Chapter 6 – How Forces Affect Motion (Part 1A)

6.1 The Concept of Force

(CBSE Class 9 | NCERT-Based | Topper Notes | Professional Edition)

Chapter Introduction

"Class 9 Science Chapter 6 How Forces Affect Motion One Shot Notes | Learn with Afsa"

Every action we perform in our daily life involves the application of a force. Whether we open a door, lift a school bag, ride a bicycle, kick a football, or simply push a chair, we are applying a force. Sometimes the force makes an object move, while at other times it stops a moving object, changes its speed, alters its direction, or even changes its shape.

Have you ever wondered why a football starts moving when it is kicked? Why does a moving bicycle stop when the brakes are applied? Why is it easier to push an empty trolley than a loaded one?

The answer to all these questions lies in one important concept of Physics—Force.

Force is one of the most fundamental ideas in science because it explains how and why objects move or change their motion. Understanding force also helps us understand many natural phenomena, from the motion of planets around the Sun to the movement of vehicles on roads.

In this chapter, you will explore the concept of force, its different types, the role of friction, balanced and unbalanced forces, and Newton's Laws of Motion, which form the foundation of classical mechanics.

Learning Outcomes

After studying this chapter, you will be able to:

✔ Define force using the NCERT concept.

✔ Explain how force affects the motion of an object.

✔ Distinguish between contact and non-contact forces.

✔ Understand balanced and unbalanced forces.

✔ Describe the role of friction in everyday life.

✔ Measure force using a spring balance.

✔ Explain Newton's First and Second Laws of Motion.

✔ Apply the formula F = ma to solve numerical problems.

✔ Relate scientific concepts to real-life situations.

6.1 The Concept of Force

Think About It

Imagine the following situations:

You kick a football, and it starts moving.
You push a shopping trolley in a supermarket.
You pull a drawer to open it.
You squeeze a sponge, and its shape changes.
You apply brakes on a bicycle, and it slows down.

Although these actions appear different, they all involve one common factor—Force.

A force is responsible for producing a change in an object's motion or shape.

What is Force?

NCERT Definition

Force is a push or a pull acting on an object that can change its state of rest, state of motion, speed, direction of motion, or shape.

This definition highlights two important ideas:

A force is always a push or a pull.
A force has the ability to produce a change in an object.

Easy Definition

Force is simply a push or a pull applied to an object.

Although this definition is simple, remember that not every push or pull produces motion. Sometimes the applied force is balanced by another force, and no visible change occurs.

Understanding Force Through Daily Life

We experience force throughout the day, often without realizing it.

Activity	Force Applied
Opening a door	Push or Pull
Lifting a school bag	Upward muscular force
Kicking a football	Muscular force
Pulling a suitcase	Pulling force
Writing with a pencil	Muscular force with friction
Stretching a rubber band	Pulling force
Applying brakes	Frictional force
Closing a window	Push

These examples show that force is an essential part of almost every activity we perform.

Characteristics of Force

Every force has certain important characteristics.

1. Force is a Push or Pull

Every force originates from either pushing or pulling an object.

Examples

Pushing a table across the floor
Pulling a rope during a tug of war

2. Force Can Change Motion

Force can make a stationary object move or stop a moving object.

Example

A football lying on the ground remains at rest until someone kicks it.

3. Force Can Change Shape

Some forces do not move an object but change its shape.

Examples

Squeezing a sponge
Pressing clay
Stretching a rubber band

4. Force Can Act in Different Directions

A force may act:

Upward
Downward
Left
Right
Forward
Backward

The direction in which a force acts is just as important as its strength.

Real-Life Connection

Suppose two students are carrying a heavy table.

If both lift it together, the table rises easily.

If one student suddenly lets go, the table tilts downward.

This happens because the forces acting on the table are no longer balanced.

This simple example shows that both the magnitude and direction of force determine its overall effect.

Why is Force Important?

Force is responsible for almost every motion we observe around us.

Without force:

Vehicles would not start moving.
We would not be able to walk.
A ball would never change its direction.
Machines would not work.
Objects could not be lifted or pushed.

In simple words, force is the cause of every change in motion.

Everyday Science

When you walk, your feet push the ground backward. In return, the ground exerts an equal forward force on your feet, allowing you to move ahead.

This interaction between your feet and the ground demonstrates that force always acts through interaction between two objects.

Did You Know?

🚀 Even astronauts experience forces in space. Although they appear to float, gravitational force still acts on them. They remain in continuous free fall while orbiting the Earth, creating the sensation of weightlessness.

Think Like a Scientist

Question: Can an object move without applying any force?

Answer: Yes, if it is already moving and no unbalanced external force acts on it, it can continue moving with constant velocity. This idea was first explained by Galileo Galilei and later formalized by Sir Isaac Newton in the First Law of Motion.

Common Misconception

❌ "Force is always needed to keep an object moving."

✅ Correct Concept:
Force is not required to maintain the motion of an object. A force is required only to change its state of motion (speed or direction). This concept forms the basis of Newton's First Law of Motion.

Key Points to Remember

Force is a push or a pull.
Force arises due to the interaction between two objects.
A force can change the state of rest, state of motion, speed, direction, or shape of an object.
Every force has both magnitude and direction, making it a vector quantity.
Force is measured in Newton (N).

NCERT Keywords

Force, Push, Pull, State of Rest, State of Motion, Interaction, Magnitude, Direction, Vector Quantity, Newton (N)

Chapter Quote

"Force does not create motion; it changes motion."

This single idea is the foundation of the entire chapter.

How Does Force Affect an Object?

Applying a force does not always produce the same result. The effect of a force depends on:

The magnitude (strength) of the force.
The direction in which the force is applied.
The mass of the object.
Whether other forces acting on the object are balanced or unbalanced.

When a force acts on an object, it may change its motion, speed, direction, or shape. In some situations, however, a force may produce no visible change if it is balanced by another force.

Major Effects of Force

A force can produce one or more of the following effects.

1. Force Can Start the Motion of an Object

A stationary object remains at rest until an unbalanced force acts on it.

Examples

Kicking a football lying on the ground.
Pushing a shopping trolley to make it move.
Pulling a drawer to open it.

Scientific Explanation

When the applied force is greater than the opposing forces (such as friction), the object begins to move.

Key Idea: An object at rest requires an unbalanced force to start moving.

2. Force Can Stop a Moving Object

A moving object can be brought to rest when a force acts opposite to its direction of motion.

Examples

Applying brakes on a bicycle.
A goalkeeper stopping a moving football.
Catching a cricket ball with your hands.

Scientific Explanation

The stopping force reduces the object's velocity. If the force continues, the velocity becomes zero, and the object comes to rest.

Key Idea: A force opposite to the direction of motion slows down or stops an object.

3. Force Can Change the Speed of an Object

A force may increase or decrease the speed of a moving object.

(a) Increasing Speed

Examples

Pedalling a bicycle faster.
Pushing a swing repeatedly.
Hitting a hockey ball harder.

(b) Decreasing Speed

Examples

Applying brakes to a car.
Rolling a ball on grass, where friction slows it down.

Scientific Explanation

If the force acts in the direction of motion, the speed increases. If it acts opposite to the direction of motion, the speed decreases.

Key Idea: Force changes the velocity of an object by increasing or decreasing its speed.

4. Force Can Change the Direction of Motion

A force can alter the path along which an object moves.

Examples

A cricket bat changes the direction of a moving ball.
A tennis player returns a serve.
Steering a bicycle around a corner.

Scientific Explanation

Although the object may continue moving, the direction of its velocity changes due to the applied force.

Key Idea: Changing direction is also a change in motion.

5. Force Can Change the Shape or Size of an Object

Not all forces produce motion. Some forces only deform an object.

Examples

Squeezing a sponge.
Stretching a rubber band.
Pressing clay into different shapes.
Compressing a spring.

Scientific Explanation

The applied force changes the arrangement of particles inside the material, causing a temporary or permanent change in shape.

Key Idea: A force can deform an object even if it does not move.

Can More Than One Effect Occur at the Same Time?

Yes.

A single force may produce multiple effects simultaneously.

Example: Kicking a Football

When a player kicks a football:

It starts moving.
Its speed increases.
Its direction may change.
The foot and the ball are briefly compressed at the point of contact.

Thus, one force can produce several effects at the same time.

Everyday Applications of Force

Situation	Effect of Force
Opening a door	Starts motion
Applying brakes	Stops motion
Pedalling a bicycle	Increases speed
Steering a car	Changes direction
Stretching a rubber band	Changes shape
Kneading dough	Changes shape
Catching a ball	Stops motion

Force Does Not Always Cause Motion

Many students think that every force makes an object move. This is not always true.

