Forces are all around us in our daily lives, from pushing a door open to riding a bicycle against the wind. When we cycle up a hill or feel the pull of gravity as we come down a slope, we are experiencing different types of forces at work.
What Is a Force?
Force is basically a push or pull that we apply on objects around us. Whenever we want to move something, change how fast it’s going, change its direction, or even change its shape, we need to apply a force.
Think about moving a heavy cardboard box – you can push it, pull it, or even lift it up. In all these cases, you are applying force on the box. Without applying any push or pull, the box would just stay where it is. This shows us that force is needed to make things happen.
The push or pull applied on an object is called force in science. Force always involves at least two objects – one that applies the force and another that receives it. When you push a table with your hand, your hand is applying force on the table.
What Can a Force Do to the Bodies on Which It Is Applied?
Forces can do many different things to objects, and we see these effects in our daily lives all the time. Let’s study what forces can achieve when they act on different objects.
Effects of Forces
Making Objects Move from Rest
- When you kick a football that is lying still on the ground, your foot applies force and the ball starts moving
- A car starts moving when the engine applies force through the wheels
- You can make a book slide across a table by pushing it with your hand
Changing Speed of Moving Objects
- When you apply brakes on a bicycle, you slow it down by applying force
- A cricket bat hits a moving ball and makes it go faster in a different direction
- Pressing the accelerator in a car makes it go faster by applying more force
Changing Direction of Motion
- A hockey stick hits a moving ball and changes its direction
- When you turn the steering wheel of a car, you change the direction the car is moving
- A tennis racket hits a ball coming towards you and sends it back in the opposite direction
Changing Shape of Objects
- When you press an inflated balloon, it changes shape
- Stretching a rubber band changes its shape from short to long
- Rolling chapati dough with a rolling pin flattens it and changes its shape
Force as Interaction
It’s imp to understand that forces result from interaction between two or more objects. When you push a table, both your hand and the table are involved in this interaction. Your hand applies force on the table, and interestingly, the table also applies an equal force back on your hand (though you might not always notice this).
Are Forces an Interaction Between Two or More Objects?
Yes, forces always involve interaction between objects. There cannot be a force without at least two objects being involved. One object applies the force, and another object receives it.
When you think about all the examples of forces around you, you’ll notice that there are always two objects interacting. A person pushing a car involves the person and the car. A magnet attracting iron involves the magnet and the iron piece. Even when you walk, your foot interacts with the ground to create the force that moves you forward.
The SI unit of force is newton, written with a small ‘n’, and its symbol is N. This unit is named after the scientist Isaac Newton who made many imp discoveries about forces and motion.
What Are the Different Types of Forces?
Forces can be classified into two main categories based on whether the objects need to touch each other or not.
Contact Forces
Contact forces are those forces where physical contact is necessary between the objects. The contact can be direct (like using your hands) or indirect (like using a stick or rope).
Muscular Force
Muscular force is the force that comes from the action of muscles in our body. Whenever we walk, run, lift something, push something, jump, or stretch, we are using muscular force.
How Muscular Force Works
- Muscles contract (get shorter) and elongate (get longer) to create force
- When you bend your arm, some muscles contract while others elongate
- This coordinated action of muscles creates the force needed for movement
Examples of Muscular Force in Daily Life
- Walking and running require muscular force from leg muscles
- Lifting your school bag uses muscular force from arm and back muscles
- Even simple actions like writing use small muscular forces in your fingers
Muscular Force in Animals
- Animals, birds, fish, and insects all use muscular forces for movement and survival
- Birds use muscular force to flap their wings and fly
- Fish use muscular force to swim through water
- Even tiny insects use muscular force to crawl and jump
Human Use of Animal Muscular Force
- For thousands of years, humans have used the muscular force of animals for various tasks
- Horses, oxen, and buffaloes have been used to pull carts and plough fields
- Elephants have been used to lift heavy logs and move large objects
- Even today, in many parts of the world, animals help humans with heavy work
Internal Body Functions
Muscular force is not just used for external movements. It plays an imp role in many functions inside our body too:
- Digestion: Muscles in our digestive system push food through the alimentary canal
- Circulation: The heart muscle contracts and expands to pump blood throughout our body
- Breathing: Muscles in our chest help us breathe in and out
- Chewing: Jaw muscles help us chew food and break it down
Friction
Friction is another type of contact force that we encounter everywhere in our daily lives. Have you noticed that when you roll a ball on the ground, it eventually stops on its own? Or when you stop pedaling your bicycle on a flat road, it slows down and comes to a stop? This happens because of friction.
