Chapter 8 Pressure
Class 10 Science Notes
Summary: Pressure Theory
1. Introduction to Pressure
Pressure: The perpendicular force (or thrust) applied over a specific area. Pressure is exerted by solids, liquids, and gases (collectively called fluids).
Pascal’s Law: Explains how pressure is transmitted equally in enclosed fluids, forming the basis for hydraulic machines like the hydraulic press.
Upthrust: The upward force a fluid exerts on an object placed in it.
2. Important Terms
Thrust
Definition: A force acting perpendicular (at 90°) to a surface.
SI Unit: Newton (N)
Pressure
Definition: The thrust acting on a unit area of a surface.
SI Unit: Pascal (Pa) = $N/m^2$
Fluids
Definition: Substances that can flow, such as liquids and gases.
Hydraulic Press
Definition: A machine that uses Pascal’s principle to multiply force, typically using a small piston and a large piston.
Upthrust
Definition: The upward force exerted by a fluid on an object that is partially or fully immersed in it.
3. Important Formulas
Pressure (P)
$$P = \frac{Thrust (T)}{Area (A)} \quad \text{or} \quad P = \frac{Force (F)}{Area (A)}$$
Pressure in Liquids (P)
$$P = h \times d \times g$$
Where:
$h$ = depth of the liquid
$d$ = density of the liquid
$g$ = acceleration due to gravity
Pascal’s Law (for hydraulic machines)
Pressure on the small piston equals pressure on the large piston:
$$P_1 = P_2$$
$$\frac{F_1}{A_1} = \frac{F_2}{A_2}$$
Where:
$F_1$ = Force on the small piston
$A_1$ = Area of the small piston
$F_2$ = Force on the large piston
$A_2$ = Area of the large piston
1. Choose the correct option for the following questions:
(a) On which law/principle is a lactometer based?
(b) What is the condition for flying a hydrogen balloon upwards in the air?
(c) In a hydraulic machine, if the cross-sectional area of the larger piston is twice the area of the smaller piston, what is the correct group of the weights on the smaller piston $(W_{1})$ and the larger piston $(W_{2})$ to keep the machine balanced?
(d) When an object is suspended using spring balance in air, water and glycerin, the weight is founded to be $W_{1}$, $W_{2}$, and $W_{3}$ respectively. What will be the weight of the object in those mediums in increasing order?
(e) When a ship enters a river from a sea, it was found to sink more than before. What is the reason for this?
(f) Which one of the following statements is correct for a hand pushing a ball into the water as shown in the given figure?
(g) If the same magnitude of upthrust acts on the three cubical balls made up of different materials on keeping them in water, which one of the following quantities is equal for them?
(h) What is the upthrust when the cork shown in the figure is placed in the liquid?
2. Differentiate between:
(a) pressure and upthrust
| Feature | Pressure | Upthrust |
|---|---|---|
| Definition | The force acting perpendicularly on a unit area of a surface. | The upward force exerted by a fluid on an object partially or wholly submerged in it. |
| Cause | Caused by the continuous collision of fluid particles or the application of a force (like weight). | Caused by the pressure difference between the top and bottom surfaces of a submerged object (pressure is greater at the bottom). |
| Formula | $P = F / A$ (Force per unit Area) | $U = V \cdot \rho \cdot g$ (Weight of displaced fluid) |
| Direction | Acts in all directions in a fluid, and always perpendicular (normal) to any surface. | Acts in a single direction: vertically upwards. |
| SI Unit | Pascal (Pa) or Newtons per square meter ($N/m^2$). | Newton (N), as it is a force. |
(b) the reason for a steel pin sinking in water and a steel plate floating on water
| Feature | Sinking Steel Pin | Floating Steel Plate |
|---|---|---|
| Object | A small, solid steel pin. | A large, broad, and often concave (boat-shaped) steel plate. |
| Displaced Fluid | Displaces a small volume of water, equal to its own small volume. | Displaces a very large volume of water due to its shape. |
| Upthrust vs. Weight | The upthrust (weight of the small volume of water) is less than the pin’s weight. | The upthrust (weight of the large volume of water) is equal to the plate’s weight. |
| Density Principle | The density of steel is greater than the density of water ($d_{\text{steel}} > d_{\text{water}}$). | The average density of the plate (including the air it contains in its shape) is less than the density of water. |
