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⚡ GRADE 9 PHYSICS — CHAPTER: MOTION & FORCE

Master Physics.Interactively.

Learn, revise, practice, and self-test — all in one extraordinary learning experience. Concepts come alive with animations, interactive graphs, and instant feedback.

Learning Sections

50+

Practice Questions

Live Animations

100%

Interactive

01 · LEARNING OBJECTIVES

What You Will Learn

🚗

Define velocity and acceleration and distinguish between them clearly.

⚡

Explain inertia and state all three of Newton’s Laws of Motion.

📈

Interpret distance-time and velocity-time graphs accurately.

🔄

Differentiate between elasticity and plasticity with real-life examples.

🔢

Solve numerical problems using the three equations of motion.

🧠

Apply physics concepts to real-world situations and everyday observations.

02 · CONCEPT MAP

Chapter Overview

Motion

→

Distance / Displacement

→

Speed / Velocity

→

Acceleration

→

Equations of Motion

Force

→

Inertia

→

Newton’s 1st Law

Newton’s 2nd Law

Newton’s 3rd Law

Materials

→

Elasticity

→

Springs / Rubber

Plasticity

→

Clay / Putty

03 · QUICK NOTES

Key Concepts

📍 Distance vs Displacement

Distance — total path length covered (scalar). Displacement — shortest path from start to end point (vector).

⚡ Speed vs Velocity

Speed = Distance/Time (no direction). Velocity = Displacement/Time (has direction). Velocity can be negative; speed cannot.

v = Δs / Δt

🚀 Acceleration

Rate of change of velocity. Can be positive (speeding up) or negative (slowing down, called deceleration/retardation).

a = (v − u) / t

🌀 Inertia

The tendency of an object to resist any change in its state of rest or motion. More mass = more inertia. It is NOT a force.

🔵 Uniform Motion

Object travels equal distances in equal time intervals. Velocity is constant, acceleration = 0. Distance-time graph is a straight line.

a = 0, v = const

🔴 Non-Uniform Motion

Velocity changes over time. Acceleration ≠ 0. Distance-time graph is curved. Velocity-time graph is a straight line (for uniform acceleration).

🧲 Elasticity

Property of a material to regain its original shape after the deforming force is removed. Example: rubber band, spring.

🏺 Plasticity

Property of a material that does NOT regain its original shape after the deforming force is removed. Example: clay, putty, wax.

04 · VISUAL INFOGRAPHIC

Newton’s Laws of Motion

⚖️ The Three Laws That Govern All Motion

Law of Inertia

An object at rest stays at rest and an object in motion stays in motion at constant velocity, unless acted upon by a net external force.

🚗 Seatbelt saves you when car brakes suddenly

Law of Acceleration

The acceleration of an object is directly proportional to the net force and inversely proportional to its mass. F = ma

🏋️ Heavier objects need more force to accelerate

III

Law of Reaction

For every action, there is an equal and opposite reaction. Forces always occur in pairs on different objects.

🚀 Rocket exhaust pushes down → rocket goes up

05 · FORMULA CARDS

Interactive Formula Cards

Click each card to reveal the formula details

ACCELERATION

a = (v−u)/t

Rate of change of velocity over time

Units: m/s² | SI

1ST EQUATION

v = u + at

Final velocity from initial velocity and acceleration

v=final, u=initial, a=accel, t=time

2ND EQUATION

s = ut + ½at²

Displacement when initial velocity and acceleration known

s=displacement (m)

3RD EQUATION

v² = u² + 2as

Velocity-displacement relationship (no time)

Useful when t is unknown

NEWTON’S 2ND LAW

F = ma

Net force equals mass times acceleration

F(N), m(kg), a(m/s²)

MOMENTUM

p = mv

Product of mass and velocity

Units: kg·m/s

AVERAGE VELOCITY

v̄ = (u+v)/2

Average of initial and final velocities (uniform accel)

Units: m/s

HOOKE’S LAW

F = −kx

Spring force proportional to extension/compression

k=spring constant (N/m)

06 · ANIMATION SECTION

See Physics in Action

07 · INTERACTIVE GRAPH EXPLORER

Draw Your Own Graphs

Graph Controls

Select a graph type and click a button above to visualise different motion patterns.

08 · THINK & PREDICT

Can You Explain It?

What happens when a bus suddenly stops? ▾

Passengers lurch forward due to inertia. Their bodies were moving with the bus; when the bus stops, no force acts on the passengers immediately, so they continue forward (Newton’s 1st Law).

Why do passengers lean backward when a bus starts? ▾

The bus accelerates forward, but the passengers’ bodies (due to inertia) remain behind. They appear to lean backward relative to the bus.

Why does a rocket move upward? ▾

Newton’s 3rd Law! The rocket engine burns fuel and expels hot gases downward (action). The equal and opposite reaction force pushes the rocket upward.