What is Friction
- Friction is the force that comes into play when an object moves or tries to move over another surface
- It always acts in a direction opposite to the direction in which the object is moving or trying to move
- Friction is a contact force because it arises due to two surfaces being in contact
Why Friction Occurs
- Friction occurs due to irregularities in the surfaces that are in contact
- Even surfaces that appear very smooth to our eyes have many tiny bumps and valleys
- When two surfaces are placed in contact, these irregularities lock into each other
- This locking opposes any effort to move one surface over the other
Factors Affecting Friction
- Nature of surfaces: Rough surfaces create more friction than smooth surfaces
- Force pressing surfaces together: More pressure between surfaces increases friction
- Area of contact: Though this might seem obvious, the area of contact has less effect than you might think
Types of Surfaces and Friction
Different surfaces create different amounts of friction:
| Surface Type | Friction Level | Examples |
|---|---|---|
| Very smooth | Low friction | Ice, polished marble |
| Moderately smooth | Medium friction | Wood, ceramic tiles |
| Rough | High friction | Sandpaper, rough concrete |
| Very rough | Very high friction | Sand, gravel |
Friction in Different Media
Friction doesn’t just occur on solid surfaces. It also acts when objects move through liquids and gases:
- Air resistance: When planes, cars, or trains move through air, they experience friction
- Water resistance: Ships, boats, and swimmers experience friction when moving through water
- Design considerations: Vehicles are designed with special shapes to reduce friction and move more efficiently
Non-contact Forces
Non-contact forces are those forces whose effects can be experienced even when objects are not touching each other. These forces can act across empty space and are quite common in nature.
Magnetic Force
We have learned about magnets and their properties earlier. Magnets can attract objects made of magnetic materials like iron, nickel, and cobalt. They can also attract or repel other magnets.
Properties of Magnetic Force
- Like poles (North-North or South-South) repel each other
- Unlike poles (North-South) attract each other
- Magnetic force can act through non-magnetic materials like paper, plastic, or glass
- The strength of magnetic force decreases as the distance between magnets increases
Demonstrating Magnetic Force
When you place two ring magnets with like poles facing each other, you can see one magnet floating above the other. If you try to push the upper magnet down, you can feel the force pushing back against your hand. This shows that magnets can exert force without touching.
Uses of Magnetic Force
- Magnetic force is used in many devices like electric motors, generators, and speakers
- Magnetic compasses use Earth’s magnetic field to show direction
- MRI machines in hospitals use strong magnetic fields to create images of inside our body
- Magnetic force is used in maglev trains that float above tracks
Electrostatic Force
Electrostatic force is the force exerted by objects that have electric charges on them. This might sound complicated, but it’s actually quite easy to observe and understand.
Creating Static Charges
When certain materials are rubbed together, they acquire electrical charges on their surfaces. These are called static charges because they don’t move by themselves. The object that gets these charges is called a charged object.
Observing Electrostatic Force
- When you rub a plastic scale with a piece of cloth, it becomes charged
- If you then bring this charged scale close to small pieces of paper, the paper pieces jump towards the scale
- This happens because the charged scale exerts an electrostatic force on the uncharged paper pieces
Properties of Electric Charges
There are two types of electric charges: positive and negative.
- Like charges repel: Two objects with the same type of charge push each other away
- Unlike charges attract: Objects with different types of charges pull towards each other
- Uncharged objects: Charged objects can attract uncharged objects made of certain materials
Examples of Electrostatic Force
- When you rub a balloon on your hair and then bring it close to your hair, the hair strands move towards the balloon
- Sometimes when you take off a woolen sweater, you hear crackling sounds – this is due to static charges
- Lightning is a dramatic example of electrostatic force in nature
Connection to Electric Current
When static charges start moving, they create electric current in electrical circuits. This is the same current that makes bulbs glow, creates heating effects in appliances, and produces magnetic effects in electromagnets.
