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Chapter 6: How Forces Affect Motion

Grade 9 Science | Chapter 6

How Forces Affect Motion

Why do objects start, stop or change direction? This chapter introduces force, inertia and momentum, and sets out Newton’s three laws with clear reasoning and diagrams.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: What Is a Force?

Balanced and Unbalanced Forces

Newton’s First Law and Inertia

Newton’s Second Law and Momentum

Newton’s Third Law

Free Body Diagrams

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: What Is a Force?

A force is a push or a pull that can change the speed, direction or shape of an object. Force is a vector, so it has both size and direction, and its SI unit is the newton (N). Forces never act alone in our experience; several usually act on an object at once, and it is their combined effect, the net force, that decides how the object moves.

This chapter explains how forces affect motion through the work of Isaac Newton, whose three laws connect force, mass and motion. Along the way we meet inertia, the reluctance of objects to change their motion, and momentum, a measure of how much motion an object has.

Core idea

A single force, or an unbalanced set of forces, changes an object’s motion. The net force is what remains after all the forces on an object are combined.

2. Balanced and Unbalanced Forces

When the forces on an object cancel out, they are balanced and the net force is zero; the object stays at rest or keeps moving steadily. When they do not cancel, they are unbalanced and the net force changes the object’s motion, speeding it up, slowing it down or turning it.

Forces

Net Force

Effect on Motion

Balanced

Zero

No change: stays at rest or moves steadily

Unbalanced

Not zero

Motion changes: speeds up, slows down or turns

3. Newton’s First Law and Inertia

Newton’s first law states that an object stays at rest, or keeps moving in a straight line at steady speed, unless an unbalanced force acts on it. This tendency to resist a change in motion is called inertia, and it is greater for objects of greater mass. A heavy lorry is harder to start or stop than a light bicycle because it has more inertia.

4. Newton’s Second Law and Momentum

Momentum is the product of mass and velocity, p = m v, a measure of how much motion an object has. Newton’s second law says that the net force equals the rate of change of momentum, which for constant mass gives the familiar form F = m a: the net force equals mass times acceleration. A larger force gives a larger acceleration, and a heavier object accelerates less for the same force.

Watch out

F = ma uses the net force, not just one of the forces acting. Always combine the forces first to find the net force before using the equation.

5. Newton’s Third Law

Newton’s third law states that for every action there is an equal and opposite reaction. When one object pushes on a second, the second pushes back equally hard in the opposite direction. These two forces act on different objects, which is why they do not simply cancel. A swimmer pushes water backward and the water pushes the swimmer forward.

Diagram 1 – Action and Reaction

Two blocks in contact with equal and opposite force arrows showing action and reaction

Fig 1. Block A pushes on block B; block B pushes back on A with an equal force in the opposite direction. The two forces act on different objects.

6. Free Body Diagrams

A free body diagram shows all the forces acting on a single object as labelled arrows. For a box resting on the ground there is its weight pulling down, the normal force pushing up, an applied force if it is pushed, and friction opposing motion. Drawing these arrows makes it easy to see whether the forces are balanced and to find the net force.

Diagram 2 – Free Body Diagram

Free body diagram of a box on the ground showing weight, normal force, applied force and friction

Fig 2. The forces on a box: weight downward, normal force upward, an applied force forward and friction backward. Their combined effect is the net force.

7. Key Reasoning (Principles)

Principle 1: Net force decides the motion

Only the net force, found by combining all the forces, changes an object’s motion. If the net force is zero, the motion does not change.

Principle 2: F = ma links force, mass and acceleration

For a given net force, a heavier object accelerates less, and for a given mass, a larger force gives a larger acceleration. Force and acceleration are in the same direction.

Principle 3: Action and reaction act on different bodies

The equal and opposite forces of Newton’s third law act on two different objects, so they do not cancel each other and can still produce motion.

8. Worked Examples

Example 1

Q: A force of 6 N acts on a 2 kg object. Find its acceleration.

▶ Show Solution

a = F ÷ m = 6 ÷ 2.

= 3 m/s².

Answer: 3 m/s².

Example 2

Q: Find the force needed to give a 4 kg object an acceleration of 5 m/s².

▶ Show Solution

F = m a = 4 × 5.

= 20 N.

Answer: 20 N.

Example 3

Q: Find the momentum of a 5 kg object moving at 4 m/s.

▶ Show Solution

p = m v = 5 × 4.

= 20 kg m/s.

Answer: 20 kg m/s.

Example 4

Q: Two forces act on a box: 10 N right and 4 N left. Find the net force.

▶ Show Solution

Net force = 10 minus 4 = 6 N, to the right.

Answer: 6 N to the right.

Example 5

Q: A net force of 12 N gives an object an acceleration of 2 m/s². Find its mass.

▶ Show Solution

m = F ÷ a = 12 ÷ 2.

= 6 kg.

Answer: 6 kg.

Example 6

Q: Find the weight of a 10 kg object (take g = 9.8 m/s²).

▶ Show Solution

Weight = m g = 10 × 9.8.

= 98 N.

Answer: 98 N.

Example 7

Q: An object of mass 3 kg speeds up from 2 m/s to 5 m/s. Find its change in momentum.

▶ Show Solution

Change in momentum = m (v minus u) = 3 × (5 minus 2).

= 3 × 3 = 9 kg m/s.

