Selina Concise Solutions for ICSE Class 8 Physics Chapter 3 Force and Pressure

FORCE : “Is the cause which changes the state of a body (rest or state of motion) or it changes the size or shape of a body”.
Force acts as a push or a pull that can make things move or stop. It can also stretch or squash an object to change how it looks.
Teacher's Tip: Think of force as the "Action Hero" that makes objects do something different.
Exam Tip: To score full marks, always mention that force can change both the 'state of motion' and the 'shape' of an object.

Weight of a body -> The force with which a body is attracted towards the centre of earth mg = force of gravity.
Weight is actually the pull of the Earth's gravity on your mass. It changes if you go to another planet because gravity changes there.
Teacher's Tip: Mass stays the same everywhere, but weight is a "moody" force that changes with gravity.
Exam Tip: Remember the formula W = m ×s g where m is mass and g is gravity.

A force does not change the mass of the body that is why mass of a body on earth and moon in same but weight -> force exerted on body is different.
Mass is the amount of matter inside you, which doesn't disappear just because you are on the moon. Since the moon pulls you less than the Earth, you weigh less there even though you are the same size.
Teacher's Tip: You have the same "stuff" inside you on the moon, you just feel lighter.
Exam Tip: Use this point to explain the difference between mass and weight in long-form answers.

Force cannot be seen but it is felt.
You can't see the wind pushing a sail, but you can see the boat move. We observe force through the changes it causes in objects around us.
Teacher's Tip: Force is like an invisible ghost that leaves footprints you can see.
Exam Tip: When asked about the nature of force, mention that it is known by its 'effects'.

-> represents force, length of arrow gives magnitude and arrow points the direction.
In physics, we use arrows called vectors to show how strong a force is and which way it is pushing. A longer arrow means a bigger, stronger force.
Teacher's Tip: Length = Strength and Tip = Way to go!
Exam Tip: Always draw arrows carefully when representing forces in diagrams to show the correct direction.

S.I. unit of force is Newton (N).
This unit is named after the famous scientist Isaac Newton who studied how things move. One Newton is roughly the weight of a small apple.
Teacher's Tip: Remember "Newtons for Force" like you remember "Meters for Length".
Exam Tip: Always use the capital letter 'N' when writing the unit symbol.

Newton: “Is that much force, which when acting on a body of mass 1 kg produces in it (increases) a speed of 1 M s^-1 in the direction of its motion.
This definition explains exactly how a force of one Newton affects a specific weight. It links the force directly to the movement or acceleration it causes.
Teacher's Tip: One Newton is the "kick" needed to make 1 kg move at 1 meter per second.
Exam Tip: Memorize the '1 kg' and '1 M s^-1' values to define Newton accurately.

1 kgf = 9.8 N = 10 N (nearly)
Kilogram-force is another way to measure force based on Earth's gravity. It is a very common unit used in engineering and daily life measurements.
Teacher's Tip: Multiply kgf by 10 to get the approximate value in Newtons.
Exam Tip: Use 9.8 N for exact calculations and 10 N only if the question says "nearly" or "approximately".

RIGID body: “When a force is applied on a body and inter-spacing between its constituent particles do not change is called RIGID body” force can cause only the motion in it.
A rigid body is like a solid brick that doesn't bend or squash when you push it. All parts of the object move together as one single unit.
Teacher's Tip: Rigid = "Super Strong" and stays in shape.
Exam Tip: Define a rigid body by the fact that its 'internal particles' do not move closer together.

NON-RIGID body: “When force applied changes inter-spacing.” Force causes both change in its size (shape) and the motion in body.
Non-rigid bodies are like sponges or rubber balls that change their look when squeezed. The particles inside them can shift around under pressure.
Teacher's Tip: Non-rigid = "Flexible" like playdough or a balloon.
Exam Tip: Remember that for non-rigid bodies, force causes both 'motion' and 'deformation'.

TURNING EFFECT: “When force is applied on a pivoted (at a point) body, it can turn it and turning of body about point of rotation is called TURNING EFFECT.” or Moment of force.
This is the effect you see when you push a door and it swings on its hinges. It involves a fixed point that doesn't move while the rest of the object rotates.
Teacher's Tip: Turning effect is the "Secret of the See-saw".
Exam Tip: The 'pivot' is the most important part of this definition; don't forget to mention it.

This is measured as: TURNING EFFECT = MOMENT OF FORCE = Force x perpendicular distance from point of rotation. Moment of force = F x OP. S.I. unit of moment of force = N ×s m = Nm
To calculate the turning power, you multiply how hard you push by how far away you are from the hinge. Pushing farther away from the center makes it much easier to turn something.
Teacher's Tip: Torque (Moment) = How hard ×s How far.
Exam Tip: Always check that the distance is 'perpendicular' (at 90 degrees) to the force for the formula to work.

THRUST: “Force acting normally on a surface.” Smaller the area of surface, larger is thrust.
Thrust is a special kind of force that pushes straight down or perpendicular to a surface. It is the total weight or push acting on an object.
Teacher's Tip: Thrust is just a "Normal Force" - normal means at a right angle.
Exam Tip: Don't confuse thrust with pressure; thrust is the *total* force, while pressure is the force *per bit of area*.

PRESSURE : “Thrust per unit area. p = Thrust/area = F/A S.I unit of area A pressure is Nm^-2 or pascal (Pa)
Pressure tells us how spread out a force is over a surface. A sharp needle has high pressure because all the force is at one tiny point.
Teacher's Tip: Pressure is like "Force's concentration".
Exam Tip: Mention both Nm^-2 and Pascal (Pa) as units to show comprehensive knowledge.

If Thrust is measured in kgf and area in Cm², then pressure is expressed as kgf Cm^-2.
This is a non-S.I. unit often used in older textbooks or specific industries. It simply combines the gravitational unit of force with a smaller unit of area.
Teacher's Tip: Units are like languages; kgf Cm^-2 is just a different dialect of pressure.
Exam Tip: Be ready to convert these units to S.I. if the numerical problem asks for the answer in Pascals.

ATMOSPHERIC Pressure: 1 atm = 76 cm of mercury column 1 atm = 1.013 ×s 10⁵ Pa
This is the heavy weight of the air above us pressing down on everything. We don't feel it because the fluids inside our body push back with the same force.
Teacher's Tip: You are carrying the weight of a small car on your head in air pressure!
Exam Tip: Memorize the value 1.013 ×s 10⁵ Pa for standard atmospheric pressure.

FACTORS AFFECTING THE PRESSURE : P = F/A
 

(i) Area: Greater the area, lesser is pressure and lesser area, greater is pressure.
(ii) Magnitude of thrust acting: greater thrust, greater pressure.
Pressure changes based on how much force you use and how big the surface is. This explains why walking on snow is easier with wide snowshoes than with thin heels.
Teacher's Tip: Sharp things = Small Area = High Pressure. Flat things = Big Area = Low Pressure.
Exam Tip: Use the term 'inversely proportional' for area and 'directly proportional' for thrust.