Example

Two equally strong teams pull a rope in opposite directions during a tug of war.

Although both teams apply force, the rope does not move because the forces balance each other.

This situation will be studied later under Balanced Forces.

Important Note: A force produces a change in motion only when it is unbalanced.

Real-Life Connection

Imagine trying to push a heavy cupboard.

If you push gently, it may not move because friction balances your force.
If you push harder, your force becomes greater than friction, and the cupboard starts moving.

This shows that the effect of a force depends on both its magnitude and the opposing forces acting on the object.

Did You Know?

💡 Rocket launches are possible because the engines produce a huge upward force (thrust) that overcomes the downward gravitational force. Once the upward force becomes greater than the weight of the rocket, it lifts off from the ground.

Common Misconceptions

❌ A force always makes an object move.

✅ A force may not produce motion if it is balanced by another force.

❌ Stopping an object means no force is acting on it.

✅ An object stops because a force acts opposite to its motion.

❌ Only moving objects experience force.

✅ Objects at rest also experience forces, such as the weight of a book resting on a table.

Exam Tips

⭐ Learn all five effects of force with at least one example each.

⭐ In descriptive answers, always mention that an unbalanced force is required to change the state of motion.

⭐ Remember: Changing speed and changing direction are both examples of changing the object's motion.

Quick Revision

A force can start the motion of an object.
A force can stop a moving object.
A force can increase or decrease the speed of an object.
A force can change the direction of motion.
A force can change the shape or size of an object.
A force does not always produce motion; balanced forces may result in no visible change.

NCERT Keywords

State of Rest • State of Motion • Speed • Direction • Shape • Deformation • Unbalanced Force • Friction • Velocity

Chapter Insight

Force is not judged by how hard we push or pull, but by the change it produces in an object.

6.1.1 Magnitude and Direction of Force

In everyday life, we often describe a force as strong, weak, large, or small. However, in science, merely saying that a force is "strong" is not enough. To describe a force completely, we must know:

How much force is applied? (Magnitude)
In which direction is the force applied? (Direction)

Both these quantities are essential because changing either one can completely change the effect of the force on an object.

Key Idea: A force is completely described only when both its magnitude and direction are known.

Magnitude of Force

NCERT Definition

The magnitude of a force is the measure of its strength or size.

It tells us how large or how small the applied force is.

Easy Definition

Magnitude of force means how strong or how weak a force is.

A larger magnitude produces a greater effect, provided all other conditions remain the same.

Everyday Examples

Situation	Magnitude of Force
Lifting a pencil	Small force
Pushing a chair	Moderate force
Moving a loaded cupboard	Large force
Launching a rocket	Extremely large force

Why Does Magnitude Matter?

Imagine two students trying to push the same table.

One student pushes gently.
The other pushes with much greater force.

The student applying the larger force is more likely to move the table because the greater force can overcome friction more effectively.

Conclusion:
The greater the magnitude of the applied force, the greater its ability to change an object's motion.

Direction of Force

NCERT Definition

The direction of force specifies the direction in which the force acts on an object.

Easy Definition

The direction of force tells us where the force is acting.

Examples of directions include:

Upward
Downward
Left
Right
Forward
Backward

Everyday Examples

Opening a Drawer

To open a drawer, you pull it towards yourself.

Closing a Door

You usually push it away from yourself.

Lifting a School Bag

The force acts upward against gravity.

Dropping a Ball

Gravity pulls the ball downward toward the Earth.

Why is Direction Important?

Consider two students pushing a box.

Case 1: Same Direction

Student A pushes 20 N towards the east.
Student B also pushes 20 N towards the east.

The forces work together, making it easier to move the box.

Case 2: Opposite Directions

Student A pushes 20 N towards the east.
Student B pushes 20 N towards the west.

Although both apply equal forces, the box does not move because the forces cancel each other.

Conclusion:
The same magnitude of force can produce different results depending on its direction.

Force is a Vector Quantity

A physical quantity is classified as either a scalar or a vector.

Scalar Quantity

A scalar has only magnitude.

Examples:

Mass
Time
Temperature
Distance

Vector Quantity

A vector has both magnitude and direction.

Examples:

Force
Velocity
Displacement
Acceleration

Remember: Force is a vector quantity because it cannot be described completely without specifying its direction.

Representing Force

Scientists represent force using an arrow.

Arrow Length → Magnitude (Strength of the force)

Arrow Head → Direction (Where the force acts)

For example:

→ A short arrow represents a small force.

⟶⟶⟶ A longer arrow represents a larger force.

The arrowhead always points in the direction of the applied force.

Real-Life Connection

Imagine pushing a shopping trolley.

If you push it forward, it moves ahead.
If you push it sideways, it changes direction.
If you push it backward, it slows down or moves back.

Even though the same force may be applied, changing its direction changes the outcome.

SI Unit of Force

To compare forces accurately, scientists use a standard unit.

SI Unit

The SI (International System of Units) unit of force is the newton.

Symbol: N

Named After

The unit newton (N) is named in honour of Sir Isaac Newton, the English mathematician and physicist who formulated the Laws of Motion and made fundamental contributions to classical mechanics.

One Newton

One newton is the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg.

This definition is based on Newton's Second Law of Motion and will become clearer later in the chapter.

Everyday Values of Force

Situation	Approximate Force
Pressing a keyboard key	Very small
Closing a notebook	Small
Pushing a bicycle	Moderate
Moving a car	Large
Rocket launch	Extremely large

These examples help compare relative magnitudes; exact values depend on the situation.

Did You Know?

💡 Every time you stand on the ground, gravity pulls you downward, while the ground pushes upward with an equal support force. This is why you remain standing instead of sinking into the Earth.

Common Mistakes

❌ Magnitude alone completely describes a force.

✅ A force is completely described only by magnitude and direction together.

❌ Force is a scalar quantity.

✅ Force is a vector quantity because it has both magnitude and direction.

❌ The SI unit of force is kilogram.

✅ Kilogram (kg) is the SI unit of mass, while newton (N) is the SI unit of force.

Memory Trick

Force = M + D

M → Magnitude (How much?)

D → Direction (Which way?)

If either is missing, the description of the force is incomplete.

Exam Tips

⭐ Always write N (capital letter) for the symbol of newton.

⭐ In theory questions, mention that force is a vector quantity because it has both magnitude and direction.

⭐ Do not confuse mass (kg) with force (N).

Quick Revision

Magnitude tells us the strength of a force.
Direction tells us where the force acts.
Force is a vector quantity.
Force is represented by an arrow.
Arrow length shows the magnitude.
Arrowhead shows the direction.
The SI unit of force is newton (N).

NCERT Keywords

Magnitude • Direction • Vector Quantity • Scalar Quantity • Newton (N) • SI Unit • Force

Chapter Insight

A force is not completely known unless both its magnitude and direction are specified.

🔎 Improvement Suggestion (Book Quality)

Ek chhota sa point aur add karna chapter ko aur professional bana dega:

Characteristics of Force

Force is a vector quantity.
Force always acts due to the interaction between two objects.
A force cannot exist in isolation; there is always an object applying the force and another object on which it acts.
Force may or may not change the state of motion depending on whether it is balanced or unbalanced.

6.2 Types of Force (Contact & Non-contact Forces)

Introduction

Not all forces act in the same way. Some forces require physical contact between two objects, while others can act from a distance without any contact.

For example, you must touch a football to kick it, but a magnet can attract an iron nail without touching it. Similarly, the Earth pulls every object towards itself through gravity, even though there is no physical contact.

Based on how they act, forces are broadly classified into Contact Forces and Non-contact Forces.

Key Idea: The classification of force depends on whether physical contact between objects is necessary or not.

Types of Force

                     FORCE
                       │
          ┌────────────┴────────────┐
          │                         │
   Contact Forces           Non-contact Forces
          │                         │
 ┌────────┴────────┐      ┌─────────┼─────────┐
 │                 │      │         │         │
Muscular Force  Friction  Gravitational Magnetic Electrostatic

A. Contact Forces

Definition (NCERT-Based)

A contact force is a force that acts only when two objects are in direct physical contact with each other.

Without touching, a contact force cannot act.

Characteristics of Contact Forces

✔ Physical contact is necessary.

✔ The force disappears when contact ends.

✔ Common in everyday activities.

✔ Produced through pushing, pulling, rubbing, lifting, or pressing.

Types of Contact Forces

1. Muscular Force

Definition

Muscular force is the force produced by the muscles of humans or animals.