| Governing Law | Fails the law of floatation; its weight is greater than the maximum possible upthrust. | Obeys the law of floatation; it sinks just enough to displace a weight of water equal to its own weight. |
(c) the process of a hot air balloon sinking and rising in the air
| Feature | Rising Hot Air Balloon | Sinking Hot Air Balloon |
|---|---|---|
| Internal Air | The air inside the balloon is heated by the burner. | The air inside the balloon is allowed to cool (burner is off or low). |
| Density | Hot air is less dense than the cooler, surrounding (ambient) air. | The cooling air becomes denser, closer to the density of the surrounding air. |
| Upthrust vs. Weight | The weight of the displaced (cool) air is greater than the total weight of the balloon (balloon + basket + hot air). | The weight of the displaced (cool) air becomes less than the total weight of the balloon. |
| Net Force | The net force is upwards (Upthrust > Weight). | The net force is downwards (Upthrust < Weight). |
| Result | The balloon accelerates and moves upwards. | The balloon accelerates and moves downwards. |
3. Give a reason for
(a) A special type of oil is used in hydraulic brakes.
Reason: A special oil is used because it has several crucial properties: Incompressible (does not compress under pressure, allowing it to transmit force instantly and efficiently), High Boiling Point (brakes generate heat from friction, and the oil must not boil as steam bubbles are compressible and would lead to brake failure), Low Freezing Point (must remain fluid and not freeze in cold weather), and Non-Corrosive (does not rust or damage the metal parts of the brake system).
(b) A wooden cork dipped below the surface of the water by applying force with a finger comes to the surface when the finger is removed.
Reason: This is due to density and buoyant force. The density of wood (cork) is less than the density of water. When the cork is forcibly submerged, it displaces a volume of water that weighs more than the cork itself. This creates a large upward buoyant force (upthrust) that is greater than the cork’s weight. When the finger (the external downward force) is removed, the net upward force (Upthrust – Weight) pushes the cork rapidly to the surface, where it will float.
(c) We feel lighter while floating on water.
Reason: We feel lighter because the water exerts an upward force on our body, known as upthrust or buoyant force. This upward force counteracts a significant portion of our weight (the downward pull of gravity). Our “apparent weight” in the water is our real weight minus this upthrust, making us feel much lighter.
(d) On lifting a stone submerged in water, it feels heavier when it comes out of the water.
Reason: While the stone is submerged, the water provides a large upthrust, supporting some of the stone’s weight. You only have to lift the stone’s apparent weight (Real Weight – Upthrust). When you lift the stone out of the water, this buoyant force from the water disappears. You must then support the stone’s full, real weight, which feels much heavier.
(e) It is easier to float in the Dead Sea than in a swimming pool, (the density of water in the Dead Sea is $1240kg/m^{3})$.
Reason: The Dead Sea has extremely high salt content, making its water significantly denser (1240 kg/m³) than freshwater or pool water (approx. 1000 kg/m³). According to Archimedes’ principle, upthrust is proportional to the density of the fluid. The very dense water of the Dead Sea produces a much larger upthrust for a given submerged volume, making it very easy for this upthrust to equal a person’s weight, allowing them to float with very little effort.
(f) If more passengers climb a boat than its maximum capacity, the boat is likely to sink.
Reason: A boat floats by displacing a weight of water equal to its own weight plus the weight of its cargo (passengers). As more passengers climb aboard, the boat sinks lower into the water to displace more water and increase the upthrust to match the new, higher total weight. “Maximum capacity” is the point where the boat is close to the water’s edge. If the total weight exceeds the maximum possible upthrust (which is the weight of water displaced when the boat is fully submerged, i.e., full of water), the boat will sink.
4. Answers the following questions:
(a) State Pascal’s law.
Answer: Pascal’s law states that the pressure applied to an enclosed, incompressible fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.