Why is it hard to stop a moving truck vs a bicycle? ▾

The truck has much greater mass, giving it more inertia and greater momentum (p = mv). A larger force or longer time is needed to change its momentum.

A rubber band snaps back; why doesn’t clay? ▾

Rubber is an elastic material — it returns to original shape when force is removed. Clay is a plastic material — its molecules rearrange permanently.

09 · REAL-LIFE CONNECTIONS

Physics Around You

🚗

Seat Belt

Stops your body when car brakes — Newton’s 1st Law & Inertia

🚶

Walking

You push backward on Earth; Earth pushes you forward — Newton’s 3rd Law

🚀

Rocket Launch

Gas expelled down → rocket moves up. Action-Reaction pair.

🎯

Rubber Band

Stores elastic potential energy, returns to original shape — Elasticity

⚖️

Spring Balance

Uses Hooke’s Law: extension proportional to force applied.

⛵

Jumping from Boat

You jump forward → boat moves backward — Newton’s 3rd Law

🏏

Cricket Ball

Bat applies force → ball accelerates — Newton’s 2nd Law (F=ma)

🏺

Pottery Clay

Retains new shape permanently after moulding — Plasticity

10 · WORKED EXAMPLES

Step-by-Step Solutions

A car starts from rest and reaches 20 m/s in 5 seconds. Find the acceleration.

Identify: u = 0 m/s (starts from rest), v = 20 m/s, t = 5 s

Use formula: a = (v − u) / t

Substitute: a = (20 − 0) / 5

∴ a = 4 m/s²

A ball is thrown with initial velocity 5 m/s and acceleration 2 m/s². Find the distance in 3 seconds.

Given: u = 5 m/s, a = 2 m/s², t = 3 s

Use: s = ut + ½at²

s = (5)(3) + ½(2)(3)² = 15 + 9

∴ s = 24 m

A 10 kg box accelerates at 3 m/s². What net force acts on it?

Given: m = 10 kg, a = 3 m/s²

Use: F = ma

∴ F = 30 N

11 · FLASHCARDS

Flip & Remember

Click each card to flip and reveal the definition

12 · KEY POINTS TO REMEMBER

Exam Essentials

💡 Important Facts

Speed is scalar; velocity is vector. Distance ≥ displacement. g = 9.8 m/s². Area under V-T graph = displacement. Slope of D-T graph = speed.

⚠️ Watch Out

Don’t confuse retardation with negative acceleration (they’re the same thing). Inertia is NOT a force — it is a property. Both action and reaction forces act on different bodies.

❌ Common Mistakes

Using wrong sign for deceleration in equations. Forgetting that momentum changes when mass OR velocity changes. Assuming that zero velocity means zero acceleration.

📐 Unit Reminder

Distance/Displacement: metre (m). Speed/Velocity: m/s. Acceleration: m/s². Force: Newton (N). Momentum: kg·m/s. Mass: kilogram (kg).

13 · FILL IN THE BLANKS

Test Your Memory

The rate of change of velocity is called .

An object at rest has tendency to remain at rest due to .

The unit of force is (N).

is the inability of a material to regain its original shape.

Momentum = mass × and its unit is kg·m/s.

The slope of a velocity-time graph gives .

14 · TRUE OR FALSE

Instant Feedback

15 · MULTIPLE CHOICE QUESTIONS

Test Your Knowledge

16 · MATCH THE FOLLOWING

Connect Concepts

Click a law on the left, then its matching example on the right.

Law / Quantity

Newton’s 1st Law

Newton’s 2nd Law

Newton’s 3rd Law

Elasticity

Plasticity

Example / Description

Rocket propulsion

Clay retains new shape

Seatbelt in sudden stop

Rubber band returns to shape

Heavier object needs more force

18 · ASSERTION & REASON

Board Exam Practice

Assertion (A): A person in a moving bus falls forward when the bus stops suddenly.

Reason (R): This happens due to the property of inertia.

Assertion (A): The slope of a velocity-time graph gives acceleration.

Reason (R): Acceleration = change in velocity / time = slope of V-T graph.

19 · CASE STUDY QUESTIONS

Real Situations, Physics Thinking

🚗 Case Study 1: The Braking Car
Rajan is driving his car at 60 km/h. He suddenly applies the brakes. The car decelerates uniformly and stops after travelling 50 m. Rajan’s body lurches forward before the seatbelt holds him.

Questions:

Q1. Why does Rajan’s body lurch forward when the car brakes? Name the law involved.

Q2. Convert 60 km/h to m/s. (Hint: 1 km/h = 1000/3600 m/s)

Q3. Find the deceleration using v² = u² + 2as (v=0, u=60km/h, s=50m).

Q4. What is the role of the seatbelt in this situation?

🚀 Case Study 2: The Rocket Launch
A rocket of mass 500 kg is launched vertically. The engine generates thrust of 8000 N. Hot gases are expelled downward at very high speed.

Questions:

Q1. Which of Newton’s Laws explains why the rocket moves upward?

