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Class 9 Science Chapter 6
How Forces Affect Motion

Chapter Summary

📘 Chapter Overview

Chapter Name Chapter 6 : How Forces Affect Motion
Subject Physics
Main Theme Understanding how different types of forces affect the motion of objects and how Newton's Laws explain these changes.
Real-Life Applications Playing sports, driving vehicles, launching rockets, pushing carts, braking bicycles, seat belts, elevators, and everyday movement of objects.

🌍 Why Should You Study This Chapter?

Have you ever wondered why a football starts moving when kicked, why a moving bicycle eventually stops, or why passengers move forward when a car suddenly applies brakes? The answer lies in the action of forces. This chapter helps you understand how forces influence the motion of objects and explains the scientific principles behind many everyday events using Newton's Laws of Motion.


🎯 In This Chapter, You Will Learn

✓ Meaning of force and its effects on objects.
✓ Difference between balanced and unbalanced forces.
✓ How force changes the speed, direction, or shape of an object.
✓ Newton's First Law of Motion and the concept of inertia.
✓ Newton's Second Law of Motion and the relationship between force, mass, and acceleration.
✓ Newton's Third Law of Motion and action-reaction force pairs.
✓ Meaning of momentum and how it depends on mass and velocity.
✓ Understanding weight as the gravitational force acting on an object.
✓ Applications of Newton's Laws in daily life and technology.

⚡ Important Formulae Covered

Concept Formula
Force F = ma
Weight W = mg
Momentum p = mv
Acceleration a = (v − u)/t
Second Equation of Motion s = ut + ½at²

💡 Everyday Examples

  • ⚽ Kicking a football to make it move.
  • 🚗 Seat belts protecting passengers during sudden braking.
  • 🚲 Applying brakes to stop a bicycle.
  • 🚀 Rocket launch based on Newton's Third Law.
  • 🏏 Hitting a cricket ball with a bat.
  • 🛒 Pushing a shopping trolley.
  • 🚪 Opening or closing a door by applying force.
  • 🪂 Skydiving and the effect of gravity.

📚 Learning Outcomes

After studying this chapter, you will be able to:
  • ✔ Explain what force is and describe its effects.
  • ✔ Differentiate between balanced and unbalanced forces.
  • ✔ State and explain Newton's Three Laws of Motion.
  • ✔ Solve numerical problems involving force, mass, acceleration, and momentum.
  • ✔ Calculate the weight of an object using gravitational acceleration.
  • ✔ Explain the concept of inertia with everyday examples.
  • ✔ Describe action-reaction force pairs in practical situations.
  • ✔ Apply the concepts of force and motion to real-life situations.

🌟 Quick Facts

Did You Know?
  • ⚡ A force can change the speed, direction, or shape of an object.
  • ⚡ Balanced forces do not change the state of motion of an object.
  • ⚡ Unbalanced forces always produce acceleration.
  • ⚡ Every action has an equal and opposite reaction.
  • ⚡ The SI unit of force is Newton (N).
  • ⚡ Momentum depends on both the mass and velocity of an object.
  • ⚡ Weight changes from one planet to another because gravity is different.

🚀 Ready to Explore?

Let's discover how forces control motion, why objects move the way they do, and how Newton's Laws explain everything from walking and cycling to launching rockets into space.

Important Concepts

Concept Description
Force A force is a push or pull acting on an object. It can change the state of motion, speed, direction, or shape of an object.
Effects of Force
  • Can start the motion of a stationary object.
  • Can stop a moving object.
  • Can increase or decrease the speed of an object.
  • Can change the direction of motion.
  • Can change the shape or size of an object.
Balanced Forces Balanced forces are equal in magnitude and opposite in direction. Their net force is zero, so they do not change the state of motion of an object.
Unbalanced Forces When the forces acting on an object are not equal, the net force is non-zero. Such forces change the speed, direction, or both, causing acceleration.
Inertia Inertia is the property of an object by which it resists any change in its state of rest or uniform motion. Greater the mass of an object, greater is its inertia.
Newton's First Law of Motion An object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an unbalanced external force. This law is also known as the Law of Inertia.
Newton's Second Law of Motion The acceleration produced in an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship is expressed by the equation F = ma.
Newton's Third Law of Motion For every action, there is an equal and opposite reaction. Action and reaction forces always act on different objects.
Momentum Momentum is the quantity of motion possessed by a moving object. It depends on both the mass and velocity of the object.
Weight Weight is the gravitational force acting on an object. It depends on the mass of the object and the acceleration due to gravity.
Net Force Net force is the resultant of all the forces acting on an object. It determines whether the object will accelerate, slow down, or continue moving with constant velocity.
Acceleration Due to Gravity The acceleration produced by Earth's gravitational pull is represented by g. Near the Earth's surface, its value is approximately 9.8 m/s².
Relation Between Force, Mass and Acceleration For the same force, a lighter object accelerates more than a heavier object. For the same mass, a larger force produces greater acceleration.
Action-Reaction Pair Action and reaction forces are equal in magnitude and opposite in direction. Since they act on different objects, they never cancel each other.