It is the most common force we use in daily life.

Examples

Lifting a school bag.
Kicking a football.
Pulling a bucket from a well.
Writing with a pen.
Opening a door.
Climbing stairs.

Scientific Explanation

Our muscles contract and relax to produce force. This force is transmitted to the object through direct contact.

Without muscles, most daily activities would not be possible.

Everyday Applications

Activity	Muscular Force Used
Walking	Legs push against the ground
Cycling	Legs rotate the pedals
Rowing a boat	Hands pull the oars
Carrying luggage	Arms lift the load

Did You Know?

💡 Even animals use muscular force. A horse pulling a cart, an elephant lifting logs, and a bird flapping its wings are all examples of muscular force.

2. Frictional Force

Definition (NCERT-Based)

Friction is the force that opposes the relative motion or the tendency of motion between two surfaces in contact.

Easy Definition

Friction is a force that always acts opposite to the direction of motion.

Why Does Friction Occur?

Even smooth surfaces are not perfectly smooth. Under a microscope, they contain tiny irregularities called surface roughness.

When two surfaces touch, these irregularities interlock and resist motion. This resistance is called friction.

Direction of Friction

Friction always acts opposite to the direction of motion.

Example:

Motion of box
──────────────►

Friction
◄──────────────

Everyday Examples

Walking on the road.
Writing with a pencil.
Applying brakes in a vehicle.
Lighting a matchstick.
Holding a glass without slipping.

Advantages of Friction

✔ Helps us walk without slipping.

✔ Enables vehicles to move safely.

✔ Allows us to write on paper.

✔ Helps nails and screws remain fixed.

✔ Provides grip while holding objects.

Disadvantages of Friction

✘ Produces heat.

✘ Causes wear and tear of machine parts.

✘ Reduces efficiency of machines.

✘ Slows down moving objects.

Scientist Connection – Guillaume Amontons

Who was he?

Guillaume Amontons (1663–1705) was a French physicist who carried out some of the earliest systematic studies on friction.

Contribution

Explained that friction depends on the nature of the surfaces in contact.
His work laid the foundation for the laws of friction, which were later refined by Charles-Augustin de Coulomb.

Exam Note: While NCERT mainly discusses friction conceptually, knowing Amontons' contribution adds scientific depth and enriches understanding.

B. Non-contact Forces

Definition (NCERT-Based)

A non-contact force is a force that acts on an object without any physical contact.

These forces act through a field, allowing one object to influence another from a distance.

Characteristics of Non-contact Forces

✔ No physical contact is required.

✔ Can act over a distance.

✔ Invisible but observable through their effects.

Types of Non-contact Forces

1. Gravitational Force

Definition

Gravitational force is the attractive force between any two objects having mass.

The Earth exerts a gravitational force on all objects, pulling them towards its centre.

Examples

Fruits falling from a tree.
Rain falling to the ground.
A dropped pen falling to the floor.
The Moon revolving around the Earth.
Planets revolving around the Sun.

Key Features

✔ Always attractive.

✔ Acts between all objects with mass.

✔ Responsible for weight.

Scientist Spotlight – Sir Isaac Newton

Introduction

Sir Isaac Newton (1642–1727) was an English mathematician, physicist, and astronomer whose discoveries transformed modern science.

Major Contributions

Proposed the Three Laws of Motion.
Formulated the Universal Law of Gravitation.
Explained the motion of planets and satellites.
The SI unit of force (newton, N) is named in his honour.

Interesting Fact

According to a popular story, Newton began thinking deeply about gravity after observing an apple fall from a tree. Although the story is likely simplified, it symbolizes his curiosity about why objects fall toward Earth.

2. Magnetic Force

Definition

Magnetic force is the force exerted by a magnet on magnetic materials or on another magnet.

Examples

A magnet attracting iron nails.
Refrigerator magnets sticking to the door.
A compass needle pointing north.

Characteristics

✔ Can attract or repel.

✔ Acts without contact.

✔ Mainly affects magnetic materials such as iron, nickel, cobalt, and steel.

Scientist Spotlight – William Gilbert

William Gilbert (1544–1603), an English physician and scientist, is often called the Father of Magnetism. He showed that the Earth behaves like a giant magnet and laid the foundation of modern magnetic science.

3. Electrostatic Force

Definition

Electrostatic force is the force of attraction or repulsion between electrically charged objects.

Examples

A rubbed plastic comb attracting tiny pieces of paper.
A balloon rubbed on hair sticking to a wall.
Clothes crackling after drying.

Characteristics

✔ Acts without contact.

✔ Can attract or repel.

✔ Depends on electric charges.

Scientist Spotlight – Benjamin Franklin

Benjamin Franklin (1706–1790) made important contributions to the study of electricity. His famous kite experiment helped demonstrate that lightning is an electrical phenomenon.

Comparison: Contact vs Non-contact Forces

Feature	Contact Force	Non-contact Force
Physical contact required	✔ Yes	✘ No
Acts through touch	✔ Yes	✘ No
Acts from a distance	✘ No	✔ Yes
Examples	Muscular force, Friction	Gravitational, Magnetic, Electrostatic

Real-Life Applications

Situation	Type of Force
Lifting a suitcase	Muscular force
Walking on the road	Frictional force
Magnet attracting pins	Magnetic force
Apple falling from a tree	Gravitational force
Balloon sticking to a wall	Electrostatic force

NCERT Activity 6.1 – Exploring Contact and Non-contact Forces

Aim

To observe the difference between contact and non-contact forces.

Materials Required

A small magnet
Iron pins or paper clips
Wooden pencil
Plastic ruler
Small pieces of paper

Procedure

Bring the magnet close to the iron pins without touching them.
Observe what happens.
Now use the wooden pencil to push the paper pieces.
Compare both observations.

Observation

The magnet attracts the iron pins without touching them.
The pencil moves the paper pieces only when it touches them.

Conclusion

Magnetic force is a non-contact force.
Pushing with a pencil is a contact force.

Exam Tips

⭐ Remember only two contact forces at this level:

Muscular force
Frictional force

⭐ Remember three non-contact forces:

Gravitational force
Magnetic force
Electrostatic force

⭐ In board exams, always mention whether contact is required or not while defining these forces.

Quick Revision

Forces are classified into contact and non-contact forces.
Contact forces require physical contact.
Non-contact forces act from a distance.
Muscular force and friction are contact forces.
Gravitational, magnetic, and electrostatic forces are non-contact forces.
Friction always opposes motion.
Gravity always attracts objects toward one another.

NCERT Keywords

Contact Force • Non-contact Force • Muscular Force • Friction • Gravitational Force • Magnetic Force • Electrostatic Force • Surface Roughness • Attraction • Repulsion

6.3 Net Force, Balanced Forces & Unbalanced Force

6.3 Net Force (Resultant Force)

Introduction

In many real-life situations, more than one force acts on an object at the same time. For example, when two people push a box together or two teams pull a rope in a tug of war, the object experiences multiple forces simultaneously.

The motion of the object depends not on each individual force, but on their combined effect. This combined effect is known as the net force or resultant force.

Key Idea: The motion of an object is determined by the net force, not by a single force acting alone.

NCERT Definition

The net force (or resultant force) is the single force that has the same effect as all the individual forces acting together on an object.

Easy Definition

Net force is the overall force obtained after combining all the forces acting on an object, taking both magnitude and direction into account.

Why Do We Need Net Force?

Imagine pushing a heavy cupboard.

You push with 50 N towards the right.
Your friend pushes with 20 N towards the right.

The cupboard does not experience two separate motions.

Instead, it behaves as if one force of 70 N is acting on it.

This single equivalent force is called the net force.

Rules for Finding Net Force

Case 1: Forces Acting in the Same Direction

When all forces act in the same direction, they are added.

Formula

Net Force = F₁ + F₂ + F₃ + ...

Example

A boy pushes a box with 25 N towards the east.

His friend also pushes with 15 N towards the east.

Net Force = 25 N + 15 N = 40 N (East)

Conclusion: The box moves eastward under a net force of 40 N.

Case 2: Forces Acting in Opposite Directions

When forces act in opposite directions, subtract the smaller force from the larger force.

Formula

Net Force = Larger Force − Smaller Force

The direction of the net force is the direction of the larger force.

Example

A boy pushes a box with 30 N towards the east.

Another boy pushes it with 18 N towards the west.

Net Force = 30 N − 18 N = 12 N (East)

The box moves towards the east because the eastward force is greater.