Gravitational Force
Gravitational force is probably the most familiar force to us, even though we might not always think about it. This is the force that keeps us on the ground and makes things fall when we drop them.
What is Gravitational Force
- Gravitational force is the force with which the Earth attracts objects towards itself
- This force is also called force of gravity or simply gravity
- It is always an attractive force – it always pulls objects towards the Earth’s center
Characteristics of Gravitational Force
- Gravitational force acts on all objects, regardless of their size, shape, or material
- It acts without any contact between the Earth and the object
- The force always acts vertically downward towards the center of the Earth
- Gravitational force is always attractive, unlike magnetic or electrostatic forces which can be repulsive too
Vertical Motion Under Gravity
When objects move under the influence of gravitational force, they follow predictable patterns:
Dropping Objects
- When you drop an object from a height, it falls straight down
- The object starts from rest and speeds up as it falls
- All objects fall at the same rate in the absence of air resistance
Throwing Objects Upward
- When you throw an object straight up, it first moves up while slowing down
- At the highest point, it stops for a moment
- Then it starts falling back down, speeding up as it comes down
- The path is always straight and vertical
Why Objects Slow Down Going Up
When you throw something upward, gravitational force acts downward (opposite to the motion), causing the object to slow down until it stops. Then the same gravitational force makes it fall back down, causing it to speed up.
Weight and Its Measurement
Weight is a concept that is closely related to gravitational force, and it’s imp to understand the difference between weight and mass.
What is Weight?
Weight is the force with which the Earth pulls an object towards itself. When we say something is heavy, we’re really talking about how strongly the Earth attracts it.
Important Points about Weight
- Weight is a force, so it is measured in newtons (N)
- Weight depends on gravitational force, which can vary slightly from place to place
- The same object will have different weights on different planets
- Weight is always directed towards the center of the Earth
Difference Between Weight and Mass
This is a very imp concept that many people find confusing:
| Aspect | Mass | Weight |
|---|---|---|
| Definition | Amount of matter in an object | Gravitational force on an object |
| Unit | Kilogram (kg) or gram (g) | Newton (N) |
| Variation | Remains same everywhere | Changes with gravitational field |
| Measurement tool | Beam balance | Spring balance |
| Nature | Scalar quantity | Vector quantity (has direction) |
Mass
- Mass is the amount of matter in an object
- It remains the same whether you are on Earth, Moon, or anywhere else
- Mass is measured using a beam balance by comparing with known masses
Weight
- Weight is the gravitational force acting on an object
- It changes when gravitational force changes (like on different planets)
- Weight is measured using a spring balance
Weight on Different Planets
Here’s how the weight of a 1 kg object would vary on different celestial bodies:
| Location | Mass | Weight |
|---|---|---|
| Earth | 1 kg | 10 N |
| Moon | 1 kg | 1.6 N |
| Mars | 1 kg | 3.8 N |
| Venus | 1 kg | 9 N |
| Jupiter | 1 kg | 25.4 N |
This table shows that while mass remains constant, weight changes dramatically based on the gravitational pull of each celestial body.
Measuring Weight with Spring Balance
A spring balance is a device used to measure weight (force). It works on a simple principle:
How Spring Balance Works
- When you hang an object from the hook, the spring stretches
- The amount of stretching is proportional to the weight of the object
- A scale on the balance shows the weight in newtons
- Usually, there’s also a scale showing mass in grams (calculated assuming Earth’s gravity)
Using Spring Balance Properly
- Never exceed the maximum weight limit of the balance
- Read the scale carefully at eye level to avoid errors
- Make sure the balance is hanging freely and not touching anything
- Check that the pointer reads zero when nothing is hanging from it
Everyday Use of Weight and Mass
In daily life, we often use the terms weight and mass interchangeably, which can be confusing:
- When we say “the weight of this bag is 5 kg,” we’re actually talking about its mass
- Scientifically, we should say “the mass of this bag is 5 kg” or “the weight of this bag is 50 N”
- For practical purposes on Earth, this confusion doesn’t matter much
- But in scientific work, it’s imp to use the correct terms with correct units
Floating and Sinking
When we place objects in water, some float while others sink to the bottom. This happens because of the interaction between gravitational force and another force called buoyant force.