Key Points to Remember

  • ✔ Force is a push or pull that can change the motion of an object.
  • ✔ Balanced forces do not change the state of motion.
  • ✔ Unbalanced forces always produce acceleration.
  • ✔ Inertia depends on the mass of the object.
  • ✔ Newton's First Law is also called the Law of Inertia.
  • ✔ Newton's Second Law establishes the relationship between force, mass, and acceleration.
  • ✔ Newton's Third Law states that every action has an equal and opposite reaction.
  • ✔ Momentum depends on both mass and velocity.
  • ✔ Weight is a force caused by gravity and is measured in Newtons (N).
  • ✔ The SI unit of force is Newton (N).
  • ✔ The SI unit of momentum is kg·m/s.
  • ✔ The value of acceleration due to gravity near Earth's surface is approximately 9.8 m/s².

Quick Revision Table

Concept Remember This
Force A push or pull
Balanced Forces Net Force = 0
Unbalanced Forces Cause acceleration
Inertia Resistance to change in motion
Newton's First Law Law of Inertia
Newton's Second Law F = ma
Newton's Third Law Every action has an equal and opposite reaction
Momentum Mass × Velocity
Weight Gravitational Force (W = mg)
Net Force Resultant of all forces acting on an object

Remember These Facts

Important Fact Value / Information
SI Unit of Force Newton (N)
SI Unit of Momentum kg·m/s
Acceleration Due to Gravity 9.8 m/s² (approximately 10 m/s²)
Force Formula F = ma
Momentum Formula p = mv
Weight Formula W = mg

Science Class 9 Chapter - 6 NCERT Solution

Real-Life Applications

The concepts of Force, Motion, Newton's Laws, Momentum, and Weight are used in almost every activity of our daily lives. From walking and driving to launching satellites and playing sports, these principles help explain how objects move and interact with one another.


Real-Life Situation Application of the Concept
🚗 Driving a Car When a driver presses the accelerator, the engine produces a force that accelerates the car. Applying brakes creates an opposing force that slows down or stops the vehicle.
🛡️ Seat Belts in Vehicles Seat belts protect passengers due to Newton's First Law of Motion (Law of Inertia). During sudden braking, passengers tend to continue moving forward, and the seat belt prevents injury.
⚽ Playing Football or Cricket A football or cricket ball starts moving only when a player applies force. A stronger kick or hit produces greater acceleration and changes the ball's motion.
🚀 Rocket Launch A rocket moves upward because hot gases are pushed downward. This is an application of Newton's Third Law of Motion, where every action has an equal and opposite reaction.
🚲 Riding a Bicycle Pedalling applies force to the wheels, causing the bicycle to accelerate. Applying brakes creates friction that slows it down.
🛒 Pushing a Shopping Trolley An empty trolley accelerates more easily than a loaded one because a heavier object has greater mass and requires more force to produce the same acceleration.
🏏 Catching a Fast Ball A player moves their hands backward while catching a ball to increase the stopping time, reducing the force experienced by the hands.
🪂 Skydiving A parachute increases air resistance, reducing the skydiver's acceleration and allowing a safe landing.
🏋️ Weightlifting Weightlifters apply an upward force greater than the weight of the barbell to lift it against gravity.
🛗 Elevators Elevators use motors to apply force that lifts or lowers passengers while overcoming gravitational force.
🚪 Opening a Door Applying force to a door changes its state of rest and makes it rotate about its hinges.
🏃 Running and Walking While walking or running, your feet push the ground backward, and the ground pushes you forward with an equal and opposite force, illustrating Newton's Third Law.
🏗️ Construction Work Cranes and heavy machines apply large forces to lift and move construction materials safely and efficiently.
🎾 Tennis and Badminton The racket applies force to the ball or shuttlecock, changing its speed and direction according to Newton's Laws.
🚚 Transporting Heavy Goods Heavy trucks require greater force to start moving because they have larger mass and greater inertia.
🌍 Space Exploration Spacecraft and satellites move according to Newton's Laws of Motion, which are fundamental to modern space missions.
🧳 Pulling a Suitcase Applying a pulling force overcomes friction and sets the suitcase into motion.
🏍️ Riding a Motorcycle The engine provides the force needed for acceleration, while brakes apply an opposite force to reduce speed.
⚖️ Measuring Body Weight A weighing machine measures the gravitational force (weight) acting on a person's body.
🎡 Amusement Park Rides Roller coasters and other rides demonstrate changes in speed, direction, inertia, and the effects of forces during motion.