(b) Write any two applications of Pascal’s law in daily life.
Answer:
1. Hydraulic brakes in vehicles (cars, trucks)
2. Hydraulic lifts (car jacks, garage lifts)
(c) What is a hydraulic machine?
Answer: A hydraulic machine is a device that uses an enclosed fluid (usually a liquid like oil) to transmit and multiply force, based on Pascal’s law. It typically consists of two pistons of different cross-sectional areas connected by the fluid.
(d) State Archimedes’ principle.
Answer: Archimedes’ principle states that when a body is immersed wholly or partially in a fluid, it experiences an upthrust (or buoyant force) equal to the weight of the fluid displaced by it.
(e) What is upthrust?
Answer: Upthrust, or buoyant force, is the upward force exerted by a fluid (a liquid or a gas) on any object that is partially or wholly submerged in it.
(f) Mention the forces acting on a floating object and their directions.
Answer:
1. Weight (Gravitational Force): Acts vertically downwards, through the object’s center of gravity.
2. Upthrust (Buoyant Force): Acts vertically upwards, through the center of buoyancy (the center of gravity of the displaced fluid).
(g) State the law of floatation.
Answer: The law of floatation states that a floating object displaces a weight of fluid equal to its own total weight.
(h) Write any two applications of Archimedes’ Principle.
Answer:
1. The design of ships and submarines (controlling buoyancy to float or submerge)
2. The use of hydrometers and lactometers to measure the density and purity of liquids
(i) Explain, with a figure, the cause of the production of upthrust.
Answer: Upthrust is caused by the pressure difference in a fluid. Pressure in a fluid increases with depth.
Imagine a cylindrical object submerged in water:
– The top surface of the object is at a certain depth and experiences a downward force from the pressure of the water above it.
– The bottom surface of the object is at a greater depth, so it experiences a greater pressure from the water below it. This creates a larger upward force on the bottom surface.
– The forces on the sides cancel each other out. The upthrust is the net upward force, which is the difference between the strong upward force on the bottom and the weaker downward force on the top. This net upward force is equal to the weight of the fluid the object displaces.
(j) When a stone is dropped into the water, it sinks. Does this happen because of the absence of upthrust?
Answer: No, this is false. The stone sinks despite the presence of upthrust.
The stone sinks because its density is greater than the density of water. This means its weight is greater than the maximum possible upthrust it can receive (which is the weight of a volume of water equal to the stone’s own volume). Since the downward force (Weight) is stronger than the upward force (Upthrust), the net force is downwards, and the stone sinks.
(k) A person is trying to lift a stone of 500N keeping it on the smaller piston of a hydraulic machine by applying a force on the larger piston. What suggestion would you give him/her so that he/she can lift the load easily? Explain…
Answer: Suggestion: The person is using the machine incorrectly. To lift a heavy load, the load (the 500N stone) must be placed on the larger piston, and the effort (force) must be applied to the smaller piston.
Explanation: A hydraulic machine is a force multiplier. Based on Pascal’s law ($F_1/A_1 = F_2/A_2$), the force on the larger piston ($F_2$) is related to the force on the smaller piston ($F_1$) by the equation: $F_2 = F_1 \cdot (A_2/A_1)$.
Since the area of the larger piston ($A_2$) is greater than the area of the smaller piston ($A_1$), the ratio $(A_2/A_1)$ is a large number. This means the output force $F_2$ (on the large piston) is many times greater than the input force $F_1$ (on the small piston). By applying a small force to the small piston, the person can generate a large force on the large piston to lift the 500N stone.
(l) Substances with a density greater than the density of the liquid sink in it. Is this statement always true? Justify with an example.
Answer: No, this statement is not always true. It is true for solid, non-hollow objects.
Justification: The statement is false for objects that are shaped to be hollow, like a boat. A ship is made of steel, which has a density much greater than water. However, the ship is shaped so that it has a hollow hull filled with air. This shape allows the ship to displace a very large volume of water. This makes the ship’s average density (the total mass of the ship divided by the total volume it occupies) less than the density of water, allowing it to float according to the law of floatation.