Understanding Buoyant Force
When you try to push an empty plastic bottle into water, you can feel it pushing back against your hand. This upward push that you feel is called buoyant force or upthrust.
What is Buoyant Force
- Buoyant force is the upward force applied by a liquid on objects placed in it
- All liquids apply this upward force on objects immersed in them
- The force is always directed upward, opposite to the direction of gravitational force
Why Do Some Objects Float and Others Sink?
Whether an object floats or sinks depends on the balance between two forces:
Forces Acting on Objects in Liquids
- Gravitational force: Acts downward, trying to pull the object down
- Buoyant force: Acts upward, trying to push the object up
Conditions for Floating and Sinking
- If gravitational force > buoyant force → Object sinks
- If gravitational force = buoyant force → Object floats
- If gravitational force < buoyant force → Object rises to surface
Factors Affecting Buoyant Force
The buoyant force depends on several factors:
- Volume of liquid displaced: Larger objects displace more liquid and experience greater buoyant force
- Density of the liquid: Denser liquids provide greater buoyant force
- Depth of immersion: How much of the object is underwater affects the buoyant force
Archimedes’ Principle
Archimedes, a famous Greek scientist, discovered an imp principle about buoyant force:
Archimedes’ Principle States
When an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid it displaces.
Applications of Archimedes’ Principle
- If weight of displaced liquid < weight of object → Object sinks
- If weight of displaced liquid = weight of object → Object floats
- If weight of displaced liquid > weight of object → Object rises until equilibrium
Practical Examples
Why Ships Float
- Large ships are made of steel, which is denser than water
- However, ships have hollow spaces filled with air
- The overall density of the ship (including air spaces) is less than water
- Therefore, ships float even though they’re made of materials denser than water
Why Some Rocks Float
- Pumice rock is formed during volcanic eruptions
- When lava with lots of gas and water vapor cools quickly, it traps tiny air bubbles
- These air pockets make pumice less dense than water
- Therefore, pumice rocks can float on water despite being made of rock material
Floating and Sinking
Objects behave differently when placed in liquids – some float on the surface while others sink to the bottom. This interesting phenomenon can be explained by understanding the forces acting on objects in liquids.
Forces Acting on Objects in Liquids
When any object is placed in a liquid, two main forces act on it:
Gravitational Force (Downward)
- The Earth pulls the object downward with gravitational force
- This force tries to make the object sink
- The strength of this force depends on the mass of the object
Buoyant Force (Upward)
- The liquid pushes the object upward with buoyant force
- This force tries to make the object float
- The strength of this force depends on how much liquid the object pushes away
Understanding Buoyant Force Better
What Creates Buoyant Force
- When an object is placed in liquid, it pushes some liquid out of the way
- The liquid pushes back with an upward force
- This upward push from the liquid is called buoyant force or upthrust
Observing Buoyant Force
- When you try to push an empty bottle underwater, you feel resistance
- The bottle seems to want to pop back up to the surface
- This upward push you feel is the buoyant force of water
Conditions for Floating and Sinking
The behavior of objects in liquids depends on which force is stronger:
| Condition | Result | Example |
|---|---|---|
| Weight > Buoyant force | Object sinks | Stone in water |
| Weight = Buoyant force | Object floats | Wood piece in water |
| Weight < Buoyant force | Object rises up | Air-filled balloon underwater |
Real-Life Applications
Ship Design
- Ships are made of steel, which is much denser than water
- However, ships have large hollow areas filled with air
- These air spaces reduce the overall density of the ship
- The ship displaces a large volume of water, creating enough buoyant force to keep it afloat
Life Jackets
- Life jackets are filled with materials that trap air
- This makes the overall density of a person wearing a life jacket less than water
- Therefore, the person floats even if they cannot swim
Hot Air Balloons
- Hot air balloons work on the same principle
- Hot air inside the balloon is less dense than cool air outside
- This creates enough buoyant force to lift the balloon and basket
Questions and Answers
What is the difference between push and pull forces?
- Push force is when you apply force away from your body to move an object in the direction opposite to yourself, like pushing a door to open it or pushing a cart forward from behind
- Pull force is when you apply force towards your body to move an object in your direction, like pulling a rope to draw water from a well or pulling a drawer to open it
- Both push and pull are types of forces that can produce the same effects like changing motion, speed, direction, or shape of objects
- The main difference is the direction in which force is applied relative to the person applying it, but both can achieve similar results in moving objects
- Examples of push forces include pressing a button, pushing a swing, or kicking a ball, while examples of pull forces include opening a door towards yourself, pulling a suitcase, or drawing a bow string
Why do we feel lighter when we are in water?