Real-Life Examples of Net Force

Situation	Net Force
Two friends push a car in the same direction	Forces add up
Tug of war	Difference between the two pulls
Two people carrying a table	Combined upward force supports the table
Boat rowed by two people	Combined rowing force moves the boat faster

Balanced Forces

NCERT Definition

Balanced forces are equal in magnitude but opposite in direction, so their net force is zero.

Easy Definition

Balanced forces cancel each other, producing no change in the state of motion.

Characteristics of Balanced Forces

✔ Equal magnitude

✔ Opposite direction

✔ Net force = 0 N

✔ No acceleration is produced

✔ The object's state of motion remains unchanged

Example 1 – Tug of War

Two teams pull a rope with equal force.

Team A → 500 N
Team B ← 500 N

Since both forces are equal and opposite,

Net Force = 500 N − 500 N = 0 N

The rope remains stationary.

Example 2 – Book on a Table

A book resting on a table experiences two forces:

Gravitational force (weight) acting downward.
Normal reaction force from the table acting upward.

These forces are equal in magnitude and opposite in direction.

Therefore,

Net Force = 0 N

The book remains at rest.

Important Note: An object at rest can still have forces acting on it. If those forces are balanced, the object remains at rest.

Unbalanced Forces

NCERT Definition

Unbalanced forces are forces whose resultant (net) force is not zero.

Easy Definition

When the forces acting on an object are unequal, they do not cancel each other. The object experiences a net force, causing its motion to change.

Characteristics of Unbalanced Forces

✔ Unequal magnitude (or not completely cancelled)

✔ Net force ≠ 0 N

✔ Produces acceleration

✔ Can start, stop, speed up, slow down, or change the direction of motion

Example 1 – Tug of War

Team A → 650 N
Team B ← 500 N

Net Force = 650 N − 500 N = 150 N (towards Team A)

The rope moves towards Team A because the forces are unbalanced.

Example 2 – Pushing a Shopping Trolley

If you push a trolley harder than the friction opposing it, the trolley accelerates forward because the forward force is greater than the opposing force.

Comparison: Balanced vs Unbalanced Forces

Feature	Balanced Forces	Unbalanced Forces
Magnitude	Equal	Unequal
Direction	Opposite	May be same or opposite
Net Force	0 N	Not equal to 0 N
Acceleration	No	Yes
Change in Motion	No	Yes
Example	Book on a table	Football being kicked

Relation with Newton's First Law

Balanced and unbalanced forces help explain Newton's First Law of Motion.

Balanced forces → No change in motion.
Unbalanced forces → Change in motion.

Thus, only an unbalanced external force can change the state of rest or uniform motion of an object.

Real-Life Connections

Walking

When you walk, your feet push the ground backward. The ground exerts a forward frictional force on your feet. This unbalanced force helps you move forward.

Cycling

When you pedal, the bicycle moves because the forward driving force is greater than the opposing forces (friction and air resistance).

Car Braking

When brakes are applied, friction creates an unbalanced force opposite to the car's motion, causing it to slow down and eventually stop.

Think Like a Scientist

Question

A person pushes a wall with great force, but the wall does not move. Does this mean no force is acting?

Answer

No. A force is certainly applied by the person, but the wall exerts an equal and opposite force. These balanced forces produce zero net force, so the wall does not move.

Common Mistakes

❌ Balanced forces mean no forces are acting.

✅ Balanced forces mean forces are acting, but they cancel each other.

❌ An object moving with constant speed has no forces acting on it.

✅ It may have several forces acting, but if the net force is zero, its motion remains unchanged.

❌ Only stationary objects can have balanced forces.

✅ Moving objects can also experience balanced forces if they continue moving with constant speed in a straight line.

Memory Trick

B = Balance = 0

Balanced Force → Net Force = 0

No change in motion.

U = Unbalanced = Unequal

Unbalanced Force → Net Force ≠ 0

Motion changes.

Exam Tips

⭐ Always write "net force = 0 N" while defining balanced forces.

⭐ Mention "net force ≠ 0 N" while defining unbalanced forces.

⭐ Use book on a table and tug of war as standard examples—they are commonly asked in exams.

Quick Revision

Net force is the combined effect of all forces acting on an object.
Forces in the same direction are added.
Forces in opposite directions are subtracted.
Balanced forces have zero net force.
Unbalanced forces have non-zero net force.
Only unbalanced forces can change the state of motion of an object.

NCERT Keywords

Net Force • Resultant Force • Balanced Forces • Unbalanced Forces • Acceleration • State of Motion • Resultant • External Force

🌟 Topper Note

A question often asked in exams is:

"Can an object move even when the net force acting on it is zero?"

Answer: Yes. If an object is already moving, it will continue to move with constant velocity in a straight line when the net force is zero. This is exactly what Newton's First Law of Motion states.

6.4 Force of Friction

Introduction

Imagine trying to walk on a perfectly smooth sheet of ice. You would find it difficult to walk because your feet would continuously slip. Now think about writing with a pencil on a sheet of paper. Why does the pencil leave a mark? Why do vehicles stop when brakes are applied? Why do our shoes gradually wear out?

The answer to all these questions is friction.

Friction is one of the most common forces experienced in everyday life. It is always present whenever two surfaces come in contact. Although it often opposes motion, friction is also essential for many daily activities such as walking, writing, driving, and holding objects.

Key Idea: Friction is a contact force that always opposes the relative motion (or tendency of motion) between two surfaces in contact.

6.4.1 What is Friction?

NCERT Definition

Friction is the force that opposes the relative motion or the tendency of motion between two surfaces in contact.

Easy Definition

Friction is a force that acts opposite to the direction of motion and tries to slow down or stop a moving object.

Why Does Friction Occur?

At first glance, many surfaces appear perfectly smooth. However, under a microscope, they are found to have tiny irregularities or rough projections.

When two surfaces come into contact, these irregularities interlock with each other. This interlocking resists motion and produces a force called friction.

Scientific Explanation

Smooth surfaces have fewer irregularities, so they produce less friction.
Rough surfaces have more irregularities, so they produce greater friction.

Remember: Friction depends on the nature of the surfaces in contact.

Direction of Friction

One of the most important properties of friction is its direction.

Friction always acts opposite to the direction of motion or the tendency of motion.

Example

A box is pushed towards the right.

Applied Force
────────────►

Box

◄────────────
Friction

The applied force tries to move the box forward, while friction opposes this motion.

Everyday Examples of Friction

Situation	Role of Friction
Walking	Prevents slipping by providing grip
Writing with a pencil	Allows graphite to stick to paper
Applying brakes	Slows down and stops vehicles
Holding a glass	Prevents it from slipping
Lighting a matchstick	Produces heat to ignite the match
Climbing stairs	Provides grip between shoes and steps

Advantages of Friction

Although friction opposes motion, it is extremely useful.

1. Helps Us Walk

Walking is possible because friction provides grip between our feet and the ground.

Without friction, we would continuously slip.

2. Enables Vehicles to Move

The tyres of a vehicle grip the road due to friction.

Without friction, the wheels would spin without moving the vehicle forward.

3. Makes Writing Possible

The graphite from a pencil sticks to paper because of friction.

Similarly, chalk writes on a blackboard due to friction.

4. Helps in Holding Objects

Friction between our fingers and an object allows us to hold it firmly.

5. Allows Brakes to Work

Brakes use friction to reduce the speed of moving vehicles.

6. Keeps Nails and Screws Fixed

Friction between the nail (or screw) and the surrounding material prevents it from slipping out easily.

Disadvantages of Friction

While friction is useful, excessive friction has some harmful effects.

1. Produces Heat

Rubbing two hands together generates heat because of friction.

Similarly, machine parts become hot during operation.

2. Causes Wear and Tear

Continuous friction gradually damages surfaces.

Examples:

Shoe soles wear out.
Tyres become smooth after long use.
Machine parts require replacement.

3. Wastes Energy

Part of the useful energy is converted into heat due to friction, reducing the efficiency of machines.

4. Slows Down Moving Objects

A rolling ball eventually stops because friction acts opposite to its motion.

How Can Friction Be Reduced?

In many machines, excessive friction is undesirable. It can be reduced by:

✔ Applying lubricants such as oil or grease.

✔ Using ball bearings to reduce rubbing between moving parts.

✔ Polishing surfaces to make them smoother.

✔ Using streamlined shapes in vehicles to reduce air resistance (a type of friction).

Did You Know?
Air and water also exert friction. This is called fluid friction or drag.

Activity 6.1 – Investigating Friction (NCERT-Based)

Aim

To observe how friction affects the motion of an object on different surfaces.