(m) Two balloons, one filled with air and the other with hydrogen, look identical. What difference can be noticed when they are released into the air? Explain with reasons.
Answer: Difference: When released, the hydrogen-filled balloon will rise quickly, while the air-filled balloon will fall to the ground.
Reason:
Hydrogen Balloon: Hydrogen gas is much less dense than the surrounding air. Because of this, the total weight of the hydrogen balloon (balloon material + hydrogen) is less than the weight of the air it displaces (the upthrust). Since Upthrust > Weight, the net force is upwards, and it rises.
Air-filled Balloon: This balloon is filled with air, which has (almost) the same density as the air outside. The total weight of this balloon (balloon material + inside air) is greater than the upthrust (weight of outside air displaced). Since Weight > Upthrust, the net force is downwards, and it falls.
(n) An object is suspending /floating just below the surface of the water. If the amount of salt dissolving in the water goes on increasing, what change will occur in the position of the balloon? Explain with reasons.
Answer: Change: The object will rise and float higher, possibly emerging partially above the surface.
Reason: The object is initially floating, which means its weight is exactly balanced by the upthrust ($Weight = Upthrust$). When salt is dissolved in the water, the density of the water increases. Upthrust is calculated as $U = V_{\text{submerged}} \cdot \rho_{\text{fluid}} \cdot g$. Since the fluid density ($\rho_{\text{fluid}}$) increases, the upthrust on the object also increases. The upward force (Upthrust) now becomes greater than the object’s constant downward force (Weight), causing the object to rise. It will float higher, displacing a smaller volume of the new, denser water until the upthrust once again equals its weight.
(o) When a person puts a lactometer into the milk, the whole part of the narrow stem sinks into the milk. What conclusions can be drawn from this event?
Answer: Conclusion: The milk is of very low density and is likely adulterated with a large amount of water.
Reason: A lactometer is calibrated to float at a specific level (on the narrow stem) in pure milk. If the entire stem sinks, it means the lactometer is not buoyant enough. This happens because the liquid (the “milk”) is less dense than pure milk. Water is less dense than milk, so adding water lowers the milk’s overall density, causing the lactometer to sink.
5. Solve the following mathematical problems:
(a) In the figure, a sample of a hydraulic machine constructed by using syringes is shown. How much force is to be applied through syringe B to balance the force on piston A?
Given:
Force on piston A, $F_A = 20~N$
Area of piston A, $A_A = 0.5~cm^2$
Area of piston B, $A_B = 5~cm^2$
Formula (Pascal’s Law): $F_A / A_A = F_B / A_B$
Solution: We need to find the force on syringe B ($F_B$).
$F_B = F_A \cdot (A_B / A_A)$
$F_B = 20~N \cdot (5~cm^2 / 0.5~cm^2)$
$F_B = 20~N \cdot 10$
$F_B = 200~N$
Answer: A force of 200 N must be applied to piston B to balance the 20 N force on piston A.
(b) Pressure of 30000 Pa is generated in the liquid of a hydraulic lift. If the cross-sectional area of the piston used to lift a weight is $0.1~m^{2}$ how much load can it lift?
Given:
Pressure, $P = 30,000~Pa$ (or $30,000~N/m^2$)
Area, $A = 0.1~m^2$
Formula: Pressure = Force / Area ($P = F / A$)
Solution: We need to find the load (Force, $F$).
$F = P \cdot A$
$F = 30,000~N/m^2 \cdot 0.1~m^2$
$F = 3,000~N$
Answer: The lift can lift a load of 3,000 N.
(c) Calculate the upthrust acting on the object in situations b and c shown in the figure. Mention the cause for the occurrence of different upthrusts under these two situations.
Given:
Weight in air (from figure a) = 20 N
Apparent weight in situation b (partially submerged) = 14 N
Apparent weight in situation c (fully submerged) = 10 N
Formula: Upthrust = Weight in air – Apparent weight in fluid
Calculation for situation b:
Upthrust (b) = 20 N – 14 N
Upthrust (b) = 6 N
Calculation for situation c:
Upthrust (c) = 20 N – 10 N
Upthrust (c) = 10 N
Cause for difference:
Upthrust is equal to the weight of the fluid displaced (Archimedes’ Principle), which is proportional to the submerged volume of the object.