Key Takeaways

  • ✔ Every push or pull is an example of a force.
  • ✔ Force can start, stop, speed up, slow down, or change the direction of motion.
  • ✔ Newton's Laws explain the motion of everything from bicycles to rockets.
  • ✔ Momentum depends on both the mass and velocity of an object.
  • ✔ Weight is the gravitational force acting on an object.
  • ✔ Balanced forces do not change the state of motion, while unbalanced forces produce acceleration.
  • ✔ Understanding force and motion helps engineers design safer vehicles, stronger machines, efficient transport systems, and advanced space technology.

Memory Tricks to Remember Important Formulae

Use these simple memory tricks to quickly remember the important formulae and concepts of Chapter 6: How Forces Affect Motion.


Memory Trick Formula / Concept
👉 "Force Makes Acceleration" F = ma
Force = Mass × Acceleration
👉 "Weight Means Gravity" W = mg
Weight = Mass × Gravity
👉 "Mass Moves" p = mv
Momentum = Mass × Velocity
👉 "Velocity Changes Over Time" a = (v − u)/t
Acceleration = Change in Velocity ÷ Time
👉 "Start, Move, Speed" s = ut + ½at²
Second Equation of Motion
👉 "Gravity is Always 9.8" g = 9.8 m/s² (approximately 10 m/s²)

🧠 Newton's Laws Memory Trick

Law Easy Memory Trick
First Law "No Push, No Change"
An object remains at rest or continues moving unless acted upon by an unbalanced force.
Second Law "More Force, More Acceleration"
Force depends on mass and acceleration (F = ma).
Third Law "Every Action Gets a Reaction"
Every action has an equal and opposite reaction.

🚀 Super Memory Sentence

"Force Makes Acceleration, Weight Needs Gravity, Mass Moves Fast, Velocity Changes with Time, Every Action Gets a Reaction."


⚡ One-Line Formula Chain

Force → F = ma

Weight → W = mg

Momentum → p = mv

Acceleration → (v − u)/t

Motion → s = ut + ½at²


🎯 Quick Revision Code

  • 🟢 FMA → Force = Mass × Acceleration
  • 🟢 MG → Weight = Mass × Gravity
  • 🟢 MV → Momentum = Mass × Velocity
  • 🟢 VUT → Acceleration = (Final Velocity − Initial Velocity) ÷ Time
  • 🟢 UTA² → Second Equation of Motion
  • 🟢 9.8 → Acceleration due to Gravity

💡 Easy Word Association

Word Remember
Force Push or Pull
Inertia Resists Change
Momentum Mass in Motion
Weight Gravity's Pull
Balanced Forces No Change in Motion
Unbalanced Forces Cause Acceleration
Newton's Third Law Action ↔ Reaction

📚 Exam Magic Trick

Remember the sequence:

F → W → P → A → N

Force → Weight → Momentum → Acceleration → Newton's Laws

One Minute Revision

Revise the complete chapter in just one minute using the important concepts, formulae, and key facts given below.