Q2. Find the net upward force if weight = 5000 N and thrust = 8000 N.

Q3. Find the initial acceleration of the rocket (mass = 500 kg).

Q4. Why does the rocket accelerate more as fuel burns?

20 · HOTS

Higher Order Thinking

APPLICATION

A glass falls onto a hard floor and breaks, but the same glass placed on foam doesn’t break. Explain using Newton’s laws and the concept of impulse (F × t).

ANALYSIS

Two objects A and B have the same kinetic energy. A has twice the mass of B. Which has greater momentum? Show your reasoning mathematically.

REASONING

A ball thrown vertically upward has zero velocity at the top. Is it in equilibrium? Justify with reference to forces and acceleration.

EVALUATION

If Newton’s 3rd Law says every action has an equal and opposite reaction, why does a horse pulling a cart actually make the cart move? Shouldn’t the reaction cancel the action?

21 · NUMERICAL PRACTICE

Solve & Check

🔵 Basic — Acceleration Calculation

Given: u = 2 m/s, v = 14 m/s, t = 4 s Find: acceleration (a)

a = m/s²

🟡 Intermediate — Second Equation of Motion

Given: u = 0, a = 5 m/s², t = 6 s Find: displacement (s)

s = m

23 · SUMMARY NOTES

Chapter at a Glance

🚗 Motion

Distance (scalar) ≠ Displacement (vector)
Speed (scalar) ≠ Velocity (vector)
Acceleration = rate of change of velocity
Three equations connect u, v, a, s, t
D-T slope = speed; V-T slope = acceleration

⚖️ Newton’s Laws

1st: Objects resist change → Inertia
2nd: F = ma → more force, more acceleration
3rd: Action = equal and opposite Reaction
Reaction forces act on DIFFERENT bodies
Momentum = mv; changes with net force

🧲 Elasticity & Plasticity

Elastic: returns to original shape (rubber, spring)
Plastic: permanent deformation (clay, putty)
Hooke’s Law: F = kx (within elastic limit)
Spring balance uses elastic property
Beyond elastic limit → permanent deformation

24 · FORMULA SHEET

Complete Reference

Quantity / Law	Formula	Units	Notes
Speed	v = d/t	m/s	Scalar quantity
Velocity	v = s/t	m/s	Vector quantity
Acceleration	a = (v−u)/t	m/s²	Vector quantity
1st Equation	v = u + at	m/s	Velocity-time relation
2nd Equation	s = ut + ½at²	m	Displacement-time relation
3rd Equation	v² = u² + 2as	m²/s²	No time involved
Newton’s 2nd Law	F = ma	N (kg·m/s²)	Net force
Momentum	p = mv	kg·m/s	Vector quantity
Impulse	J = F·t = Δp	N·s	Change in momentum
Hooke’s Law	F = kx	N/m (k)	Within elastic limit
Average Velocity	v̄ = (u+v)/2	m/s	Uniform acceleration only
Weight	W = mg	N	g = 9.8 m/s²

27 · SELF-ASSESSMENT QUIZ

Auto-Scoring Quiz

28 · CHALLENGE ZONE

Olympiad Level

Brain Teaser #1

A bullet fired from a gun has mass 10 g and velocity 400 m/s. The gun has mass 2 kg. Find the recoil velocity of the gun.

By conservation of momentum: 0.01 × 400 = 2 × v → v = 2 m/s (backward)

Brain Teaser #2

Two forces act on a body: 6N East and 8N North. Find the resultant force and its direction.

F = √(6² + 8²) = √100 = 10 N. Angle = tan⁻¹(8/6) ≈ 53.1° from East toward North.

Puzzle #3

An astronaut in space pushes a wall. What happens to the astronaut and the wall? Explain all forces in detail.

Newton’s 3rd Law: astronaut pushes wall (action) → wall pushes astronaut backward (reaction). The astronaut drifts backward.

Olympiad #4

A ball dropped from height h bounces and rises to height h/2. Find the ratio of speed just after bounce to speed just before.

v_before = √(2gh). v_after = √(gh). Ratio = 1/√2 ≈ 0.707 (coefficient of restitution).

26 · EXAM BOOSTER

Last-Minute Revision

📌 Top Definitions

Acceleration: rate of change of velocity
Inertia: resistance to change in motion
Force: push or pull, F = ma
Momentum: p = mv
Elasticity: regains original shape

🔢 Must-Know Equations

v = u + at
s = ut + ½at²
v² = u² + 2as
F = ma
p = mv

📈 Graph Facts

D-T slope = speed
Flat D-T = rest
V-T slope = acceleration
V-T area = displacement
Flat V-T = uniform motion

❓ Frequent Questions

Explain Newton’s 3rd Law with example
Difference: elastic vs plastic
What is inertia? Give 2 examples
Derive v = u + at
Why do we need seatbelts?

30 · DOWNLOAD CORNER

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