In situation (b), the object is only partially submerged, so it displaces a smaller volume of water. This results in a smaller upthrust (6 N).
In situation (c), the object is fully submerged, so it displaces its maximum possible volume of water. This results in a larger upthrust (10 N).
Multiple Choice Questions:
1. Which law is given by the statement, “Any pressure applied on an enclosed liquid is transmitted equally to all parts.”
2. Which formula shows the ratio of the cross sectional area of the two pistons and the forces applied on them in the correct form?
3. Which of the following formulae can give the part of a solid immersed in a liquid?
4. Which statement is true about the law of floatation?
5. Which statement is true about the law of floatation?
6. Sarita made a model for presenting Pascal law. But when she pushed one of the pistons inward, the rest pistons did not move outwards. What mistake might she have made in the installation of the apparatus?
Short Answer Questions:
7. Differentiate between:
(a) Law of floatation and Archimedes’ principle
| Archimedes’ Principle | Law of Floatation |
|---|---|
| Archimedes’ Principle explains how objects behave in water or other fluids. | The Law of Flotation is a specific case derived from Archimedes’ Principle. |
| It says that when you put something in water, it pushes some water out of the way (displaces it), and this displaced water pushes back on the object. | It tells us that a floating object displaces (moves aside) an amount of water that’s equal to its own weight. |
| It’s a general rule that helps us understand why things float or sink in any fluid. | It’s a more focused rule that describes how objects float specifically by displacing their weight in the fluid they are in. |
(b) Upward and downward thrust
| Upward Thrust | Downward Thrust |
|---|---|
| Upward thrust is the force that pushes an object from below, making it feel lighter. | Downward thrust is the force that pushes an object from above, making it feel heavier. |
| This force helps objects float and reduces the weight they seem to have. | Gravity is a common example of downward thrust-it pulls everything towards the center of the Earth. |
| It’s like a helpful hand pushing you up when you’re in a swimming pool. | When you drop something, it’s gravity that pulls it down. |
(c) Thrust and upthrust
| Thrust | Upthrust |
|---|---|
| Thrust is a force that makes things move forward or in a particular direction. | Upthrust, also known as buoyant force, is a force that pushes things upwards when they are in a fluid (like water or air). |
| For example, when a rocket engine propels gases backward, it generates a thrust that propels the rocket forward. | When you’re in a swimming pool, the water pushes you upwards, making you feel lighter. That’s upthrust at work. |
| Thrust helps in moving things, like when you push a swing, you’re giving it a thrust to move. | Upthrust is why objects like boats float in water it’s like a gentle push from below, helping them stay afloat. |
(d) Upward thrust and upthrust
| Upward Thrust | Upthrust |
|---|---|
| Upward thrust is the force that pushes something in an upward direction, opposing the force of gravity pulling things down. | Upthrust, also called buoyant force, is the specific upward push experienced by an object submerged in a fluid (like water or air). |
| Imagine jumping off the ground. the force pushing you up is the upward thrust. | When you’re in a pool, the water pushes you upwards, making you feel lighter than usual. This is upthrust in action. |
8. State Pascal’s law.
Pascal’s law says if you press on a fluid in a closed space, that pressure is transmitted everywhere in the fluid and even to the walls of the container. It’s like a push that spreads out in all directions.
9. Mention two factors which affect pressure due to liquid contained in a vessel.
Two factors which affect pressure due to liquid contained in a vessel:
1. Depth: The deeper you go in a liquid, the more pressure there is. It’s like when you dive into a pool, and you feel more pressure as you go deeper.
2. Density of the Liquid: Thicker or denser liquids can exert more pressure. It’s similar to how a heavy book on your hand feels like more pressure compared to a lighter one.
10. Name any two instruments based on Pascal’s law.
Any two instruments are:
i) Hydraulic Jack
ii) Hydraulic Brake System
11. State the law of floatation.
An object will float in a liquid if it is light or less dense than the liquid. Conversely, the object will sink if it is heavier or denser than the liquid.