Topic Quick Revision
Force A push or pull that can change the state of motion, speed, direction, or shape of an object.
Balanced Forces Equal and opposite forces. Net Force = 0. No change in the state of motion.
Unbalanced Forces Produce a net force and cause acceleration or change in motion.
Inertia The tendency of an object to resist any change in its state of rest or motion.
Newton's First Law An object remains at rest or moves uniformly unless acted upon by an unbalanced force.
Newton's Second Law Force is directly proportional to mass and acceleration.
F = ma
Newton's Third Law Every action has an equal and opposite reaction.
Momentum Momentum depends on the mass and velocity of an object.
p = mv
Weight Weight is the gravitational force acting on an object.
W = mg
Gravity Acceleration due to gravity near Earth's surface is 9.8 m/s² (approximately 10 m/s²).

Formula Flash Cards

Concept Formula
Force F = ma
Weight W = mg
Momentum p = mv
Acceleration a = (v − u)/t
Second Equation of Motion s = ut + ½at²

Exam Booster Points

  • ✔ Force can start, stop, speed up, slow down, or change the direction of an object.
  • ✔ Balanced forces do not change the state of motion.
  • ✔ Unbalanced forces always produce acceleration.
  • ✔ Inertia depends on the mass of the object.
  • ✔ Newton's First Law is also known as the Law of Inertia.
  • ✔ Newton's Second Law explains the relationship between force, mass, and acceleration.
  • ✔ Action and reaction forces are equal in magnitude but act on different objects.
  • ✔ Momentum increases if either mass or velocity increases.
  • ✔ Weight depends on gravity, while mass remains constant everywhere.
  • ✔ The SI unit of force is Newton (N) and the SI unit of momentum is kg·m/s.

30-Second Formula Recap

F = ma

W = mg

p = mv

a = (v − u)/t

s = ut + ½at²


Quick Concept Chain

Force → Motion → Inertia → Newton's Laws → Acceleration → Momentum → Weight


Remember These Keywords

Keyword Remember
Force Push or Pull
Balanced Force No Change in Motion
Unbalanced Force Changes Motion
Inertia Resists Change
Momentum Mass × Velocity
Weight Force due to Gravity
Newton's Third Law Action ↔ Reaction

🎯 Final Revision Tip

Remember the sequence:

Force → Motion → Newton's Laws → F = ma → Momentum → Weight

If you understand this sequence, you can solve most conceptual and numerical questions from this chapter.

Think Like a Scientist

A scientist does not simply observe motion—they ask why objects move, how forces act on them, and what happens if the conditions change. This chapter encourages you to observe your surroundings carefully and apply the concepts of Force, Motion, Inertia, Newton's Laws, Momentum, and Weight to explain real-life events scientifically.


🔬 Think Like a Scientist
1 Why do passengers move forward when a moving bus stops suddenly, but move backward when the bus starts moving suddenly? Explain using the concept of inertia.
2 A football and a stone are kicked with the same force. Why does the football accelerate much more than the stone? Which law of motion explains this?
3 Why is it easier to push an empty shopping trolley than a fully loaded one? Relate your answer to mass, inertia, and force.
4 A rocket moves upward even though there is no surface to push against in space. Which scientific principle explains this phenomenon?
5 Why do athletes bend their knees immediately after landing from a jump? How does this reduce the force acting on their body?
6 Two cars are moving at the same speed, but one has a much larger mass. Which car has greater momentum? Explain your reasoning.
7 Why is wearing a seat belt important even when driving at a moderate speed? Which law of motion is involved?
8 Why does a person feel heavier while standing in a fast-moving lift that is accelerating upward and lighter when it accelerates downward?
9 If there were no friction between your shoes and the ground, would you be able to walk? Explain scientifically.
10 Why do racing cars have powerful engines compared to ordinary cars? Relate your answer to force and acceleration.

Scientific Challenge

Challenge
Imagine you are a safety engineer designing a new school bus.
  • How would you use Newton's Laws to make the bus safer?
  • Why should every seat have a seat belt?
  • How can you reduce injuries during sudden braking?
  • Suggest one additional safety feature based on the concepts of force and motion.