Factors Affecting LIQUID PRESSURE = hdg
(i) High of liquid column: increases with height
(ii) Density of liquid: increases with density of liquid.
(iii) Gravity constant.
The deeper you dive into water, the more water is above you, which makes the pressure much higher. Heavier liquids like mercury also create much more pressure than water.
Teacher's Tip: Deep = High Pressure. Thick liquid = High Pressure.
Exam Tip: Remember the formula P = h ×s d ×s g where h is depth, d is density, and g is gravity.

ATMOSPHERIC PRESSURE : “Pressure exerted by the air of atmosphere around us.”
STANDARD ATMOSPHERIC PRESSURE 1 Atm = 76 cm of Hg column = 1.013 ×s 10⁵ Pa
The atmosphere is a layer of air hundreds of kilometers thick, and its weight exerts pressure on the Earth's surface. Scientists use a column of mercury to measure this pressure in a tool called a barometer.
Teacher's Tip: Hg is the symbol for Mercury, the liquid metal used in barometers.
Exam Tip: Use '76 cm of Hg' as a landmark measurement for sea-level pressure.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The S.I. unit of force is kgf.
Answer: False.
The S.I. unit of force is newton.
While kgf is used for convenience, Newton is the standard unit used by scientists worldwide. It ensures that calculations remain consistent regardless of the local gravity.
Teacher's Tip: S.I. units are the "Science International" standard.
Exam Tip: If a statement is false, always write the correct version to get full marks.

(b) A force always produces both the linear and turning motions.
Answer: False.
A force only produces a turning motion if the object is pivoted or fixed at a point. If an object is free to move, the force will only produce linear (straight) motion.
Teacher's Tip: A push can make a ball roll straight OR make a door swing, but usually not both at once.
Exam Tip: Look for the word 'always' in true/false questions; it often signals a false statement.

(c) Moment of force = force x perpendicular distance of force - from the pivoted point.
Answer: True.
The "Moment" is the scientific measure of how effective a force is at rotating an object. Both the strength of the force and its distance from the pivot determine this value.
Teacher's Tip: This is why it's easier to open a door by pushing near the handle than near the hinges.
Exam Tip: Ensure you include the word 'perpendicular' to define the distance correctly.

(d) Less force is needed when applied at a farther distance from the pivoted point.
Answer: True.
When you increase the distance, you increase the leverage, which creates more turning power with less effort. This is the basic principle behind tools like wrenches and crowbars.
Teacher's Tip: Distance is your friend when you need to turn something heavy.
Exam Tip: This concept is often asked in the context of "Why spanners have long handles."

(e) For a given thrust, pressure is more on a surface of large area.
Answer: False.
For a given thrust, pressure is less on a surface of large area.
When force is spread out over a large space, it becomes "diluted" and exerts less pressure. This is why wide tires prevent trucks from sinking into soft ground.
Teacher's Tip: Big area = "Comfortable" pressure. Small area = "Stinging" pressure.
Exam Tip: Remember that Pressure is inversely proportional to Area.

(f) The pressure on a surface increases with an increase in the thrust on the surface.
Answer: True.
Pushing harder on the same area will always result in more pressure being felt. More force concentrated in the same space makes the effect stronger.
Teacher's Tip: Thrust and pressure are "Best Friends" - they go up together.
Exam Tip: Use the formula P=F/A to show that if F increases, P must also increase.

(g) A man exerts same pressure on the ground whether he is standing or he is lying.
Answer: False.
A man exerts different pressure on the ground whether he is standing or he is lying.
When standing, all weight is on two small feet, creating high pressure. When lying down, the weight is spread over the whole body area, creating much lower pressure.
Teacher's Tip: This is why you sink in mud when standing, but can stay on top if you lie flat.
Exam Tip: Explain this using the 'Area' factor of the pressure formula.

(h) It is easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.
Answer: False.
It is not easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.
A sharp point has a tiny area, which creates massive pressure to cut through the wood fibers easily. A blunt nail spreads the force out, making it hard to penetrate.
Teacher's Tip: Sharp = Small Area = Huge Pressure.
Exam Tip: Use this as an example of an application of high pressure in daily life.

(i) The S.I. unit of pressure is pascal.
Answer: True.
One Pascal is equal to one Newton of force acting on one square meter of area. It is named after Blaise Pascal, a scientist who studied fluid mechanics.
Teacher's Tip: Pascal is the "Main Name" for pressure units.
Exam Tip: Be prepared to define 1 Pascal as 1 N/m².

(j) Water in a lake exerts pressure only at its bottom.
Answer: False.
Liquids exert pressure in all directions, including the sides of the lake and even upwards. This is because liquid particles move freely and push against every surface they touch.
Teacher's Tip: If you poke a hole in the side of a cup, water squirts out; that's side pressure!
Exam Tip: State that 'Liquid pressure acts in all directions' for a full score.

(k) A liquid exerts pressure in all directions.
Answer: True.
Unlike solids which only push down, fluids (liquids and gases) push against anything they are in contact with. This pressure increases as you go deeper into the liquid.
Teacher's Tip: Think of being underwater; the water presses on your ears from every side.
Exam Tip: This is a fundamental property of fluids that distinguishes them from solids.

(l) Gases exert pressure in all directions.
Answer: True.
Gas molecules move at high speeds and collide with the walls of their container. These constant collisions create pressure that acts equally in every direction.
Teacher's Tip: A balloon stays round because air pushes out equally in all directions.
Exam Tip: Mention 'molecular collisions' if asked for the reason behind gas pressure.

(m) The atmospheric pressure is nearly 10⁵ Pa.
Answer: True.
This is a very large amount of pressure, roughly equal to 100,000 New

(n): Higher we go, greater is the air pressure.
Answer: False.
Air pressure actually decreases as we go higher because the atmosphere becomes thinner and there is less air above to press down. This is why mountain climbers often need oxygen tanks at very high altitudes.
Teacher's Tip: Think of altitude like a pile of blankets; the higher you are in the pile, the fewer blankets (air) are on top of you.
Exam Tip: Remember that atmospheric pressure and altitude have an inverse relationship; one goes up while the other goes down.

2. Fill in the blanks

(a) 1 kgf = 10 N (nearly).
This conversion helps us switch between units of force based on gravity and the standard scientific unit. It is a simplified value used to make mathematical calculations easier in the classroom.
Teacher's Tip: "kgf" stands for kilogram-force, which is roughly ten times a Newton.
Exam Tip: Use the value 10 N only when the question asks for an approximate or "nearly" value.

(b) Moment of force = force x distance of force from the point of turning
This formula calculates the turning effect or torque produced when you apply force to a pivoted object. The 'distance' must be measured from the hinge or pivot to where the force is being applied.
Teacher's Tip: This is why it is much easier to open a heavy gate by pushing at the very edge.
Exam Tip: Always ensure the distance is measured in meters (m) to get the moment in Nm.