Materials Required

Four identical coins
Rubber band
Smooth table
Rough surface

Procedure

Stack four coins one above the other.
Stretch a rubber band and release the coins.
Measure the distance travelled by the coins on a smooth surface.
Repeat the activity on a rough surface.
Compare the distances travelled.

Observation

The coins travel farther on the smooth surface.
The coins travel a shorter distance on the rough surface.

Conclusion

Rough surfaces produce greater friction.
Smooth surfaces produce less friction.
Greater friction opposes motion more strongly.

Think Like a Scientist

Question

Why is it easier to pull a suitcase with wheels than to drag it across the floor?

Answer

When a suitcase has wheels, rolling friction acts, which is much smaller than sliding friction. Therefore, less force is required to move the suitcase.

Scientist Spotlight – Guillaume Amontons

Guillaume Amontons (1663–1705) was a French physicist who carried out pioneering studies on friction. He observed that friction depends on the nature of the surfaces in contact and helped establish the early laws of friction.

His work was later expanded by Charles-Augustin de Coulomb, whose studies further improved our understanding of friction.

Common Misconceptions

❌ Friction is always harmful.

✅ Friction is both useful and harmful. Without friction, walking, writing, driving, and holding objects would not be possible.

❌ Smooth surfaces have more friction.

✅ Smooth surfaces generally produce less friction, while rough surfaces produce more friction.

❌ Friction acts in the direction of motion.

✅ Friction always acts opposite to the direction of motion or the tendency of motion.

Exam Tips

⭐ Always mention "between two surfaces in contact" while defining friction.

⭐ Remember at least four advantages and four disadvantages of friction with examples.

⭐ In descriptive answers, clearly state that friction opposes motion.

Quick Revision

Friction is a contact force.
It acts opposite to the direction of motion.
It is caused by irregularities on surfaces.
Rough surfaces produce more friction.
Smooth surfaces produce less friction.
Friction helps in walking, writing, gripping, and braking.
Friction also causes heat, wear and tear, and energy loss.

NCERT Keywords

Friction • Contact Force • Surface Irregularities • Rough Surface • Smooth Surface • Grip • Wear and Tear • Lubrication • Drag • Rolling Friction

🌟 Topper Insight

Many students memorize "friction opposes motion", but the complete NCERT concept is:

Friction opposes the relative motion or the tendency of relative motion between two surfaces in contact.

This means friction can act even before an object starts moving. For example, when you gently push a heavy cupboard and it does not move, friction is already acting to oppose the tendency of motion. This deeper understanding helps in conceptual and HOTS-based questions.

Chapter 6 – How Forces Affect Motion (Part 2C)

6.5 Measuring the Magnitude of a Force

(CBSE Class 9 | NCERT-Based | Topper Notes | Professional Edition)

Introduction

In our daily life, we often describe a force as small, large, weak, or strong. However, these descriptions are only qualitative and cannot tell us the exact amount of force applied.

In science, every physical quantity must be measured accurately. Just as we measure length using a metre scale and mass using a balance, force is measured using a special instrument called a spring balance.

Key Idea: Scientific measurements require standard units and reliable measuring instruments.

Why Do We Measure Force?

Suppose two students claim that they can push a box with a "strong force."

How can we decide whose force is actually greater?

Words like strong or weak are based on personal opinion. Therefore, scientists measure force using standard units and instruments.

Measuring force helps us to:

Compare different forces accurately.
Perform scientific experiments.
Design machines and vehicles.
Ensure safety in engineering and construction.

Magnitude of Force

NCERT Definition

The magnitude of a force is the measure of its strength or size.

Easy Definition

The magnitude of force tells us how much force is acting on an object.

It does not tell us the direction of the force.

Examples

Situation	Magnitude of Force
Pressing a keyboard key	Very small
Closing a notebook	Small
Pushing a chair	Moderate
Pulling a loaded cart	Large

Measuring Force

To measure force accurately, scientists use an instrument called a spring balance.

Spring Balance

NCERT Definition

A spring balance is an instrument used to measure the magnitude of a force or the weight of an object.

Construction of a Spring Balance

A spring balance consists of:

A strong metal spring
A transparent casing with a graduated scale
A pointer attached to the spring
A hook for holding or pulling objects

When a force is applied, the spring stretches and the pointer moves along the scale.

Principle of a Spring Balance

A spring balance works on Hooke's Law.

Within its elastic limit, the extension of a spring is directly proportional to the applied force.

This means:

Small force → Small extension
Large force → Large extension

As the spring stretches, the pointer indicates the force on the scale.

SI Unit of Force

The SI unit of force is the newton (N).

It is named after Sir Isaac Newton, whose work on the laws of motion revolutionized physics.

One Newton

Definition

One newton is the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg.

This definition comes from Newton's Second Law of Motion, which you will study later.

Uses of a Spring Balance

A spring balance is commonly used to:

✔ Measure force in laboratory experiments.

✔ Measure the weight of small objects.

✔ Compare the force applied in different situations.

✔ Demonstrate scientific concepts in classrooms.

Everyday Applications

Situation	Use of Spring Balance
School laboratory	Measuring force
Science exhibition	Demonstrating weight
Physics experiment	Comparing different forces
Industrial testing	Measuring pulling force

Activity 6.2 – Measuring Friction Using a Spring Balance

Aim

To compare the force of friction on different surfaces.

Materials Required

Spring balance
Wooden block
Smooth surface
Rough surface

Procedure

Tie the wooden block to the hook of the spring balance.
Place the block on a smooth surface.
Pull the spring balance gently.
Observe the reading just before the block starts moving.
Repeat the activity on a rough surface.
Compare the readings.

Observation

The spring balance shows a smaller reading on the smooth surface.
The spring balance shows a larger reading on the rough surface.

Conclusion

A rough surface produces greater friction, so a larger force is required to move the block.

A smooth surface produces less friction, so a smaller force is sufficient.

Think Like a Scientist

Question

Why does a spring balance show a larger reading when pulling a block on a rough surface?

Answer

A rough surface produces greater friction because its irregularities interlock more strongly with those of the block. Therefore, a larger force is required to overcome friction and start the motion.

Scientist Spotlight – Robert Hooke

Who was Robert Hooke?

Robert Hooke (1635–1703) was an English scientist known for his work in physics, biology, and microscopy.

Major Contributions

Proposed Hooke's Law, which explains the behaviour of springs.
Improved scientific instruments, including microscopes.
Coined the term "cell" after observing cork under a microscope.

Why is Hooke Important in This Chapter?

The working of a spring balance is based on Hooke's Law, making his contribution fundamental to the measurement of force.

Common Mistakes

❌ A spring balance measures mass.

✅ A spring balance measures force or weight, not mass.

❌ Kilogram (kg) is the SI unit of force.

✅ The SI unit of force is newton (N), while kilogram (kg) is the SI unit of mass.

❌ A larger spring extension means a smaller force.

✅ A larger extension indicates a greater applied force (within the elastic limit of the spring).

Exam Tips

⭐ Learn the definition of a spring balance exactly.

⭐ Remember that a spring balance works on Hooke's Law.

⭐ Do not confuse mass with weight:

Mass is measured in kilograms (kg).
Weight is a force measured in newtons (N).

Quick Revision

Force is measured using a spring balance.
The magnitude of force indicates its strength.
A spring balance works on Hooke's Law.
The SI unit of force is newton (N).
Greater force produces greater extension of the spring.
Rough surfaces require greater force because they produce more friction.

NCERT Keywords

Spring Balance • Magnitude of Force • Hooke's Law • Newton (N) • Weight • Elastic Limit • Measurement of Force

🌟 Topper Insight

Many students think that weight and mass are the same, but they are different.

Mass	Weight
Amount of matter in an object	Gravitational force acting on the object
SI Unit: kilogram (kg)	SI Unit: newton (N)
Remains constant everywhere	Changes slightly with gravity

This distinction is important because a spring balance measures weight (a force), whereas a beam balance measure

Chapter 6 – How Forces Affect Motion (Part 3A)

6.6 Galileo Galilei and Newton's First Law of Motion (Law of Inertia)

(CBSE Class 9 | NCERT-Based | Topper Notes | Professional Edition)

Introduction

Have you ever noticed that:

A moving bicycle gradually stops if you stop pedalling?
Passengers move forward when a bus stops suddenly?
Dust comes out of a carpet when it is beaten?

These common observations raise an important question:

Why do objects change or resist changing their motion?

For many centuries, people believed that a continuous force was necessary to keep an object moving. However, the experiments of Galileo Galilei challenged this idea. Later, Sir Isaac Newton explained this behaviour through the First Law of Motion, also known as the Law of Inertia.

