced water represents the apparent weight loss of the body.
Apparent weight loss of body \( = 100 \, \text{g} \)
Weight of jar and water after immersion \( = \text{Weight of water and jar before immersion} - \text{Apparent weight loss} \)
\( = 700 \, \text{g} - 100 \, \text{g} \)
\( = 600 \, \text{g} \)
In simple words: When an object is put into water, it pushes some water aside. The weight of this pushed-aside water is how much the object seems to lose weight. So, if the jar and water together weighed 700g, and 100g of water was pushed aside, the new combined weight will be 600g.

🎯 Exam Tip: Remember Archimedes' principle: the buoyant force (or apparent weight loss) is equal to the weight of the fluid displaced by the object. This is key to solving such problems.

VII. Assertion and Reason :

 

Question. Assertion (A) : The buoyant force on submerged rigid object can be considered to be acting at the centre of mass of object. Reason (R) : In rigid body, force distributed uniformly through its volume can be considered to be acting at the centre of mass of the body.
Answer: (c) Assertion is true but reason is false
The buoyant force acts at the center of buoyancy, which is the center of gravity of the displaced fluid, not necessarily the center of mass of the object itself. The reason given incorrectly links the buoyant force's action point to the object's center of mass, rather than the displaced fluid's center of gravity.
In simple words: The push from water on a submerged object happens at a special point called the center of buoyancy, which is related to the water pushed away. It is not always the same as the object's own center point. So, the first statement is true, but the reason given for it is wrong.

🎯 Exam Tip: Distinguish carefully between the center of mass of an object and the center of buoyancy (center of gravity of the displaced fluid); they are distinct points.

 

Question. Assertion (A): The weight of the truck exerts less pressure on road. Reason (R): The truck has six to eight wheels. As area increases pressure decreases.
Answer: (a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion
A truck uses many wheels to spread its heavy weight over a larger area. This increased contact area reduces the pressure exerted on the road, preventing damage. This design choice directly demonstrates the inverse relationship between pressure and area when force is constant.
In simple words: A heavy truck has many wheels. This makes the total contact area with the road very big. Because pressure becomes less when the area is bigger (for the same weight), the truck puts less pressure on the road, which helps prevent it from damaging the road. So both statements are true, and the reason explains why the assertion is correct.

🎯 Exam Tip: Remember the formula \( \text{Pressure} = \frac{\text{Force}}{\text{Area}} \). A larger area for the same force will always result in less pressure.

 

Question. Assertion (A): Air gets thinner with increasing altitude. Reason (R): The atmospheric pressure increases as we go up in mountains.
Answer: (c) Assertion is true but the reason is false
As you go higher up in mountains, the air indeed gets thinner because there is less air above you. This means the atmospheric pressure actually decreases, not increases, with altitude. So, the assertion is correct, but the reason provided is incorrect.
In simple words: The air at higher places, like on mountains, has fewer air particles, making it "thinner." This means the air pressure goes down as you go up, not up. So the first part is true, but the reason given is false.

🎯 Exam Tip: Atmospheric pressure always decreases with increasing altitude because the column of air above is shorter and less dense.

 

Question. Assertion (A) : Lactometer is used to check the purity of milk. Reason (R) : Lactometer measures the cream content of milk.
Answer: (b) Both assertion and reason are true but the reason is not the correct explanation of the assertion
A lactometer is used to check milk purity by measuring its density. While it does measure the density related to cream content, this is not the fundamental principle that explains how it checks purity. The purity check is based on the overall density change, not solely cream content. Both statements are factually correct, but the reason doesn't fully explain the assertion.
In simple words: A lactometer helps check if milk is pure. It works by measuring how dense the milk is, which is often affected by how much cream is in it. Both statements are true, but the second one doesn't fully explain why the lactometer checks purity; it's more about density as a whole.

🎯 Exam Tip: While cream content influences milk density, the lactometer primarily relies on the overall density principle to assess purity, as adulteration also changes density.

 

Question. Assertion (A): The force acting on the surface of a liquid at rest, under gravity, in a container is always horizontal. Reason (R) : The forces acting on a fluid at rest have to be normal to the surface.
Answer: (d) Assertion is false but reason is true
For a liquid at rest, the pressure exerted by the liquid at any point acts perpendicular (normal) to the surface. This means the force is usually vertical (downwards or upwards) or sideways, depending on the surface. The reason correctly states that forces on a static fluid must be normal to the surface, which contradicts the assertion that the force is always horizontal.
In simple words: When water is still in a container, the push it gives on any surface is always straight out from that surface (at a 90-degree angle). So, the force is not always sideways (horizontal). The second statement, saying the push is always at a 90-degree angle to the surface, is correct.