(c) In a door, handle is provided farthest from the hinges.
Placing the handle far away increases the distance from the pivot, which maximizes the turning effect. This allows us to open large doors with very little physical effort.
Teacher's Tip: Maximum distance equals minimum effort for turning.
Exam Tip: Use the term 'lever arm' or 'perpendicular distance' to explain this design in long answers.

(d) The unit of thrust is newton.
Thrust is simply a force acting perpendicular to a surface, so it shares the same unit as any other force. It represents the total push acting on an entire area.
Teacher's Tip: Thrust is just a "Force with a direction" (straight down).
Exam Tip: Do not write Pascal for thrust; Pascal is for pressure, while Newton is for thrust.

(e) Thrust is the normal force acting on a surface.
The word 'normal' in physics means at a right angle or 90 degrees. When you stand on a floor, your weight acts as a normal force or thrust on that floor.
Teacher's Tip: Normal = Perpendicular = 90° angle.
Exam Tip: Always include the word 'normal' or 'perpendicular' when defining thrust to get full marks.

(f) Pressure is the thrust acting on a surface of unit area.
Pressure measures how concentrated a force is by looking at how much push is on every single square centimeter or meter. A high pressure means the force is squeezed into a very small space.
Teacher's Tip: Pressure is like "Force Density" on a surface.
Exam Tip: The keyword here is 'unit area'; make sure it is in your definition.

(g) The unit of pressure is pascal
Named after Blaise Pascal, this unit is equivalent to one Newton per square meter (1 N/m²). It is the standard S.I. unit used to measure pressure in fluids and on solids.
Teacher's Tip: 1 Pa is actually a very small amount of pressure, like the weight of a piece of paper spread over a table.
Exam Tip: Capitalize the unit 'Pascal' but use a lowercase 'p' for the symbol 'Pa'.

(h) Pressure is reduced if area of surface increases.
By spreading a force over a larger area, each individual point on the surface feels less of the push. This is why wide straps on a school bag are more comfortable than thin strings.
Teacher's Tip: Big Area = Small Pressure (Inversely proportional).
Exam Tip: Use this principle to explain why camels can walk on sand without sinking.

(i) Pressure in a liquid increases with the depth.
As you go deeper into a liquid, there is more weight of the liquid above you pressing down. This is why deep-sea divers need special suits to withstand the massive pressure at the bottom of the ocean.
Teacher's Tip: Think of it like a human pyramid; the person at the bottom feels the most pressure.
Exam Tip: Remember that liquid pressure depends on depth (h) and density (d).

(j) The atmospheric pressure on earth surface is nearly 10⁵ Pa.
This huge number represents the weight of the miles of air above us pressing down on every square meter of the ground. We are used to it, so we don't feel crushed by this immense force.
Teacher's Tip: 10⁵ is the same as 100,000.
Exam Tip: Memorize this numerical value as it is a common fill-in-the-blank question.

3. Match the following

Column A                                 Column B
(a) Camel                               (i) broad and deep foundation
(b) Truck                                (ii) broad feet
(c) Knife                                 (iii) six or eight tyres
(d) High building                    (iv) sharp cutting edge
(e) Thrust                               (v) atm
(f) Moment of force                (vi) N
(g) Atmospheric pressure      (vii) N m

Answer:
(a) Camel - (ii) broad feet
(b) Truck - (iii) six or eight tyres
(c) Knife - (iv) sharp cutting edge
(d) High building - (i) broad and deep foundation
(e) Thrust - (vi) N
(f) Moment of force - (vii) N m
(g) Atmospheric pressure - (v) atm
These pairings show how nature and engineers use the principles of area and force to solve problems. For example, broad feet help a camel stay on top of soft sand by reducing pressure.
Teacher's Tip: Match the biological or structural feature to its scientific purpose (reducing or increasing pressure).
Exam Tip: Write the full correct pairs in your answer sheet rather than just drawing lines to ensure clarity.

4. Select the correct alternative

(a) SI. unit of moment of force is
Answer: 4. N m
Since the moment is calculated by multiplying force (Newtons) by distance (meters), the unit combines both as Newton-meters. It measures the rotational strength of a force.
Teacher's Tip: Moment = Force ×s Distance, so Unit = Newton ×s Meter.
Exam Tip: Do not confuse Nm with N/m (division); always use the multiplication dot or space.

(b) To obtain a given moment of force for turning a body, the force needed can be decreased by
Answer: 3. applying the force farthest from the pivoted point
Increasing the distance from the pivot gives you more leverage, meaning you don't have to push as hard to get the same result. This is why long-handled wrenches are used for tight bolts.
Teacher's Tip: Distance is your "Mechanical Advantage" friend.
Exam Tip: The farther the force is from the pivot, the smaller the force required for rotation.

(c) The unit of thrust is
Answer: 1. kgf
(Note: Newton is the S.I. unit, but kgf is a valid gravitational unit of thrust provided in the options). Thrust is the total perpendicular force, and in many practical cases, we measure this force in terms of kilograms of weight.
Teacher's Tip: kgf is a common unit for "Force due to gravity".
Exam Tip: If both Newton and kgf are in the options, Newton is usually the preferred S.I. choice, but follow your textbook's options.

(d) The unit of pressure is
Answer: 3. N m^-2
This unit shows that pressure is force (Newtons) divided by area (square meters). It is the mathematical way of writing Pascals.
Teacher's Tip: The -2 exponent just means "per square meter".
Exam Tip: N m^-2 and Pa are identical; learn both for your exams.

(e) The pressure and thrust are related as
Answer: 3. Pressure = Thrust / Area,
This is the fundamental formula of the chapter, showing that pressure is the result of dividing total force by the space it covers. This relationship helps us calculate how much a surface will be affected by a weight.
Teacher's Tip: Pressure is the "Average Push" on every tiny bit of a surface.
Exam Tip: Be able to rearrange this formula to Thrust = Pressure ×s Area as well.

(f) A body weighing 5 kgf, placed on a surface of area 0.1 m², exerts a thrust on the surface equal to
Answer: 2. 5 kgf
The thrust is simply the weight of the object acting vertically on the surface. Unless the object is moving or being pushed, the thrust is always equal to its weight.
Teacher's Tip: Thrust is the "Weight" in this scenario.
Exam Tip: Don't do any math if the question just asks for "Thrust" and gives you the "Weight".

P.Q. A body weighing 5 kgf, placed on a surface of area 0.1 m², exerts a pressure on the surface equal to
Answer: 3. 50 kgf m^-2
To find this, we use the formula P = Thrust / Area = 5 / 0.1 = 50. It shows that the weight is concentrated tenfold because the area is only a tenth of a square meter.
Teacher's Tip: Dividing by 0.1 is the same as multiplying by 10.
Exam Tip: Check your units; since the inputs were kgf and m², the answer must be in kgf m^-2.