12. State Archimedes’ principle.
Archimedes’ principle tells us that when something is put into water or any liquid, it experiences an upward force that helps it float. This force is equal to the weight of the liquid that the object pushes out of the way.
13. While dipping a solid object in a liquid, upthrust on the object due to the liquid and the weight of the displaced liquid are shown to be equal in an experiment. Which principle does the experiment prove?
This experiment demonstrates Archimedes’ principle. Archimedes’ principle states that when you put an object in a liquid, the upward force (upthrust) on the object is equal to the weight of the liquid that the object pushes aside or displaces. In this experiment, when the upthrust on the object matches the weight of the displaced liquid, it confirms Archimedes’ principle, showing that the liquid’s force pushing up on the object is related to the amount of liquid it displaces.
14. If a freely floating ship has a weight of $5 \times 10^5$N, what will be the weight of the water displaced by the ship?
“If a ship is floating freely, it’s like a dance between the ship’s weight and the weight of the water it pushes aside. The ship weighs $5\times10^5$ Newtons. The weight of the water it moves out of the way (displaces) is also $5\times10^5$ Newtons. It’s like a balancing act in water.”
15. Define upthrust.
“Upthrust is the gentle push or force that a liquid, like water, gives to an object placed in it. It’s the feeling of being lighter when you’re in the water, and it’s what helps objects float. The more an object pushes the liquid out of the way (displaces it), the stronger the upthrust it experiences.”
16. Mention one factor that can affect upthrust.
Certainly, one factor that can affect upthrust is:
Volume or Size of the Object: If you change the size or volume of an object placed in a liquid, it will affect the upthrust it experiences. A larger object will displace more liquid, resulting in a greater upthrust force.
17. What is the role of density in floatation of bodies?
The role of density in flotation bodies are:
| Less Dense Objects Float | More Dense Objects Sink |
|---|---|
| Objects with lower density than the fluid will float | Objects with higher density than the fluid will sink |
| They displace a weight of fluid equal to their own weight | They cannot displace enough fluid to equal their weight |
| Examples: Wood in water, oil on water | Examples: Stone in water, metal objects |
18. Mention the factors that affect the upthrust applied on a body.
The factors that affect the upthrust applied on a body:
– Volume of the Body: Larger volume means more fluid displaced
– Density of the Fluid: Denser fluids provide greater upthrust
– Density of the Body: Determines how much of the body is submerged
– Gravity: Upthrust depends on gravitational acceleration
19. When a body is fully or partially immersed in a liquid, name the forces acting on the body.
When something is in water or any liquid, it’s a battle between gravity pulling the object down and the upthrust from the liquid pushing the object up. If upthrust is stronger, the object floats. If gravity is stronger, the object sinks. If they balance, the object stays where it is in the liquid.
20. State Pascal’s law. Name any two instruments based on Pascal’s law.
Pascal’s Law: When you push or squeeze a liquid in an enclosed space, that pressure spreads equally everywhere in the liquid.
Two instruments based on Pascal’s Law are:
1. Hydraulic Jack
2. Hydraulic Brakes in Cars
21. How do animals use upthrust in their daily life?
Floating and Swimming: Fish use upthrust to help them swim and stay afloat in the water. Their bodies are designed to be a bit less dense than water, so they experience an upward push that makes swimming easier.
Staying on the Surface: Some animals, like ducks or water bugs, use upthrust to stay on the water’s surface without sinking. Their bodies and shapes are such that the water’s upthrust force keeps them from going under.
Diving and Surfacing: Marine mammals like seals and whales use upthrust to control their depth in the water. By adjusting their body position and using their lungs to change buoyancy, they can dive down or come up to the surface.
22. Describe the experimental verification of Archimedes’ principle.
To prove Archimedes’ principle, you can do a fun experiment in your kitchen sink or bathtub:
Gather Your Tools: You need a few things – a measuring cup, a small object (like a toy or a small fruit), and a bathtub or sink filled with water.