Before Galileo: Aristotle's Idea

The ancient Greek philosopher Aristotle (384–322 BCE) believed that:

"A continuous force is required to keep an object in motion."

According to this view, if the applied force was removed, the object would immediately stop.

For many centuries, this idea was accepted as true because people observed that moving objects eventually come to rest.

However, Aristotle did not realize that friction was responsible for stopping the objects.

Galileo Galilei (1564–1642)

Introduction

Galileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher. He is widely known as the Father of Modern Science because he introduced the experimental method into scientific investigations.

Instead of accepting old beliefs, Galileo relied on careful observations and experiments.

Galileo's Experiment on Motion

Galileo rolled a ball down a smooth inclined plane and allowed it to move up another inclined plane.

Observation

When the second plane was rough, the ball stopped after travelling a short distance.
As the surface became smoother, the ball travelled farther.
If friction could be completely removed, the ball would continue moving indefinitely.

Galileo's Conclusion

Galileo concluded that:

An object does not need a continuous force to keep moving. It continues in uniform motion unless an external force acts on it.

This revolutionary idea became the foundation of Newton's First Law of Motion.

Scientist Spotlight – Galileo Galilei

Major Contributions

Introduced the experimental approach in science.
Studied the motion of falling bodies.
Explained the concept of inertia.
Laid the foundation for Newton's Laws of Motion.
Improved the telescope and made important astronomical discoveries.

Interesting Fact

Galileo discovered the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. Their discovery provided strong evidence that not everything revolves around the Earth.

Newton's First Law of Motion

NCERT Statement

An object at rest remains at rest, and an object in motion continues to move with uniform velocity in a straight line unless acted upon by an unbalanced external force.

Easy Definition

A body does not change its state of rest or uniform motion by itself. Its state changes only when an unbalanced external force acts on it.

Understanding the First Law

Newton's First Law describes two situations.

1. An Object at Rest

If an object is at rest and no unbalanced external force acts on it, it remains at rest.

Example

A book placed on a table remains there until someone pushes or lifts it.

Reason: The forces acting on the book are balanced.

2. An Object in Motion

If an object is already moving and no unbalanced external force acts on it, it continues moving with constant speed in a straight line.

Example

A hockey puck slides a long distance on smooth ice because friction is very small.

Why Do Moving Objects Stop in Daily Life?

Many students think that moving objects stop because the applied force disappears.

This is incorrect.

Objects stop because friction and air resistance act as external forces opposite to the direction of motion.

If these forces were absent, the object would continue moving indefinitely.

Conditions for Newton's First Law

The law is valid when:

✔ The net external force is zero.

✔ Forces acting on the object are balanced.

✔ No unbalanced force changes the motion.

Law of Inertia

Newton's First Law is also called the Law of Inertia because it explains the property of inertia.

Important Fact: The First Law does not define force; it explains when an object's motion changes.

What is Inertia?

NCERT Definition

Inertia is the property of an object by which it resists any change in its state of rest or uniform motion in a straight line.

Easy Definition

Inertia is the natural tendency of an object to resist changes in its motion.

This means an object prefers to remain in its current state unless acted upon by an unbalanced external force.

Why Does Inertia Exist?

Every object has mass.

Mass is a measure of the amount of matter in an object.

The greater the mass, the greater the inertia.

Therefore:

A loaded truck has greater inertia than a bicycle.
A cricket ball has greater inertia than a table tennis ball.

Key Points to Remember

Galileo showed that force is not required to maintain motion.
Newton explained when motion changes.
Friction is the main reason moving objects stop in everyday life.
Newton's First Law is called the Law of Inertia.
Inertia depends on the mass of the object.

Did You Know?

🚀 In outer space, where friction is almost absent, a spacecraft can continue moving for a very long time without continuously running its engines. Small engine firings are mainly used to change its speed or direction, not to keep it moving.

Common Misconceptions

❌ A force is needed to keep an object moving.

✅ A force is needed only to change the state of motion.

❌ Objects stop because motion naturally disappears.

✅ Objects stop because friction and air resistance act as unbalanced external forces.

❌ Heavier objects move because they have more force.

✅ Heavier objects have greater inertia, meaning they resist changes in motion more strongly.

Exam Tips

⭐ Write the NCERT statement of Newton's First Law accurately.

⭐ Always mention the phrase "unbalanced external force" in descriptive answers.

⭐ Remember that Newton's First Law = Law of Inertia.

Quick Revision

Galileo challenged Aristotle's idea using experiments.
Motion continues unless an unbalanced external force acts.
Newton's First Law explains when motion changes.
Inertia is the resistance to changes in motion.
Greater mass means greater inertia.

NCERT Keywords

Galileo Galilei • Newton's First Law • Law of Inertia • External Force • Uniform Motion • State of Rest • Inertia • Friction • Air Resistance

🌟 Author's Note (for book quality)

One improvement I'd recommend over your original draft: don't introduce the three types of inertia immediately after definintion

6.7 Inertia and Types of Inertia

What is Inertia?

NCERT Definition

Inertia is the property of an object by which it resists any change in its state of rest or of uniform motion in a straight line.

Easy Definition

Inertia is the natural tendency of an object to resist any change in its state of motion or rest.

In simple words, objects do not change their state by themselves. They require an unbalanced external force to change their motion.

Remember: Greater the mass, greater is the inertia.

Why Does Inertia Depend on Mass?

Mass is a measure of the amount of matter in an object.

A loaded truck has more inertia than a bicycle.
A cricket ball has more inertia than a table tennis ball.

Therefore, heavier objects are more difficult to start, stop, or change direction.

Types of Inertia

Inertia is classified into three types.

1. Inertia of Rest

Definition

The tendency of an object at rest to remain at rest unless acted upon by an unbalanced external force.

Examples

A book remains on a table until someone moves it.
Dust comes out of a carpet when it is beaten.
A coin falls into a glass when the card beneath it is pulled quickly.

Reason: The dust and coin tend to remain at rest due to inertia.

2. Inertia of Motion

Definition

The tendency of a moving object to continue moving with the same speed in a straight line unless acted upon by an unbalanced external force.

Examples

Passengers move forward when a moving bus stops suddenly.
A rolling football continues moving for some distance even after being kicked.

Reason: The body tends to continue its motion.

3. Inertia of Direction

Definition

The tendency of an object to resist any change in the direction of its motion.

Examples

Passengers move sideways when a bus takes a sharp turn.
A stone tied to a string moves in a circular path because the string continuously changes its direction.

Everyday Applications

Situation	Type of Inertia
Dust removed from carpet	Inertia of Rest
Passenger moves forward when bus stops	Inertia of Motion
Passenger moves sideways on turning	Inertia of Direction
Coin falls into glass	Inertia of Rest

Importance of Seat Belts

When a moving car stops suddenly, the passengers tend to keep moving forward due to inertia of motion.

A seat belt provides the necessary external force to stop the passenger safely and reduces the risk of injury.

Key Points

✔ Inertia is a property of matter.

✔ Greater mass means greater inertia.

✔ Newton's First Law is also called the Law of Inertia.

✔ An unbalanced external force is required to change the state of motion.

Exam Tips

⭐ Learn all three types of inertia with one example each.

⭐ Remember:

Rest → Carpet
Motion → Bus Stops
Direction → Bus Turns

These are the most common board exam examples.

Quick Revision

Inertia = Resistance to change in motion.
Three types: Rest, Motion, Direction.
Mass ↑ ⇒ Inertia ↑
Seat belts work due to inertia of motion.
Newton's First Law = Law of Inertia.

NCERT Keywords

Inertia • Inertia of Rest • Inertia of Motion • Inertia of Direction • Mass • Unbalanced External Force • Law of Inertia

6.8 Newton's Second Law of Motion (F = ma)

Introduction

Newton's First Law tells us when an object's motion changes, while Newton's Second Law explains how much the motion changes when a force acts on it.

It establishes the relationship between Force, Mass, and Acceleration.

Key Idea: A larger force produces greater acceleration, while a larger mass produces smaller acceleration.

Newton's Second Law (NCERT Statement)

The acceleration produced in an object is directly proportional to the net force acting on it and inversely proportional to its mass. The acceleration is in the direction of the net force.

Easy Definition

When a force acts on an object:

More Force → More Acceleration
More Mass → Less Acceleration
The object accelerates in the direction of the applied force.

Force, Mass and Acceleration

Force (F)

A push or pull that can change the state of motion of an object.

SI Unit: Newton (N)

Mass (m)

The amount of matter present in an object.

SI Unit: Kilogram (kg)

Acceleration (a)

The rate of change of velocity with time.