(g) The feet of lizards act like
Answer: 3. suction pads
Lizards can walk on walls and ceilings because their feet create tiny vacuums or use molecular forces that act like suction. This creates enough pressure from the outside air to hold their weight against gravity.
Teacher's Tip: Nature uses atmospheric pressure to help lizards climb!
Exam Tip: This is a classic biological application of pressure principles.

(h) Pressure exerted by a liquid is due to its
Answer: 1. weight
Every layer of liquid has mass, and gravity pulls that mass down onto the layers below. This accumulated weight is what creates pressure at any given point in a fluid.
Teacher's Tip: Liquid pressure is just the weight of the "water column" above you.
Exam Tip: Distinguish this from gas pressure, which is caused by molecular collisions.

(i) Pressure inside a liquid increases with :
Answer: 1. increase in depth
The further down you go, the more liquid there is above you, resulting in more weight and therefore more pressure. This is a linear relationship in most stationary liquids.
Teacher's Tip: More depth = more "liquid blankets" piling on top of you.
Exam Tip: Liquid pressure also depends on density, so keep that in mind if comparing different liquids.

(j) The atmospheric pressure at sea level is nearly
Answer: 2. 100,000 Pa
This is the standard measurement used as a baseline for air pressure calculations. It is roughly equivalent to 1 bar or 1 atmosphere (atm).
Teacher's Tip: 10⁵ is a shorter way to write 100,000.
Exam Tip: Be ready to see this written as 10⁵ Pa or 100,000 Pa; they are the same.

(k) Nose bleeding may occur at a high altitude because
Answer: 1. the atmospheric pressure decreases
At high altitudes, the external air pressure becomes lower than the blood pressure inside our bodies. This pressure difference can cause thin blood vessels in the nose to burst.
Teacher's Tip: It's like a balloon popping because the air outside it got too thin.
Exam Tip: Explain that 'internal blood pressure' becomes greater than 'external atmospheric pressure'.

B. Short/Long Answer Questions

Question 1: Define force. State its S.I. unit.
Answer: Force : Force is a physical cause that changes or may tend to change the state of rest or the state of motion of an object. The S.I. unit of force is Newton.
Force is the interaction that initiates all movement in the universe. Whether it's an apple falling or a rocket launching, a force is the reason behind it.
Teacher's Tip: Force = Push or Pull.
Exam Tip: Mention 'tend to change' to include forces that are applied but don't result in actual movement (like pushing a wall).

Question 2: State two effects of a force when applied on a body.
Answer: Two EFFECTS OF APPLIED FORCE :
(i) It can change the state of body. i.e. It can stop a moving object and also a force can move a stationary object.
(ii) A force can change the shap & (and size) of an object.
Force can change how an object moves or how it is built. For example, a goalkeeper stops a ball (motion) and a baker kneads dough (shape).
Teacher's Tip: Think "Motion" and "Makeover" (shape change).
Exam Tip: Use clear examples like 'stopping a ball' to illustrate your points for extra clarity.

Question 3: How does the effect of force differ when it is applied on (i) a rigid body, and (ii) a non-rigid body ?
Answer: (i) When a force applied on a rigid object does not change interspacing of constituent molecules and does not change dimensions and can cause motion in it.
(ii) When a force applied on a non-rigid object change interspacing between its constituent particle and cause a change in its dimensions and can produce motion in it.
In a rigid body, the force moves the whole object without squashing it, like pushing a stone. In a non-rigid body, the force can dent or stretch the object, like squeezing a sponge.
Teacher's Tip: Rigid = No squish. Non-rigid = Squish possible.
Exam Tip: Focus on 'interspacing of molecules' to give a more scientific explanation.

Question 4: State the effect of force F in each of the following diagram
Answer: (a) Ball moves in the direction of force (pushed) on applying force F.
(b) Wheel turns about the axis of rotation i.e. about pivot on applying force.
In diagram (a), the force causes linear motion because the ball is free. In diagram
(b), the force causes rotational motion because the wheel is fixed at a center point.
Teacher's Tip: Pivot points change straight pushes into turns.
Exam Tip: Label the first as 'translational/linear motion' and the second as 'rotational motion'.

Question 5: Define the term moment of force.
Answer: Movement of force - The turning effect of force acting on a body about an axis is called the moment of force.
This term describes how much "twist" a force provides to an object. It depends on where you push and how hard you push relative to the fixed center.
Teacher's Tip: Moment of force is just the scientific name for "Turning Power".
Exam Tip: Always relate the moment of force to an 'axis' or 'pivot point'.

Question 6: State the S.I. unit of moment of force.
Answer: S.I. units : N m (Newton meter)
This unit is the product of the unit of force (N) and the unit of distance (m). It correctly represents the formula for calculating moments.
Teacher's Tip: Use a small space or dot between N and m (N m).
Exam Tip: Never write J (Joule) for moment of force, even though Nm is also the unit for work; they are used for different concepts.

Question 7: State two factors on which affect moment of force.
Answer: Factors on which moment of force depends
(i) The magnitude of the force applied.
(ii) The distance of line of action of the force from the axis of rotation.
A bigger push or a push from further away will both increase the turning effect. This is why it's easier to turn a large wheel than a small one with the same effort.
Teacher's Tip: Force and Distance are the two "Gears" of a moment.
Exam Tip: Use the word 'magnitude' when referring to the size of the force.

Question 8: In Fig. a force F is applied in a direction passing through the pivoted point O of the body. Will the body rotate ? Give reason to support your answer.
Answer: No, the body will not rotate as Turning effect = Force x perp of the force from the pivoted point. = Fx 0 = 0. perp distance = O or force is parallel to the point of application of force.
If you push directly at the hinge of a door, it won't swing no matter how hard you push. This is because the distance from the pivot is zero, making the turning effect zero.
Teacher's Tip: You can't turn something if you push its "center of turning".
Exam Tip: State clearly that the 'perpendicular distance is zero' to explain why there is no rotation.

Question 9: Write the expression for the moment of force about a given axis of rotation.
Answer: Moment of force (O) = F x Perpendicular distance of force from point O. i.e. Moment of force = F x OP
This mathematical expression allows us to calculate the exact turning power of any force. It shows that force and distance are equally important in rotation.
Teacher's Tip: M = F ×s d.
Exam Tip: In your diagram, mark 'OP' as the distance and 'F' as the force at 90 degrees to OP.

Question 10: State one way to decrease the moment of a given force about a given axis of rotation.
Answer: Either decrease the force or decrease the perpendicular distance of force from axis of rotation.
By moving your hand closer to the hinges of a door, you make it harder to turn, effectively decreasing the moment. Similarly, pushing with less strength also reduces the turning effect.
Teacher's Tip: Move closer to the pivot to make the moment smaller.
Exam Tip: Mentioning 'decrease the perpendicular distance' is the most common correct answer.