🎯 Exam Tip: In a static fluid, pressure acts uniformly in all directions and always perpendicular to any surface it contacts. This is a fundamental concept in fluid statics.

 

Question. Assertion (A): A sleeping mattress is so designed that when you lie on it, a large area of your body comes in its contact. Reason (R) : This reduces the pressure on the body and sleeping becomes comfortable.
Answer: (a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion
A well-designed mattress aims to maximize the contact area with your body. By spreading your weight over a larger surface, the pressure on any single point of your body is reduced. This reduced pressure is what makes lying down more comfortable, as it prevents specific points from bearing too much force. Therefore, both the assertion and the reason are true, and the reason explains the assertion.
In simple words: A good mattress is made to touch a big part of your body. When your weight is spread out over a larger area, the push on any one spot of your body becomes smaller. This smaller push makes sleeping more comfortable. So, both parts of the statement are true, and the second part tells us why the first part is true.

🎯 Exam Tip: This principle applies to many everyday items, from wide shoe straps to snowshoes, where increasing contact area reduces pressure for comfort or stability.

 

Question. Assertion (A): Wide wooden sleepers are kept below railway lines to reduce pressure on the railway tracks and prevent them from sinking in the ground. Reason (R): Pressure is directly proportional to the area in which it is acting.
Answer: (c) Assertion is true but the reason is false
Wide wooden sleepers are indeed used under railway tracks to distribute the heavy weight of the train over a larger area. This reduces the pressure exerted on the ground, preventing the tracks from sinking. However, pressure is inversely proportional to the area (when force is constant), meaning a larger area leads to less pressure, not directly proportional. So the assertion is true, but the reason is false.
In simple words: The big wooden pieces under train tracks spread out the train's heavy weight, stopping the tracks from sinking. This is because pressure gets smaller when the area it pushes on gets bigger. The statement correctly says what the sleepers do, but incorrectly says that pressure and area are directly linked.

🎯 Exam Tip: Always remember the inverse relationship between pressure and area when the force is constant. Increasing the area decreases the pressure.

X. Define the Following

 

Question 1. Define thrust: The force which produces compression is called thrust. Its S.I units is newton
Answer: Thrust is a force that acts perpendicular to a surface, often causing compression or push. The International System of Units (S.I.) unit for thrust, like any force, is the newton (N). For example, a rocket engine produces thrust to move upward.
In simple words: Thrust is a pushing force that acts straight on a surface. Its unit of measurement is the newton.

🎯 Exam Tip: Remember that thrust is a force, so its unit is always the Newton, and it acts perpendicularly.

 

Question 2. Define pressure: Thrust acting normally to a unit area of a surface is called pressure. Its S.I. Unit is the pascal.
Answer: Pressure is defined as the force, or thrust, acting at a right angle (normally) to a specific unit area of a surface. It measures how concentrated a force is. The S.I. unit for pressure is the pascal (Pa), which is equivalent to one newton per square meter \( (N/m^2) \). For instance, a sharp knife exerts more pressure than a blunt one with the same force.
In simple words: Pressure is how much force pushes on a small area. Its unit is the pascal.

🎯 Exam Tip: Understand that pressure is force per unit area, and it is a scalar quantity, acting equally in all directions in a static fluid.

 

Question 3. Define atmospheric pressure: The pressure exerted by the atmospheric gases on its surroundings and on the surface of the earth is called atmospheric pressure. 1 atm is the pressure exerted by a vertical column of mercury of 76 cm height.
Answer: Atmospheric pressure is the force exerted by the weight of the air (atmospheric gases) above a surface, acting on everything around it and on the Earth's surface. One standard atmosphere (1 atm) is a common unit for this pressure, defined as the pressure exerted by a column of mercury that is 76 cm high. This pressure keeps our planet's atmosphere in place.
In simple words: Atmospheric pressure is the push that the air around us puts on everything. It is like the weight of all the air above us.

🎯 Exam Tip: Remember that atmospheric pressure varies with altitude, decreasing as you go higher, and is a significant factor in weather and daily life.