Step 1 – Measure the Water: Fill the measuring cup to the top with water and note the amount.
Step 2 – Check the Object’s Weight: Check the weight of the small object using a scale. Write down the weight.
Step 3 – Sink the Object: Put the small object in the water-filled sink or bathtub. Notice how much water is pushed aside (displaced) by the object.
Step 4 – Measure Displaced Water: Now, take the measuring cup and carefully submerge the object in the water. Measure the water again.
Step 5 – Compare Measurements: Compare the new water measurement with the earlier one. The difference in water levels shows how much water the object displaced.
Step 6 – Understand the Result: Archimedes’ principle says the weight of the water displaced by the object should be equal to the weight of the object. If you compare the weight of the water you measured to the weight of the object, you’ll see they’re very close!
This simple experiment proves that the water pushed aside (displaced) by the object weighs the same as the object itself, supporting Archimedes’ principle.
22. How do hydraulic machines help in developmental activities in our country?
Hydraulic machines are like strong helpers that make tough jobs easier for us. They play a big role in making our country better in many ways:
Construction Work: Imagine huge buildings and bridges being built. Hydraulic machines help lift heavy things like beams and concrete easily. They make construction faster and buildings stronger.
Agriculture: Farmers use hydraulic machines to do tasks like lifting heavy loads, plowing fields, and digging. This helps them grow more crops and food for everyone.
Waste Management: Hydraulic machines help pick up and move big bins of trash. This keeps our cities clean and helps manage waste properly.
Mining: In mines, hydraulic machines help dig deep into the Earth to find valuable minerals and resources. This is important for industries and the economy.
Transportation: Some vehicles, like buses and trucks, use hydraulics for brakes and steering. This makes driving safer and smoother.
23. Illustrate that hydraulic machines are force magnifiers.
Let’s break down the concept of hydraulic machines as force magnifiers in simple terms:
In a hydraulic machine, when you press that button or lever, it pushes a small amount of liquid (like oil) into a narrow tube. This narrow tube then pushes a bigger piston (a big part of the machine) with a lot more power because the pressure is spread over a larger area. So, with a little push, you can move or lift something much heavier using a hydraulic machine. It’s like having a magical strength-booster!
24. How does a hydraulic garage lift function? Explain with an illustration.
A hydraulic garage lift is like a super-strong table that can go up and down, helping fix cars and trucks easily. Here’s how it works:
The Pumping Action: Imagine you have a bike pump. When you press down the pump, it pushes air into a tire and makes it hard. In a garage lift, there’s a pump that’s usually operated by hand or electricity.
Pushing the Oil: Instead of air, the pump pushes a special liquid, often oil, through a pipe. This liquid goes into a narrow tube, like a straw, and pushes against a bigger plate (piston) at the other end.
Multiplying the Force: Just like pressing on a small area of a seesaw can move a much heavier person on the other end, pushing a small amount of liquid into a narrow tube pushes a bigger piston with a lot more power.
Lifting the Platform: This powerful push lifts the platform of the garage lift, which is where the car or truck goes. The platform can go up to a certain height, allowing mechanics to work underneath the vehicle.
24. It is easier to pull a bucket of water from a well until it is inside the water but it is difficult to lift the bucket when it is out of water.
Pulling a bucket of water inside a well is easier because water helps lighten the bucket, making it feel lighter. However, lifting the bucket out of the water is harder as you’re carrying both the weight of the bucket and the water inside without any support. Water provides a lifting boost when the bucket is submerged.
25. A bucket of water is filled faster in the downstair’s tap than in the upstair’s tap.
Filling a bucket downstairs is faster than upstairs because gravity helps pull the water downwards, making it flow faster. Upstairs, the water has to fight gravity, slowing down the filling process. It’s like going down a slide versus climbing up it’s faster and easier going down.
26. The blood pressure in human body is greater at the feet than at the brain.
Blood pressure is higher at the feet than at the brain because our hearts need to pump harder against gravity to push blood up to the brain. It’s like pushing a ball uphill-it takes more effort. The blood vessels also help by squeezing and pushing the blood upwards, but gravity makes it a bit harder for the blood to reach the brain, so the pressure is higher at the feet.