SI Unit: m/s²

Relationship Between Force, Mass and Acceleration

From Newton's Second Law,

Force ∝ Acceleration (when mass is constant)
Force ∝ Mass (when acceleration is constant)

Combining these,

Formula

F = m × a

Where,

F = Force (Newton)
m = Mass (kg)
a = Acceleration (m/s²)

Understanding the Formula

Case 1: Mass is Constant

If the applied force increases, the acceleration also increases.

Example: A football moves faster when kicked harder.

Case 2: Force is Constant

If the mass increases, the acceleration decreases.

Example: An empty shopping trolley is easier to push than a loaded one.

One Newton (1 N)

Definition:

One newton is the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg.

Everyday Examples

Example 1

A football accelerates more than a stone when both are kicked with the same force.

Reason: The football has less mass.

Example 2

A loaded truck requires more force to start moving than an empty truck.

Reason: Greater mass requires greater force.

Example 3

Cycling becomes easier after removing heavy luggage.

Reason: Less mass produces greater acceleration for the same force.

Activity 6.3 (Concept)

Aim

To observe the effect of force on objects of different masses.

Observation

A lighter object accelerates more.
A heavier object accelerates less.

Conclusion

Acceleration depends on both force and mass.

Key Points

✔ Greater force → Greater acceleration.

✔ Greater mass → Smaller acceleration.

✔ Force and acceleration act in the same direction.

✔ Formula: F = ma

Difference: First Law vs Second Law

Newton's First Law	Newton's Second Law
Explains when motion changes	Explains how much motion changes
Introduces inertia	Relates force, mass & acceleration
Qualitative law	Quantitative law
Law of Inertia	Gives the equation F = ma

Exam Tips

⭐ Learn the formula F = ma.

⭐ Remember the SI units:

Force → Newton (N)
Mass → Kilogram (kg)
Acceleration → m/s²

⭐ Always write net force while stating Newton's Second Law.

Quick Revision

Newton's Second Law relates Force, Mass and Acceleration.
More force → More acceleration.
More mass → Less acceleration.
Formula: F = ma
SI unit of force = Newton (N).
1 N is the force needed to accelerate a 1 kg object by 1 m/s².

NCERT Keywords

Newton's Second Law • Force • Mass • Acceleration • Net Force • Newton (N) • F = ma

Applications of Newton's Second Law

Newton's Second Law helps us understand how force affects the motion of objects in our daily life. It explains why heavier objects need more force and why lighter objects accelerate more easily.

1. Kicking a Football

A football moves quickly when kicked because it has less mass. The applied force produces a large acceleration.

2. Pushing a Loaded Trolley

A loaded trolley has greater mass than an empty trolley. Therefore, it requires more force to move.

3. Driving a Car

Pressing the accelerator increases the force produced by the engine, causing the car to accelerate.

4. Cricket and Baseball

Players move their hands backward while catching the ball. This increases the stopping time, reducing the force on the hands and preventing injury.

5. Rocket Launch

A rocket moves upward because its engines produce a very large force, giving it enough acceleration to overcome gravity.

Solved Numerical Examples

Example 1

A force of 20 N acts on a body of mass 4 kg. Find its acceleration.

Solution

Given:

Force (F) = 20 N
Mass (m) = 4 kg

Formula:

F = ma

a = F ÷ m

a = 20 ÷ 4 = 5 m/s²

Answer: Acceleration = 5 m/s²

Example 2

A body of mass 5 kg accelerates at 3 m/s². Find the force acting on it.

Solution

Given:

Mass = 5 kg
Acceleration = 3 m/s²

Formula:

F = ma

F = 5 × 3 = 15 N

Answer: Force = 15 N

Example 3

A force of 12 N acts on a body and produces an acceleration of 4 m/s². Find its mass.

Solution

Given:

Force = 12 N
Acceleration = 4 m/s²

Formula:

m = F ÷ a

m = 12 ÷ 4 = 3 kg

Answer: Mass = 3 kg

Concept Check

Q1. Which will accelerate more when the same force is applied?

A 2 kg object
A 10 kg object

Answer: The 2 kg object, because it has less mass.

Q2. If the force is doubled while the mass remains constant, what happens to the acceleration?

Answer: The acceleration also doubles.

Q3. If the mass is doubled while the force remains constant, what happens to the acceleration?

Answer: The acceleration becomes half.

Common Mistakes

❌ More mass means more acceleration.

✅ More mass means less acceleration (for the same force).

❌ Force and acceleration act in opposite directions.

✅ Force and acceleration act in the same direction.

Exam Tips

⭐ Use the correct formula:

F = ma
a = F/m
m = F/a

⭐ Always write the SI units in the final answer.

⭐ Write the formula first, then substitute the values, and finally mention the answer with its unit.

Quick Revision

Newton's Second Law explains the relationship between force, mass, and acceleration.
F = ma is the fundamental equation.
Greater force produces greater acceleration.
Greater mass produces smaller acceleration.
The direction of acceleration is the same as the direction of the net force.

NCERT Keywords

Force • Mass • Acceleration • Newton (N) • Formula • Numerical Problems • Application

Introduction

Have you ever noticed that:

A gun moves backward when fired.
A swimmer pushes water backward but moves forward.
A rocket rises upward by throwing gases downward.

These situations are explained by Newton's Third Law of Motion.

Key Idea: Forces always act in pairs. If one object exerts a force on another, the second object also exerts an equal and opposite force on the first.

Newton's Third Law (NCERT Statement)

To every action, there is an equal and opposite reaction.

Easy Definition

Whenever one object exerts a force on another object, the second object immediately exerts an equal force in the opposite direction on the first object.

Action and Reaction Forces

Action Force: The force applied by the first object.
Reaction Force: The equal and opposite force applied by the second object.

Important Characteristics

✔ Action and reaction always occur together.

✔ They are equal in magnitude.

✔ They are opposite in direction.

✔ They act on different objects, so they do not cancel each other.

Remember: Action and reaction are not balanced forces, because they act on different bodies.

Everyday Examples

1. Walking

When we walk, our feet push the ground backward (action).

The ground pushes us forward (reaction), allowing us to walk.

2. Swimming

A swimmer pushes water backward.

The water pushes the swimmer forward.

3. Rocket Launch

The rocket pushes hot gases downward.

The gases push the rocket upward, causing it to rise.

4. Recoil of a Gun

When a bullet moves forward, the gun moves backward with an equal and opposite reaction.

5. Rowing a Boat

The oar pushes water backward.

The water pushes the boat forward.

Action–Reaction Pair

Action	Reaction
Foot pushes ground backward	Ground pushes person forward
Swimmer pushes water backward	Water pushes swimmer forward
Rocket pushes gases downward	Gases push rocket upward
Gun pushes bullet forward	Bullet pushes gun backward

NCERT Activity (Concept)

Aim

To observe Newton's Third Law of Motion.

Activity

Blow up a balloon and release it without tying its mouth.

Observation

Air rushes backward, while the balloon moves forward.

Conclusion

The backward-moving air is the action, and the forward motion of the balloon is the reaction.

Common Misconceptions

❌ Action occurs first, then reaction.

✅ Action and reaction occur simultaneously.

❌ Action and reaction cancel each other.

✅ They do not cancel because they act on different objects.

Difference Between Newton's Three Laws

First Law	Second Law	Third Law
Explains when motion changes	Explains how force changes motion	Explains forces between two interacting objects
Law of Inertia	F = ma	Action = Reaction

Scientist Spotlight – Sir Isaac Newton

Sir Isaac Newton (1642–1727) was an English physicist and mathematician who formulated the Three Laws of Motion and the Universal Law of Gravitation. His work forms the foundation of classical mechanics, and the SI unit of force (newton, N) is named in his honour.

Exam Tips

⭐ Always write the complete statement:

"To every action, there is an equal and opposite reaction."

⭐ Mention that action and reaction act on different objects.

⭐ Use examples like walking, swimming, rocket launch, and gun recoil in long-answer questions.

Quick Revision

Newton's Third Law states that every action has an equal and opposite reaction.
Action and reaction are equal in magnitude and opposite in direction.
They always act on different objects.
They occur at the same time.
Examples: Walking, swimming, rocket launch, gun recoil, rowing a boat.

NCERT Keywords

Newton's Third Law • Action • Reaction • Equal and Opposite Forces • Interaction • Rocket • Recoil • Walking

Introduction

A moving truck and a moving bicycle may have the same speed, but it is much harder to stop the truck. This is because the truck has greater momentum.

Momentum is a quantity that depends on both the mass and velocity of an object.