Observe Around You

  • 🔎 Observe how a bicycle slows down when brakes are applied.
  • 🔎 Notice how a football changes direction when it is kicked.
  • 🔎 Watch how a shopping trolley moves differently when it is empty and when it is loaded.
  • 🔎 Observe passengers inside a bus during sudden starts and stops.
  • 🔎 Look for examples of action and reaction while walking, swimming, rowing a boat, or jumping.
  • 🔎 Notice how rockets, balloons, and jet aircraft move due to Newton's Third Law.

Think Beyond the Textbook

Question
If gravity on Earth suddenly became twice as strong for one day:
  • How would your weight change?
  • Would your mass change?
  • Would walking and jumping become easier or more difficult?
  • How would vehicles and sports be affected?
Explain your answers using the concepts learned in this chapter.

Scientist's Thinking Tip 💡

"A scientist observes every movement carefully, asks what force caused it, identifies which law of motion explains it, and uses evidence—not guesses—to reach a conclusion."


🚀 Mission for Young Scientists

Observe any five activities at home, school, or on the road that involve force or motion. For each activity, identify the type of force acting, mention which of Newton's Laws explains it, and describe how the motion changes.

Frequently Asked Questions (FAQs)

These frequently asked questions will help you quickly revise the important concepts of Chapter 6: How Forces Affect Motion.


Q.No. Question Answer
1 What is force? A force is a push or pull that can change the state of motion, speed, direction, or shape of an object.
2 What are the effects of force? Force can start or stop motion, change speed, change direction, and change the shape or size of an object.
3 What are balanced forces? Balanced forces are equal and opposite forces whose net force is zero. They do not change the state of motion of an object.
4 What are unbalanced forces? Unbalanced forces produce a non-zero net force and change the speed or direction of an object.
5 What is inertia? Inertia is the property of an object to resist any change in its state of rest or uniform motion.
6 State Newton's First Law of Motion. An object remains at rest or continues to move with uniform velocity unless acted upon by an unbalanced external force.
7 Why is Newton's First Law called the Law of Inertia? Because it explains that every object has inertia and resists changes in its state of motion.
8 State Newton's Second Law of Motion. The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. It is expressed as F = ma.
9 State Newton's Third Law of Motion. For every action, there is an equal and opposite reaction.
10 What is momentum? Momentum is the quantity of motion possessed by an object and is equal to the product of its mass and velocity.
11 What is the formula for momentum? Momentum (p) = Mass × Velocity (mv).
12 What is weight? Weight is the gravitational force acting on an object.
13 How is weight different from mass? Mass is the amount of matter in an object and remains constant, whereas weight depends on gravity and can change from one planet to another.
14 What is the SI unit of force? The SI unit of force is Newton (N).
15 What is the SI unit of momentum? The SI unit of momentum is kg·m/s.
16 What is the SI unit of weight? Weight is a force, so its SI unit is Newton (N).
17 What is the value of acceleration due to gravity? Near the Earth's surface, the value of g is approximately 9.8 m/s² (or 10 m/s² for simple calculations).
18 Why do passengers move forward when a bus stops suddenly? Because of inertia, the passengers' bodies tend to continue moving forward even after the bus stops.
19 Why are seat belts important? Seat belts prevent passengers from moving forward suddenly due to inertia during sudden braking or collisions.
20 Give two real-life applications of Newton's Third Law. Rocket launches and walking are common examples of Newton's Third Law of Motion.

Quick Exam FAQs

  • Q. What is force?
    A. A push or pull.
  • Q. What is inertia?
    A. Resistance to change in the state of motion.
  • Q. What is Newton's First Law also called?
    A. The Law of Inertia.
  • Q. What is the formula for force?
    A. F = ma.
  • Q. What is the formula for momentum?
    A. p = mv.
  • Q. What is the formula for weight?
    A. W = mg.
  • Q. What is the SI unit of force?
    A. Newton (N).
  • Q. What is the SI unit of momentum?
    A. kg·m/s.
  • Q. What is the SI unit of weight?
    A. Newton (N).
  • Q. What is the value of acceleration due to gravity?
    A. 9.8 m/s² (approximately 10 m/s²).

📚 Exam Tip

Before solving any numerical problem, first identify the given values, write the correct formula, substitute the values using SI units, perform the calculation carefully, and always write the final answer with the correct unit.