Question 11: State one way to obtain greater moment of a given force about a given axis of rotation.
Answer: Increase the perpendicular distance of force from axis of rotation.
Using a longer handle on a tool or pushing at the very edge of an object increases your turning power. This is the secret behind the design of most simple machines like levers.
Teacher's Tip: Longer handle = Greater Moment.
Exam Tip: Use the term 'lever arm' if you want to sound like a physics pro!

Question 12: What do you mean by the clockwise and anti-clockwise moment of force ?
Answer: Clockwise moment of force - When the moment of force is in the clock wise direction.
Anticlockwise moment - When the moment of force acts in anticlockwise direction.
If a force makes a wheel turn like the hands of a clock, it is a clockwise moment. If it turns the opposite way, it is an anticlockwise moment.
Teacher's Tip: Clockwise is "Right", Anticlockwise is "Left".
Exam Tip: In equilibrium problems, remember that Clockwise moments must equal Anticlockwise moments.

Question 13: Explain the following:
(a) The spanner (or wrench) has a long handle.
(b) The steering wheel of a vehicle is of large diameter.
(c) The hand flour grinder is provided with a handle near the rim.
(d) It is easier to open the door by pushing it at its free end.
(e) A potter turns his wheel by applying a force through the stick near the rim of wheel.
Answer: (a) Spanner has a long handle to produce larger turning moment and small force is applied at the end of handles.
(b) The hand of large diameter has a large perpendicular distance. The moment of force depends upon perpendicular distance.
(c) In order to increase of moment of force, handle is provided near its arm. Small force is applied at the handle.
(d) Moment of force is product of force and perpendicular distance. It is easier to open the door applying force at the free end.
(e) A Polter’s wheel has a wheel pivoted at the centre. The potter turns the wheel by means of a stick at the rim of the wheel. He increases the perpendicular distance.
All these designs use the principle of increasing distance to make turning easier. By applying force far from the pivot, we create a large turning effect with very little physical strength.
Teacher's Tip: Everything "easy to turn" always has a handle far from the center.
Exam Tip: For each part, mention that 'increasing the distance' leads to a 'greater moment of force'.

Question 14: What is thrust ?
Answer: THRUST : “Force acting normally on the surface is called THRUST.”
Thrust is a special name for force when it is pushing straight against a surface. It is the total weight or push that we use to calculate pressure later on.
Teacher's Tip: Normal = Perpendicular (90°).
Exam Tip: Defining thrust as 'perpendicular force' is the key to full marks.

Question 15: State the unit of thrust
Answer: S.I. unit is newton [N].
Since thrust is just a force, it is measured in the same unit as any other push or pull. It is often measured in kilograms-force (kgf) in non-scientific contexts.
Teacher's Tip: Newton (N) is for all forces, including thrust.
Exam Tip: Don't forget that Newton is the S.I. unit, while kgf is a gravitational unit.

Question 16: On what factors does the effect of thrust on a surface depend?
Answer: The effect of thrust depends on the area of the surface on which it acts. Smaller the area of the surface on which a thrust acts, larger is its effect. But the effect of a thrust is less on a larger area.
The "effect" mentioned here is actually pressure. A small point makes the thrust feel very strong, while a broad surface makes the same thrust feel much weaker.
Teacher's Tip: Effect of Thrust = Pressure.
Exam Tip: Use the 'Area' of the surface as the primary factor in your answer.

Question 17: Define the term ‘pressure’ and state its unit.
Answer: PRESSURE : “The thrust on unit area of the surface is called PRESSURE.”
Pressure = Thrust / Area
S.I. unit = Nm^-2
Pressure tells us how the force is distributed across a surface. It is the calculation of how much thrust hits every single square meter of an object.
Teacher's Tip: Pressure is force per "room" on the surface.
Exam Tip: Mention Nm^-2 or Pascal as the S.I. unit for a complete answer.

Question 18: How is the thrust related to pressure ?
Answer: Pressure is directly proportional to thrust P propto Thrust i.e. greater the thrust, greater is the pressure and smaller the thrust, smaller is the pressure.
If you keep the area the same and push twice as hard, you will create twice as much pressure. This direct link means they increase or decrease together.
Teacher's Tip: More push = More pressure.
Exam Tip: Always specify that this relation holds true 'for a constant area'.

Question 19: Name two factors on which the pressure on a surface depends.
Answer: FACTORS AFFECTING THE PRESSURE:
(i) The surface area (A) on which thrust acts. More area, lesser is pressure.
(ii) The magnitude of thrust (F) acting on the surface. More Thrust, more is pressure.
These two factors work against each other to decide the final pressure. Engineers balance them to make sure things like tires or foundations don't fail under heavy loads.
Teacher's Tip: Pressure is a tug-of-war between Force and Area.
Exam Tip: Remember: Force is directly proportional, but Area is inversely proportional to pressure.

Question 20: When does a man exert more pressure on the floor : while standing or while walking ?
Answer: While standing, the area touching the floor is less, the pressure exerted will be more. While walking, area touching the floor is more and pressure exerted will be less.
(Note: Verbatim from text. In many other physics contexts, walking is higher pressure because only one foot is down. Follow the textbook answer provided here). When you reduce the surface area in contact with the ground, the same body weight creates a much stronger pressure on that spot.
Teacher's Tip: Smaller contact area means "sharper" pressure on the ground.
Exam Tip: Relate the change in pressure specifically to the change in 'contact area'.

Question 21: Why do camels or elephants have broad feet ?
Answer: Camels have to walk on sand. To walk fast, the feet should not sink in sand, the feet area broad i.e. increase in area. Hence, lesser pressure, to sink less, the camels have broad feet.
Elephants have broad feet in proportion to their size. Because of their weight and size, elephants need to distribute the weight better.
Broad feet act like built-in snowshoes, spreading the animal's massive weight over a huge area. This keeps the pressure low enough that they stay on the surface rather than sinking in.
Teacher's Tip: Big feet = Big Area = Low Pressure = No Sinking.
Exam Tip: Always mention 'weight distribution' and 'lowering pressure' for full credit.

Question 22: A sharp pin works better than a blunt pin. Explain the reason.
Answer: A sharp tip is provided at the end of a pin so that pressure (FORCE / AREA) exerted by it is maximum when it is pressed by a given force.
A sharp point has an almost zero area, which turns even a tiny push into a massive, piercing pressure. This allows the pin to easily cut through paper or fabric.
Teacher's Tip: Sharp things are just "Pressure Multipliers".
Exam Tip: Use the formula P = F/A to show that a very small A makes P very large.

Question 23: Why is the bottom part of the foundation of a building made wider ?
Answer: P propto 1/A i.e. pressure is inversely proportional to surface Area. Greater the area, lesser is the pressure. In order that building should exert less pressure, the foundation of building should have more area or be wider. So that building should not sink into the earth.
Buildings are incredibly heavy, and if they were built on thin pillars, they would sink into the soft dirt. A wide foundation spreads that weight out, making the pressure on the ground safe and stable.
Teacher's Tip: Wide base = Solid base.
Exam Tip: Mention that widening the area 'reduces the pressure on the ground' to prevent sinking.