27. Deep-sea divers wear diving suits.
Deep-sea divers wear diving suits to stay safe and comfortable underwater. The suits keep them warm because the deep sea is cold. They also protect their bodies from cuts, stings, and other dangers in the water. Additionally, the suits help them float and control their movement, making it easier to dive and explore the deep ocean. It’s like having a special outfit that keeps them safe and cozy in the underwater world.
28. An egg floats on saturated salt solution.
An egg floats in salty water because the salt makes the water heavier and denser than the egg. This extra weight pushes the egg up, making it float.
29. An iron nail sinks in water but a ship made of the same material floats on it.
An iron nail is small and heavy, so it sinks in water. But a big ship made of iron floats because it’s designed to spread its weight over a large area, like a big metal boat, so it can stay on top of the water.
30. It is easier to swim in ocean than in a river.
Swimming in the ocean is easier than in a river because the ocean is big and has less pushy water compared to rivers. Rivers can push you more, making it harder to swim.
31. Dead bodies float on water only after some time.
Dead bodies float on water after some time because as the body starts to decompose, gases like methane and bacteria build up inside the body, making it less dense. This makes the body float on the water instead of sinking.
32. Hydraulic machines are also named as force magnifiers.
Hydraulic machines are called force magnifiers because they help us use a small force to move a much bigger force. It’s like getting more power to lift or move heavy things by using water or another fluid to amplify our strength.
33. Aeroplanes cannot fly on the surface of the moon.
Airplanes can’t fly on the moon’s surface because the moon doesn’t have air like our Earth does. Airplanes need air to generate lift and move, but there’s no air on the moon, so they can’t fly there.
34. A balloon filled with helium does not rise up in the air indefinitely.
A balloon filled with helium doesn’t keep going up forever because the helium makes the balloon lighter, but there’s a point where the air outside the balloon is also light enough. At that point, the balloon stops going up and stays at a certain level in the sky.
35. A loaded ship sinks more than an empty ship.
A loaded ship sinks more than an empty ship because the things (cargo, items) on the loaded ship make it heavier, and heavy things sink deeper in the water.
36. Ice floats on water.
Ice floats on water because it’s lighter than water. When water freezes into ice, it becomes less dense, which means it takes up more space and is lighter. That’s why ice floats on the surface of water.
37. An iron nail sinks in water but floats on mercury.
An iron nail sinks in water because it’s heavy and denser than water. However, it can float on mercury because mercury is even denser than the iron nail, making the nail less dense than the mercury and allowing it to float.
38. The size of an air bubble increases when it comes up and up in water.
The air bubble gets bigger as it goes up in water because there’s less squishing pressure from the water, so the air inside the bubble expands and makes it larger.
39. The weight of a piece of stone, when fully immersed in water, is 18 N and it displaces 4 N of water. What is the weight of the stone in air?
To find the weight of the stone in air, we’ll use the principle of buoyancy.
The weight of the stone in air is equal to the weight of the stone in water plus the weight of the water displaced by the stone.
Weight of stone in water = 18 N
Weight of water displaced by the stone = 4 N
So, weight of the stone in air = Weight of stone in water + Weight of water displaced
= 18 N + 4 N
= 22 N
Therefore, the weight of the stone in air is 22 N.
40. A cube of wood, whose volume is 0.2 m³ and density 600 $kg/m^{3}$, is placed in a liquid of density 800 $kg/m^{3}$. What fraction of the volume of the wood will be immersed in the liquid?
Solution:
Given:
Density of wood: 600 $kg/m^{3}$
Density of liquid: 800 $kg/m^{3}$
Volume of the wood: 0.2 $m^{3}$
We’ll calculate the fraction (f) of the volume of the wood cube immersed in the liquid:
$f = \frac{\text{Density of wood}}{\text{Density of liquid}}$
$f = \frac{600~kg/m^{3}}{800~kg/m^{3}}$
$f = \frac{3}{4}$
So, $3/4$ or 75% of the volume of the wood cube will be immersed in the liquid.