 

Question 4. Define buoyant force: The upward force experienced by a body when partly or fully immersed in a fluid is called upthrust or buoyant force.
Answer: Buoyant force, also known as upthrust, is the upward push that a fluid (like water or air) exerts on an object that is partly or fully submerged in it. This force acts against gravity, making objects feel lighter in water and allowing some to float. It is what makes a boat float on water.
In simple words: Buoyant force is the upward push from a liquid or gas that makes things float or feel lighter when put inside.

🎯 Exam Tip: The buoyant force is always equal to the weight of the fluid displaced by the object, according to Archimedes' principle.

 

Question 5. Pascal's law: Pascal's law states that an increase in pressure at any point inside a liquid at rest is transmitted equally and without any change, in all directions to every other point in the liquid.
Answer: Pascal's law states that if pressure is increased at any point within a liquid that is not moving, this increase in pressure will spread out evenly and undiminished in all directions to every other part of the liquid. This principle is fundamental to how hydraulic systems work, such as car brakes or hydraulic lifts.
In simple words: Pascal's law says that if you push on a still liquid at one spot, that push spreads equally everywhere else in the liquid.

🎯 Exam Tip: Pascal's law applies specifically to incompressible fluids (liquids) at rest and is the basis for many hydraulic applications.

 

Question 6. Archimedes principle: Archimedes' principle states that when a body is partially or wholly immersed in a fluid, it experiences an up thrust or apparent loss of weight, which is equal to the weight of the fluid displaced by the immersed part of the body.
Answer: Archimedes' principle explains that when an object is placed partially or completely into a fluid, it feels an upward push (buoyant force or upthrust). This upward push, or the apparent reduction in the object's weight, is always equal to the weight of the fluid that the submerged part of the object pushes aside. This principle helps us understand why objects float or sink.
In simple words: Archimedes' principle says that when an object is in water, it gets an upward push that is equal to the weight of the water it pushes out of the way.

🎯 Exam Tip: This principle is crucial for understanding buoyancy, flotation, and the design of ships and submarines. Always relate buoyant force to the displaced fluid's weight.

 

Question 7. Density: Density is known as mass per unit volume of a body. Its S.I. unit is kg nr5.
Answer: Density is a measure of how much mass is contained in a specific volume of a substance. It tells us how tightly packed the matter is within an object. The S.I. unit for density is kilograms per cubic meter \( (kg/m^3) \). For example, a rock is denser than a sponge of the same size.
In simple words: Density tells us how heavy something is for its size. Its unit is kilograms per cubic meter.

🎯 Exam Tip: Density is a fundamental property of matter, calculated as mass divided by volume, and helps predict whether an object will float or sink.

 

Question 8. Relative density: Relative density is the ratio between the density of a substance and the density of water. The relative density of a body is a pure number and has no unit.
Answer: Relative density, also called specific gravity, is a comparison of a substance's density to the density of water. It is found by dividing the density of the substance by the density of water (usually at 4°C). Because it is a ratio of two densities, relative density is a pure number and does not have any units. This value helps us understand if an object will float or sink in water.
In simple words: Relative density compares how dense something is to how dense water is. It's just a number with no units.

🎯 Exam Tip: Since relative density is a ratio, ensure consistent units for both densities when calculating; the units will then cancel out.

 

Question 9. Hydrometer: A hydrometer is a device used to measure the relative density of liquids based on Archimedes' principle.
Answer: A hydrometer is a tool specifically designed to measure the relative density (or specific gravity) of liquids. It works by floating in the liquid, with the depth to which it sinks indicating the liquid's density. This measurement is based on Archimedes' principle, as the hydrometer displaces different amounts of liquid depending on the liquid's density. It is commonly used in various industries, from brewing to battery testing.
In simple words: A hydrometer is a device that measures how dense a liquid is compared to water. It uses a rule called Archimedes' principle to do this.

🎯 Exam Tip: Recognize that hydrometers are specialized floating devices; the lower they float, the denser the liquid, and vice versa.

 

Question 10. Lactometer: Lactometer is a device used to check the purity of milk by measuring its density using Archimedes' principle.
Answer: A lactometer is a specific type of hydrometer used to assess the purity and fat content of milk. It functions by measuring the milk's density, which changes based on its composition (like the amount of water added or cream content). This measurement relies on Archimedes' principle, where the device floats to a certain level depending on the milk's density. A lower reading can indicate adulteration or watering down of the milk.
In simple words: A lactometer is a tool that checks how pure milk is by measuring its density. It uses Archimedes' principle to see if the milk has been mixed with water.

🎯 Exam Tip: Understand that a lactometer is a practical application of the hydrometer and Archimedes' principle, specifically tailored for milk analysis.