Question 24: It is easier to cut with a sharp knife than with a blunt one. Explain.
Answer: It is easier to cut with a sharp knife than with a blunt one because for the same applied force, the pressure exerted (force/area) is more in the case of sharp knife than in the case of a blunt one.
The cutting edge of a sharp knife is very thin, meaning the area is tiny. This creates enough pressure to separate the molecules of the food with very little effort from your hand.
Teacher's Tip: Sharp = Smaller Area = Higher Pressure.
Exam Tip: Explain this by comparing the 'surface area' of the two knife edges.

Question 25: A gum bottle rests on its base. If it is placed upside down, how does the (i) thrust, (ii) pressure change ?
Answer: A gum bottle has narrow neck and wider base when placed upside down
(i) Surface area is less, larger is the effect of thrust.
(ii) Lesser area, larger is the pressure.
The weight (thrust) of the bottle remains exactly the same because no gum was added or removed. However, since the neck is smaller than the base, that weight is now squeezed into a smaller area, increasing the pressure.
Teacher's Tip: Weight (Thrust) doesn't change when you flip a bottle, but Pressure does!
Exam Tip: Clearly state that (i) Thrust remains constant, while (ii) Pressure increases.

Question 26: Explain the following:
(a) Sleepers are used below the rails.
(b) A tall building has wide foundations.
Answer: (a) Sleepers are laid below the rails to reduce pressure (force/area) on the ground. therefore Sleepers increase the area.
(b) Foundation of buildings are kept wide so that the weight of the building may act on larger area. As a result it will exert less pressure on the ground. This avoids sinking of building into the earth.
Railroad sleepers (the wooden or concrete beams) prevent the heavy train from pushing the thin metal tracks into the mud. Both examples use a larger area to make a heavy weight safe for the ground below.
Teacher's Tip: Spreading weight makes it "Lighter" for the ground to carry.
Exam Tip: For both parts, the core reason is 'increasing area to reduce pressure'.

Question 27: Describe an experiment to show that a liquid exerts pressure at the bottom of the container in which it is kept.
Answer: A LIQUID EXERTS PRESSURE AT THE BOTTOM OF ITS CONTAINER: Take a balloon and tie it at the lower end of a glass tube and hold vertically as in fig. (a) pour some water in the tube, balloon bulges out as in (b) because water column exerts pressure (therefore liquid has weight) at its bottom. Force on balloon is equal to the weight of Thrust water column which is called the thrust P = Thrust / Area. This shows that a liquid exerts pressure at the bottom of container in which it is kept.
The bulging balloon provides visible proof that the water is actively pushing down. The more water you add, the more the balloon will stretch, showing that more weight means more pressure.
Teacher's Tip: The "Bulge" is the proof of the "Push".
Exam Tip: Use the term 'weight of the liquid column' to explain why the balloon bulges.

Question 28: Describe a suitable experiment to demonstrate that a liquid exerts pressure sideways also ?
Answer: A LIQUID EXERTS PRESSURE SIDE WAYS : Set up the apparatus as shown in fig. (a). A deflated balloon is tide on the side tube of container opened at one side. Now fill water in the container, balloon connected inflats due to pressure of liquid exerted side ways. This shows that liquids exerts pressure sideways also.
Since liquid molecules can move in any direction, they don't just push down; they push against the walls of their container too. This experiment shows that water is "hungry" to escape through any side opening.
Teacher's Tip: Sideways pressure is what makes water squirt out of a hole in a bucket.
Exam Tip: Label the diagram with 'Lateral Pressure' to show advanced understanding.

Question 29: Describe a simple experiment to show that at a given depth, a liquid exerts same pressure in all directions.
Answer: Experiment: We have often seen children playing with polythene bag filled with water and having small holes at various places on this bag and sprinkling water on others. This is because water enclosed in bag exerts pressure in all directions.
If you poke several holes at the exact same height in a water bag, the water will squirt out with the same strength from every hole. This proves that at that specific level, the pressure is perfectly balanced in every direction.
Teacher's Tip: Equal height = Equal pressure in a fluid.
Exam Tip: Mention that the 'distance the water travels' from the holes is the same to prove equal pressure.

Question 30: State two factors on which the pressure at a point in a liquid depends.
Answer: FACTORS are : (hdg)
(i) h - Depth of the point below free surface.
(ii) d - Density of liquid.
(iii) g - Acceleration due to gravity, (constant)
Liquid pressure is a team effort between how deep you are, how heavy the liquid is, and the planet's gravity. If you change any of these, the pressure will change too.
Teacher's Tip: Pressure formula = h ×s d ×s g.
Exam Tip: List 'depth' and 'density' as the two primary variables that change pressure on Earth.

Question 31: Describe an experiment to show that the liquid pressure at a point increases with the increase in height of the liquid column above that point.
Answer: Experiment: Tie a balloon at one end of a glass tube opened at both ends. Pour some water and note the bulging of balloon as in (a). Now add more of water in the tube bulging also increases. Add still more water bulging also increases further as liquid pressure at a point increases with increase in height of the liquid column above that point.
Each extra inch of water added to the tube adds more weight for the balloon to carry. This increasing weight translates directly into increasing pressure at the very bottom.
Teacher's Tip: Height of water column = Amount of push.
Exam Tip: Use the word 'directly proportional' to describe how pressure relates to the height of the column.

Question 32: Which fact about liquid pressure does the diagram in fig. illustrate.
Answer: The figure shows that THE LIQUID PRESSURE INCREASES WITH THE HEIGHT OF THE LIQUID COLUMN ABOVE IT.
When we see a balloon bulging more at the bottom of a taller tube, we know the pressure has gone up. This is a fundamental law for all liquids standing still in a container.
Teacher's Tip: Tall water = Strong water pressure.
Exam Tip: Refer to the 'degree of bulging' of the balloon to explain the diagram's meaning.

Question 33: Describe an experiment to show that liquid pressure depends on the density of liquid.
Answer: LIQUID PRESSURE DEPENDS ON THE ’DENSITY OF LIQUID : Take two identical balloons attached to one end of each of two identical glass tubes opened at both ends. Fill tube (a) with milk and tube (b) with alcohol to the same height. We notice that balloon attached to tube (a) bulged more than balloon attached to tube (b). This shows that the same height of milk exerts more pressure than alcohol. Since density of milk (1.03) is more than density alcohol (0.8).
Even if the height is the same, a "thicker" or denser liquid like milk is heavier than a "thinner" liquid like alcohol. This extra weight per inch creates a stronger pressure at the bottom.
Teacher's Tip: Heavier liquid = Heavier pressure at the same depth.
Exam Tip: Use the density values (1.03 vs 0.8) to support your experimental observation.

Question 34: A dam has broader walls at the bottom than at the top. Give a reason.
Answer: To withstand the greater pressure of water which increases with increase in depth.
Since water pressure is highest at the very bottom of the lake, the dam needs to be thickest there to avoid breaking. The top only has to hold back a little bit of pressure, so it can be thinner.
Teacher's Tip: Dams are shaped like triangles to match the pressure increase.
Exam Tip: Mention 'increasing pressure with depth' as the core engineering reason.