Answer: 9 kg m/s.

Example 8

Q: A 1000 kg car accelerates at 2 m/s². Find the driving force.

▶ Show Solution

F = m a = 1000 × 2.

= 2000 N.

Answer: 2000 N.

Example 9

Q: Why is it harder to stop a loaded lorry than an empty one?

▶ Show Solution

The loaded lorry has more mass, so more inertia and more momentum, needing a larger force to stop.

Answer: It has more mass, so more inertia and momentum.

Example 10

Q: A swimmer pushes water backward. What pushes the swimmer forward?

▶ Show Solution

By Newton’s third law, the water pushes the swimmer forward with an equal and opposite force.

Answer: The water pushes the swimmer forward (reaction).

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. The SI unit of force is the: (a) joule (b) newton (c) watt (d) pascal

2. An object keeps moving steadily when the net force on it is: (a) large (b) zero (c) upward (d) backward

3. The resistance of an object to a change in motion is called: (a) momentum (b) inertia (c) friction (d) weight

4. F = ma uses which force? (a) weight only (b) friction only (c) the net force (d) the normal force

5. Action and reaction forces act on: (a) the same object (b) different objects (c) no object (d) the ground only

▶ Reveal Answers

1. (b) newton.

2. (b) zero.

3. (b) inertia.

4. (c) the net force.

5. (b) different objects.

Set B – Short Answer (Understanding)

1. Define momentum and give its unit.

2. State Newton’s first law of motion.

3. What is the difference between balanced and unbalanced forces?

4. Why do action and reaction forces not cancel each other?

5. Why does a heavier object need a larger force to give it the same acceleration?

▶ Reveal Answers

1. Momentum is mass times velocity, p = mv; its unit is kilogram metre per second (kg m/s).

2. An object stays at rest or moves steadily in a straight line unless an unbalanced force acts on it.

3. Balanced forces cancel and give zero net force; unbalanced forces do not cancel and change the motion.

4. Because they act on two different objects, not the same one, so they do not cancel.

5. Because F = ma, so for the same acceleration a larger mass needs a proportionally larger force.

Set C – Application and Reasoning

1. A 3 kg object is pushed with a net force of 9 N. Find its acceleration.

2. Find the momentum of a 2 kg ball moving at 6 m/s.

3. A 5 N force acts left and an 8 N force acts right on a box. Find the net force.

4. Find the force needed to accelerate a 0.5 kg ball at 4 m/s².

5. Why does a passenger lurch forward when a bus stops suddenly?

▶ Reveal Answers

1. a = 9 ÷ 3 = 3 m/s².

2. p = 2 × 6 = 12 kg m/s.

3. 8 minus 5 = 3 N to the right.

4. F = 0.5 × 4 = 2 N.

5. By inertia, the passenger tends to keep moving forward when the bus suddenly stops.

Set D – Higher Order (Challenge)

1. A 1200 kg car accelerates from rest to 20 m/s in 8 s. Find the driving force.

2. Explain, using Newton’s third law, how a rocket moves forward in space.

3. Two objects have the same momentum but different masses. Which moves faster, and why?

4. A 2 kg trolley changes velocity from 3 m/s to 7 m/s in 2 s. Find the force acting on it.

5. Why is wearing a seat belt important when a car stops suddenly?

▶ Reveal Answers

1. First a = (20 minus 0) ÷ 8 = 2.5 m/s²; then F = ma = 1200 × 2.5 = 3000 N.

2. The rocket pushes hot gases backward (action); the gases push the rocket forward with an equal and opposite force (reaction).

3. The lighter object moves faster, because for the same momentum p = mv a smaller mass needs a larger velocity.

4. a = (7 minus 3) ÷ 2 = 2 m/s²; F = ma = 2 × 2 = 4 N.

5. The body has inertia and tends to keep moving forward; the seat belt provides the force to stop it safely.

Chapter Summary

Force

A push or pull that changes speed, direction or shape; measured in newtons.

Balanced vs Unbalanced

Balanced forces give zero net force; unbalanced forces change the motion.

First Law and Inertia

Objects resist changes in motion; more mass means more inertia.

Second Law

Net force = mass times acceleration, F = ma; momentum is p = mv.

Third Law

Every action has an equal and opposite reaction, acting on different objects.

Free Body Diagram

Shows all forces on one object as labelled arrows to find the net force.

Quantity

Unit

Symbol

Force

newton

Momentum

kilogram metre per second

kg m/s

Acceleration

metre per second squared

m/s²

8-Point Exam Quick-Check

A force is a push or pull measured in newtons; it is a vector.

Balanced forces give zero net force and no change in motion.

Newton’s first law: objects stay at rest or move steadily unless an unbalanced force acts.

Inertia is the resistance to a change in motion and grows with mass.

Newton’s second law: net force = mass times acceleration, F = ma; momentum p = mv.

Newton’s third law: every action has an equal and opposite reaction.

Action and reaction act on different objects, so they do not cancel.

A free body diagram shows all the forces on one object as labelled arrows.

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Class 9 Science Chapter 6: How Forces Affect Motion, Complete Notes and Practice

This revision guide follows the NCERT 2026 to 27 Exploration syllabus and explains how forces change motion, covering balanced and unbalanced forces, Newton’s three laws, inertia, momentum, the equation F equals ma and free body diagrams, with two labelled diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.