- When we enter water, our body displaces some water and pushes it out of the way, creating space for our body in the water
- According to Archimedes’ principle, water pushes back on our body with an upward force called buoyant force that is equal to the weight of water displaced
- This upward buoyant force acts opposite to the downward gravitational force (our weight), partially canceling out the effect of gravity on our body
- The net downward force we experience becomes less than our actual weight, making us feel lighter and more able to move easily in water
- This is why people can float in water and why it’s easier to lift someone who has fallen into a swimming pool compared to lifting them on land
- The same principle explains why heavy ships can float on water despite being made of materials much denser than water
How is friction both helpful and harmful?
- Friction is helpful because it allows us to walk without slipping, helps car brakes stop vehicles safely, enables us to write with pencils on paper, and helps us grip objects without them sliding out of our hands
- Friction is also essential for many sports activities like running, where athletes need grip between their shoes and the track, or in cricket where bowlers need friction to spin the ball
- However, friction can be harmful because it causes wear and tear in machine parts, reduces efficiency of engines by converting useful energy into heat, causes vehicle tires to wear out, and creates resistance that makes vehicles consume more fuel
- In machines, friction leads to energy loss as heat, requiring regular maintenance and lubrication to reduce its negative effects, and can cause moving parts to get damaged over time
- Engineers try to reduce harmful friction using lubricants like oil and grease while maintaining beneficial friction where it’s needed for safety and functionality
Why do all objects fall towards Earth and not towards each other?
- All objects do actually attract each other with gravitational force, but the Earth is so much more massive than everyday objects that its gravitational pull is overwhelmingly stronger
- Earth’s mass is approximately 6 × 10²⁴ kilograms, which is billions of times more massive than any object we encounter in daily life, making its gravitational effect dominant
- The gravitational force between two objects depends on both their masses – the more massive the objects, the stronger the attraction between them
- While a book and a table do attract each other gravitationally, this force is so incredibly small compared to Earth’s pull that we cannot observe or feel it
- Earth’s gravitational field extends far beyond its surface, affecting all objects within this field and pulling them towards Earth’s center, which is why everything falls downward
- Only objects with very large masses like planets, stars, and moons have gravitational effects that we can easily observe, such as how the Moon’s gravity causes ocean tides on Earth
What happens to weight and mass when you go to the Moon?
- Your mass remains exactly the same on the Moon as it is on Earth because mass is the amount of matter in your body, and this doesn’t change based on location
- However, your weight becomes much less on the Moon because weight is the gravitational force acting on your mass, and Moon’s gravity is only about 1/6th as strong as Earth’s gravity
- If you weigh 60 kg (actually 600 N) on Earth, you would still have a mass of 60 kg on the Moon, but your weight would be only about 100 N (equivalent to 10 kg on Earth’s surface)
- This means you could jump much higher and farther on the Moon, carry heavier objects easily, and move around with much less effort than on Earth
- Astronauts on the Moon could perform feats that would be impossible on Earth, like jumping several meters high or carrying equipment that would be too heavy to lift on Earth
- This difference between mass and weight is why scientists are careful to use the correct units – mass in kilograms and weight in newtons
How do different surfaces affect the amount of friction?
- Smooth surfaces like polished marble, ice, or glass create very little friction, making it easy for objects to slide but difficult to stop or change direction quickly
- Rough surfaces like sandpaper, concrete, or gravel create much more friction, making it harder to start sliding but easier to stop and control movement
- The amount of friction depends on the microscopic bumps and valleys on surfaces – even apparently smooth surfaces have tiny irregularities that create friction
- Different materials have different coefficients of friction, meaning some material combinations naturally create more friction than others regardless of surface smoothness
- Wet surfaces generally have less friction than dry surfaces, which is why roads become slippery when it rains and why we need to be more careful when walking on wet floors
- Engineers and designers consider friction when choosing materials for different applications – using low-friction materials for bearings and high-friction materials for brake pads and shoe soles
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