Question 35: What do you mean by atmospheric pressure ?
Answer: ATMOSPHERIC PRESSURE: “The thrust due to atmospheric air on unit area” is called ATMOSPHERIC PRESSURE.
The Earth is surrounded by a massive ocean of air that has real weight. This air presses against every square inch of our bodies and everything else on the planet's surface.
Teacher's Tip: Air pressure is the weight of the "Sky" on your shoulders.
Exam Tip: Use the phrase 'thrust on unit area' for a technically perfect definition.

Question 36: Write the numerical value of the atmospheric pressure on the earth surface in pascal.
Answer: It is about 10⁵ Pa.
This value is an average taken at sea level under normal conditions. It represents 100,000 Newtons of force pressing down on every single square meter of the Earth.
Teacher's Tip: 10⁵ is the "Magic Number" for our atmosphere.
Exam Tip: Memorize this value (100,000 Pa) for multiple-choice questions.

Question 37: We do not feel uneasy even under the enormous atmospheric pressure. Give a reason.
Answer: Blood in our body exerts BLOOD PRESSURE which is slightly more than atmospheric pressure and we do not feel uneasy.
Our internal fluids are pushing outwards just as hard as the air is pushing inwards. This perfect balance prevents our bodies from being crushed like an empty tin can.
Teacher's Tip: It's an "Internal vs External" pressure match.
Exam Tip: The key word is 'blood pressure' balancing the atmospheric pressure.

Question 38: Describe a simple experiment to illustrate that air exerts pressure.
Answer: EXPERIMENT : Take a tin-can having air tight cap (screw cap). Remove cap and boil some water in it, so that steam comes out and in this way air from inside goes out. While boiling replace the cap and allow it to cool. Vapours inside condense and form water creating vacuum above them. We see the can crumbles due to air pressure from outside. This proves that air exerts pressure.
When the steam inside turns back to water, there is no air left inside to push back against the atmosphere. The heavy outside air then wins the "pressure battle" and crushes the metal can inward.
Teacher's Tip: The "Crushing Can" is the most famous proof of air's power.
Exam Tip: Mention that 'condensation creates a vacuum' to explain why the inside pressure drops.

Question 39: bescribe the crushing tin can experiment. What do you conclude from this experiment ?
Answer: EXPERIMENT : Take a tin-can having air tight cap (screw cap). Remove cap and boil somowater in it, so that steam comes out and in this way air from inside goes out. While boiling replace the cap and allow it to cool. Vapours inside condense and form water creating vacuum above them. We see the can crumbles due to air pressure from outside. This proves that air exerts pressure.
We conclude that the invisible air around us is strong enough to crush metal when there is nothing pushing back from the inside. This shows that atmospheric pressure is a very real and powerful force.
Teacher's Tip: This experiment works best if you pour cold water on the can to speed up the cooling.
Exam Tip: State the conclusion clearly: 'Air exerts pressure in all directions'.

Question 40: Give reasons for the following :
(a) A balloon collapses when air is removed from it.
(b) Water does not run out of a dropper unless its rubber bulb is pressed.
(c) Two holes are made in a sealed oil tin to take out oil from it.
Answer: (a) Atmospheric pressure which is more than inside pressure of balloon causes balloon to collapse.
(b) Atmospheric pressure acting from outside the dropper balances the pressure exerted by water and water does not come out of a dropper. On pressing the dropper inside pressure of water becomes more than outside atmospheric pressure and water runs out.
(c) Two holes are made in a sealed can so that ATMOSPHERIC AIR presses the oil at one hole and oil comes out of the second hole.
All these daily activities depend on the "Tug-of-War" between air pressure and fluids. By understanding how to manipulate this pressure, we can make tools like droppers and oil tins work efficiently.
Teacher's Tip: Air pressure is like an "Invisible Hand" that keeps water inside droppers.
Exam Tip: For part (c), explain that air must 'enter one hole to push the oil out of the other'.

Question 41: How does the atmospheric pressure change with altitude?
Answer: Pressure at sea level is taken as 76 cm of MERCURY column which is one atmosphere. But his pressure varies with alitude as density of air decreases as we rise up and air becomes rarer. For every 105 m rise in height, pressure decreases by 1 cm of mercury column.
As we go up, the column of air above us gets shorter and thinner, so the weight pressing down decreases. This drop in pressure happens very quickly at first and then more slowly as you reach outer space.
Teacher's Tip: Higher = Thinner air = Lower pressure.
Exam Tip: Mention the specific rate of '1 cm Hg drop per 105 m rise' for a perfect score.

C. Numericals

Question 1: Find the moment of force of 20 N about an axis of rotation at distance 0.5 m from the force.
Answer: F = 20 N perp distance = 0.5 m
Moment of force = Force x perpendicular distance of force from the point of rotation
= F x perp distance
= 20 N x 0.5 m
= 10Nm
To solve this, we simply multiply the push (20 N) by the length of the lever (0.5 m). The result tells us the total turning power being applied to the pivot.
Teacher's Tip: Multiplying by 0.5 is the same as dividing by 2.
Exam Tip: Always show the formula before substituting the numbers to get step-marks.

Question 2: The moment of a force of 25 N about a point is 2.5 N m. Find the perpendicular distance of force from that point
Answer: Moment of force = 2.5 N m
Force applied = 25
perp distance from the point of rotation ?
Moment of force = Force x perp distance
2.5 = 25 x x
therefore x = perp distance = 2.5 / 25 = 1/10 m
=1/10 x 100 = 10cm
Here, we rearrange the formula to find the missing distance by dividing the moment by the force. A moment of 2.5 with a 25 N force means the push was happening just 10 centimeters away.
Teacher's Tip: Distance = Moment / Force.
Exam Tip: Be careful to convert your final answer into centimeters if the question expects a more common unit.

Question 3: A spanner of length 10 cm is used to open a nut by applying a minimum force of 5.0 N. Calculate the moment of force required.
Answer: perp Distance = 10cm = 10/100 = 1/10 m
Force applied on spanner 5 N
therefore Moment of force = F x perp distance
= 5 x 1/10 = 0.5 nm
(Note: The unit is Nm for Newton-meters). We must first convert the length of the spanner into meters to match the S.I. unit system. Then, multiplying the small force by the small distance gives us the rotational effect.
Teacher's Tip: Standard S.I. units require meters (m), not centimeters (cm)!
Exam Tip: Always check if you need to divide by 100 to get meters before starting your math.