Key Idea: Greater the mass or velocity, greater is the momentum.

What is Momentum?

NCERT Definition

Momentum is the quantity of motion possessed by a moving object.

Easy Definition

Momentum is the product of the mass and velocity of an object.

Formula of Momentum


\boxed{p = mv}

Where,

p = Momentum
m = Mass (kg)
v = Velocity (m/s)

SI Unit

kg·m/s

Factors Affecting Momentum

1. Mass

If mass increases, momentum increases.

Example: A truck has more momentum than a bicycle moving at the same speed.

2. Velocity

If velocity increases, momentum also increases.

Example: A fast-moving cricket ball has more momentum than a slow-moving one.

Law of Conservation of Momentum

NCERT Statement

When no external force acts on a system, the total momentum before collision is equal to the total momentum after collision.

Formula


\boxed{m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2}

Where,

m₁, m₂ = Masses of the objects
u₁, u₂ = Initial velocities
v₁, v₂ = Final velocities

Remember:
Total Initial Momentum = Total Final Momentum

Everyday Examples

1. Gun Recoil

When a bullet moves forward, the gun moves backward so that the total momentum remains conserved.

2. Rocket Launch

Hot gases move downward, and the rocket moves upward. The total momentum of the system remains constant.

3. Collision of Two Balls

During a collision, momentum is transferred between the balls, but the total momentum remains constant if no external force acts.

Solved Numerical

Example

A body of mass 2 kg is moving with a velocity of 5 m/s. Find its momentum.

Solution

Given:

Mass = 2 kg
Velocity = 5 m/s

Formula:


p = mv


p = 2 \times 5 = 10 \text{ kg·m/s}

Answer: 10 kg·m/s

Concept Check

Q1. Which has greater momentum?

A 5 kg object moving at 2 m/s
A 2 kg object moving at 5 m/s

Solution

First object:


p = 5 \times 2 = 10 \text{ kg·m/s}

Second object:


p = 2 \times 5 = 10 \text{ kg·m/s}

Answer: Both have the same momentum.

Key Points

✔ Momentum depends on mass and velocity.

✔ Momentum is a vector quantity because velocity has direction.

✔ In the absence of external forces, momentum is always conserved.

✔ The law of conservation of momentum is widely used to explain collisions, explosions, and rocket motion.

Difference Between Force and Momentum

Force	Momentum
Push or pull	Quantity of motion
SI Unit: Newton (N)	SI Unit: kg·m/s
Can change motion	Describes motion of a moving object

Exam Tips

⭐ Learn both formulas:

Momentum:
Conservation of Momentum:

⭐ In numerical questions, always write:

Formula
Substitution
Calculation
Final answer with unit

Quick Revision

Momentum = Mass × Velocity
SI Unit = kg·m/s
Momentum increases with mass and velocity.
Total momentum remains constant if no external force acts.
Initial Momentum = Final Momentum

NCERT Keywords

Momentum • Quantity of Motion • Conservation of Momentum • Collision • Recoil • Rocket Motion • kg·m/s

Chapter Summary, Important Definitions & Formula Sheet

Chapter Summary

Force is a push or pull that can change the state of motion, speed, direction, or shape of an object.
Force is a vector quantity because it has both magnitude and direction.
Forces are of two types: Contact Forces and Non-contact Forces.
Friction is a contact force that opposes the relative motion between two surfaces.
The combined effect of all forces acting on an object is called the net force.
Balanced forces have zero net force, while unbalanced forces change the state of motion.
A spring balance is used to measure the magnitude of force.
Galileo Galilei showed that an object continues in motion unless an external force acts on it.
Newton's First Law is called the Law of Inertia.
Newton's Second Law relates force, mass, and acceleration through F = ma.
Newton's Third Law states that every action has an equal and opposite reaction.
Momentum is the product of mass and velocity.
According to the Law of Conservation of Momentum, the total momentum of an isolated system remains constant.

Important Definitions

Force

A push or pull that can change the state of motion, speed, direction, or shape of an object.

Friction

The force that opposes the relative motion between two surfaces in contact.

Net Force

The resultant of all the forces acting on an object.

Balanced Forces

Equal and opposite forces whose net force is zero.

Unbalanced Forces

Forces whose net force is not zero and can change the state of motion.

Inertia

The property of an object by which it resists any change in its state of rest or uniform motion.

Momentum

The quantity of motion possessed by a moving object.

Formula Sheet

Quantity	Formula
Force	F = ma
Acceleration	a = F/m
Mass	m = F/a
Momentum	p = mv
Conservation of Momentum	m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

SI Units

Quantity	SI Unit
Force	Newton (N)
Mass	Kilogram (kg)
Acceleration	m/s²
Velocity	m/s
Momentum	kg·m/s

Important Scientists

Scientist	Contribution
Galileo Galilei	Introduced the concept of inertia through experiments on motion.
Sir Isaac Newton	Gave the Three Laws of Motion and the Universal Law of Gravitation.
Robert Hooke	Proposed Hooke's Law, the principle behind a spring balance.

Most Important NCERT Points

✔ Force is a vector quantity.

✔ Friction always acts opposite to motion.

✔ Balanced forces do not change the state of motion.

✔ Only unbalanced forces produce acceleration.

✔ Newton's First Law is the Law of Inertia.

✔ Newton's Second Law gives F = ma.

✔ Newton's Third Law: Action = Reaction.

✔ Momentum depends on mass and velocity.

✔ Total momentum remains constant if no external force acts.

Common Mistakes

❌ Mass and weight are the same.

✅ Mass is measured in kg, while weight is a force measured in N.

❌ Action and reaction cancel each other.

✅ They act on different objects, so they do not cancel.

❌ Force is needed to keep an object moving.

✅ Force is needed only to change the state of motion.

Memory Tricks

F = ma → Force = Mass × Acceleration

p = mv → Momentum = Mass × Velocity

First Law → Inertia

Second Law → Force & Acceleration

Third Law → Action = Reaction

One-Page Quick Revision

Force → Push or Pull
SI Unit of Force → Newton (N)
Instrument to Measure Force → Spring Balance
Types of Force → Contact & Non-contact
Friction → Opposes Motion
Balanced Forces → Net Force = 0
Unbalanced Forces → Net Force ≠ 0
Newton's First Law → Law of Inertia
Newton's Second Law → F = ma
Newton's Third Law → Action = Reaction
Momentum → p = mv
Conservation of Momentum → Initial Momentum = Final Momentum

Class 9 Science Chapter 11 – Reproduction: How Life Continues | Complete Notes

Chapter 6: How Forces Affect Motion Complete NCERT Notes | Concepts • Examples • Activities • Numericals • Revision

Chapter 6 – How Forces Affect Motion (Part 1A)

6.1 The Concept of Force

Chapter Introduction

Learning Outcomes

6.1 The Concept of Force

Think About It

What is Force?

NCERT Definition

Easy Definition

Understanding Force Through Daily Life

Characteristics of Force

1. Force is a Push or Pull

2. Force Can Change Motion

3. Force Can Change Shape

4. Force Can Act in Different Directions

Real-Life Connection

Why is Force Important?

Everyday Science

Did You Know?

Think Like a Scientist

Common Misconception

Key Points to Remember

NCERT Keywords

Chapter Quote

How Does Force Affect an Object?

Major Effects of Force

1. Force Can Start the Motion of an Object

2. Force Can Stop a Moving Object

3. Force Can Change the Speed of an Object

(a) Increasing Speed

(b) Decreasing Speed

4. Force Can Change the Direction of Motion

5. Force Can Change the Shape or Size of an Object

Can More Than One Effect Occur at the Same Time?

Example: Kicking a Football

Everyday Applications of Force

Force Does Not Always Cause Motion

Example

Real-Life Connection

Did You Know?

Common Misconceptions

Exam Tips

Quick Revision

NCERT Keywords

Chapter Insight

6.1.1 Magnitude and Direction of Force

Magnitude of Force

NCERT Definition

Easy Definition

Everyday Examples

Why Does Magnitude Matter?

Direction of Force

NCERT Definition

Easy Definition

Everyday Examples

Opening a Drawer

Closing a Door

Lifting a School Bag

Dropping a Ball

Why is Direction Important?

Case 1: Same Direction

Case 2: Opposite Directions

Force is a Vector Quantity

Scalar Quantity

Vector Quantity

Representing Force

Real-Life Connection

SI Unit of Force

SI Unit

Named After

One Newton

Everyday Values of Force

Did You Know?

Common Mistakes

Memory Trick

Force = M + D

Exam Tips

Quick Revision