Question 4: A wheel of diameter 2 m can be rotated about an axis passing through its centre by a moment of force equal to 2.0 N m. What minimum force must be applied on its rim?
Answer: therefore Diameter = 2m
radius OP = l m
therefore perp distance OP = 1 m
Moment of force = 2.0 N m
F x perp distance OP = moment of force
F x 1 = 2.0
F = 2/1 = 2N
Since theRim of the wheel is one radius away from the center, the distance is exactly 1 meter. Applying a force of 2 Newtons at this distance produces the required turning effect of 2.0 Nm.
Teacher's Tip: For a wheel, the Rim distance = Radius (half of Diameter).
Exam Tip: Don't use the diameter (2 m) in the formula; always use the radius (1 m) for a central pivot.

Question 5: A normal force of 200 N acts on an area of 0.02 m². Find the pressure in pascal.
Answer: Pressure = Force / Area
= 200N / 0.02m²
= 200 x 100 / 2 = 10,000 N/m² or 10,000 Pascal
This calculation shows how much force is hitting every square meter by dividing the total push by the surface space. Concentrating 200 Newtons onto such a tiny area results in a very high pressure of 10,000 Pascals.
Teacher's Tip: To remove decimals from the bottom, multiply the top by 100.
Exam Tip: 1 N/m² = 1 Pascal; use either name for the unit.

Question 6: Find the thrust required to exert a pressure of 50000 pascals on an area of 0.05 m² ?
Answer: Pressure = Force / Area
Force = Pressure x Area
= 50,000 x 0.05 = 2,500 Newton
To find the total push (thrust), we multiply the "concentrated push" (pressure) by the total size of the surface. A pressure of 50,000 Pa over a small area requires a total force of 2,500 Newtons.
Teacher's Tip: Thrust = P ×s A.
Exam Tip: Thrust is a force, so the final unit must be Newtons (N).

Question 7: Find the area of a body which experiences a pressure of 50000 Pa by a thrust of 100 N ?
Answer: F=100N
P = 50000 Pa
A=?
therefore A = F / P = 100 / 50000 = 1 / 500
A = 10 / 5000 = 2 / 1000 = 2 ×s 10^-3 m²
By rearranging the formula, we find that to get such a high pressure from a small 100 N force, the surface area must be very tiny. The resulting area is just two thousandths of a square meter.
Teacher's Tip: Area = Force / Pressure.
Exam Tip: Use scientific notation 10^-3 for very small decimal answers to look more professional.

Question 8: Calculate the pressure in pascal exerted by a force of 300 N acting normally on an area of 30 cm².
Answer: F = 300 N
A = 30 cm² = 30 / (100 ×s 100) m² = 30 ×s 10^-4 m²
P = F / A = 300 / (30/10000) = 300 x 10000 / 30
= 100000 = 10⁵ Pa
We must convert square centimeters into square meters by dividing by 10,000 before calculating the pressure. This ensures our final answer is in the correct S.I. unit of Pascals.
Teacher's Tip: 1 m² = 10,000 cm². Always divide by 10,000 for area conversion.
Exam Tip: The conversion of cm² to m² is the most common place where students lose marks; be careful!

Question 9: How much thrust will be required to exert a pressure of 20,000 Pa on an area of 1 cm².
Answer: Pressure = 20,000 Pa
Area = 1 cm² = 10^-4 m²
Force = Pressure x area
= 20,000 x 10^-4 = 2 Newton
For a tiny area of one square centimeter, you only need a small 2 Newton force to reach a high pressure of 20,000 Pa. This is because the force is very concentrated on such a small spot.
Teacher's Tip: 10^-4 is the same as 1/10,000.
Exam Tip: Check your math by multiplying the number of zeros; 20,000 has four zeros, which cancel out with 10^-4.

Question 10: The base of a container measures 15 cm ×s 20 cm. It is placed on a table top. If the weight of the container is 60 N, what is the pressure exerted by the container on the table top ?
Answer: Area of the base of container
A = 15 cm ×s 20 cm = 300 cm²
= 300 / 10000 = 3 / 100 m²
Force = weight = 60 N
Pressure exerted = F / A = 60 / (300/10000)
= 60 ×s 10000 / 300 = 600000 / 300 = 2000 Pa
First, we find the area in cm² and then convert it to m². Dividing the 60 N weight by this area gives us a total pressure of 2,000 Pascals on the table surface.
Teacher's Tip: Always find "Area First" in these story-based problems.
Exam Tip: Clearly state your area conversion steps to show the examiner you understood the units.

Question 11: Calculate the pressure exerted on a surface of 0.5 m² by a thrust of 100 kgf.
Answer: P = ?
Thrust (F) = 100 kgf
A = 0.5 m²
P = Thrust / Area = 100 / 0.5 = 1000 / 5 = 200
P = 200 kgf m^-2
Using gravitational units, we divide the 100 kgf weight by the half-square-meter area. This results in a pressure value where each full square meter would feel 200 kilograms of force.
Teacher's Tip: Dividing by 0.5 is just doubling the number.
Exam Tip: Keep the units as kgf m^-2 if the thrust is given in kgf.

Question 12: A boy weighing 60 kgf stands on a platform of dimensions 2.5 cm x 0.5 cm. What pressure in pascal does he exert ?
Answer: Force = Thrust = Weight = 60 kgf = 60 x 10 = 600 N
Area of platform = 2.5 cm x 0.5 cm
= (2.5 / 100) ×s (0.5 / 100) m = 25 / 1000 ×s 5 / 1000 = 125 / 1,000,000 m²
= 1 / 8000 m²
Pressure P = F / A = 600 N / (1/8000) m² = 600 x 8000 / 1
= 4,800,000
= 4.8 ×s 10⁶ Pa
We first convert the weight to Newtons (600 N) and the tiny platform area to square meters. Because the platform is so small compared to the boy's weight, the resulting pressure is a massive 4.8 million Pascals.
Teacher's Tip: Use scientific notation (10⁶) for large million-level answers.
Exam Tip: Multiply by 10 to convert kgf to N before starting the pressure division.

Question 13: Figure shows a brick of weight 2 kgf and dimensions 20 cm x 10 cm × 5 cm placed in three different positions on the ground. Find the pressure exerted by the brick in each case.
Answer: Weight of brick = Thrust = F = 2 kgf
(a) Area of base = area of top L x B = 20 Cm x 10 = 200 cm²
P = Thrust / A = 2 kgf / 200 cm² = 1 / 100 = 0.01 kgf cm^-2
(b) Area of base = area of top = 5 cm x 10 cm = 50 cm²
Pressure exerted = P = F / A = 2 kgf / 50 cm² = 0.04
P = 0.04 kgf cm^-2
(c) Weight of brick F = 2 kgf. Area of base = L x B = 20 cm x 5 cm = 100 cm²
Pressure exerted = F / A = 2 / 100 = 0.02 kgf cm^-2
The brick exerts three different pressures depending on which side it is lying on. The smallest side (50 cm²) creates the highest pressure, while the largest side (200 cm²) creates the lowest pressure.
Teacher's Tip: Same Brick + Different Side = Different Pressure.
Exam Tip: Calculate the area for each specific orientation (LxB, BxH, LxH) before doing the pressure division.