Class 9 Science – Chapter 8: Motion | Full NCERT Notes | NCERT Nation

 

 Class 9 Science – Chapter 8: Motion | Full NCERT Notes | NCERT Nation

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🚀 Introduction: Understanding the Concept of Motion

Imagine standing at a bus stop and watching vehicles rush past you. Some are slow, some are fast, and some stop right in front of you. In this everyday observation lies one of the most fundamental ideas in science — motion.

Motion is everywhere — the earth revolving around the sun, a fan spinning, a train running, a ball rolling, or even the blood flowing in your body. The study of motion helps us understand how and why things move, forming the base of mechanics, one of the most essential branches of physics.

When we study motion, we are not only observing the change in position of an object with respect to time but also describing how fast, in what direction, and how regularly this change occurs. This chapter, “Motion,” from Class 9 NCERT Science, will help you decode all of that — step by step — in the simplest and most engaging way possible.

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🌍 What Is Motion?

Definition:
An object is said to be in motion if it changes its position with respect to its surroundings over time.
For example:

• A flying bird changes its position with time → it is in motion.

• A building does not change its position → it is at rest.

Both motion and rest are relative terms.
A passenger sitting in a moving train is at rest with respect to the train but in motion with respect to the ground.

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🔹 Types of Motion

Motion can be classified based on how an object moves:

1. Linear (Translatory) Motion
   When an object moves along a straight line.
   Example: a car moving on a straight road.

2. Circular Motion
   When an object moves around a fixed point in a circular path.
   Example: a ceiling fan, the motion of the moon around the Earth.

3. Periodic Motion
   When motion repeats itself after a fixed interval of time.
   Example: the pendulum of a clock.

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⚙️ Distance and Displacement

Distance

• The total path covered by an object, irrespective of direction.

• Scalar quantity (has only magnitude).

• Example: If you walk 3 km east and 4 km west, total distance = 7 km.

Displacement

• The shortest distance between the initial and final position of an object, along with direction.

• Vector quantity (has magnitude + direction).

• From the same example: displacement = 1 km (towards east).

✅ Key Difference:

• Distance tells how much ground is covered.

• Displacement tells how far you are from the starting point.

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🕒 Speed and Velocity

Speed

Speed is the rate of change of distance.

$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Unit: m/s (meter per second)

Types of speed:

• Uniform speed – covers equal distances in equal intervals of time.

• Non-uniform speed – covers unequal distances in equal intervals of time.

Velocity

Velocity is the rate of change of displacement.

$$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$$

Velocity is a vector quantity — it depends on both speed and direction.

Example:
If a car moves 100 m north in 5 seconds,
Velocity = 100 ÷ 5 = 20 m/s (north).

If direction changes, velocity changes — even if speed remains constant (like a car turning in a circular track).

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🔺 Acceleration

When velocity changes (increases, decreases, or direction changes), the motion is said to be accelerated.

$$\text{Acceleration} = \frac{\text{Change in velocity}}{\text{Time}}$$

• Positive acceleration: Speed increases

• Negative acceleration (Retardation): Speed decreases

Example:
A car’s velocity increases from 10 m/s to 30 m/s in 5 s.

$$a = \frac{30 - 10}{5} = 4 \text{ m/s}^2$$

Unit: m/s² (meter per second squared)

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📉 Graphical Representation of Motion

Graphs make it easier to visualize how motion changes with time.

1. Distance-Time Graph

• For uniform motion, it is a straight line.

• For non-uniform motion, it’s a curved line.

2. Velocity-Time Graph

• The area under the velocity-time graph gives distance travelled.

• A straight, sloping line shows uniform acceleration.

Example:
If a velocity-time graph is a straight line from 0 to 10 m/s in 5 s,
Area under it = ½ × base × height = ½ × 5 × 10 = 25 m.
So, distance = 25 m.

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📐 Equations of Uniformly Accelerated Motion

When acceleration is constant, three equations of motion apply:

1. v = u + at
   (Final velocity = initial velocity + acceleration × time)

2. s = ut + ½ at²
   (Distance = initial velocity × time + ½ × acceleration × time²)

3. v² = u² + 2as
   (Final velocity² = initial velocity² + 2 × acceleration × distance)

Example Problem:

A car starts from rest (u = 0) and accelerates uniformly at 2 m/s² for 5 seconds. Find velocity and distance covered.

1. v = 0 + 2×5 = 10 m/s

2. s = 0×5 + ½×2×25 = 25 m
   ✅ Velocity = 10 m/s, Distance = 25 m

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⚖️ Uniform Circular Motion

When an object moves in a circle with uniform speed, its velocity changes continuously because its direction keeps changing. Hence, it is an accelerated motion.

Example: The Earth revolves around the Sun in uniform circular motion.

Centripetal Force:

The force directed towards the center of the circle that keeps the body moving in a circular path.
Without this force, the object would move off in a straight line.

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🌞 Applications of Motion in Daily Life

1. Transportation systems – Cars, trains, airplanes are designed using principles of motion.

2. Sports – Understanding velocity and acceleration improves athletic performance.

3. Space exploration – Rocket motion, satellite orbits, and planetary movement all depend on motion laws.

4. Engineering and robotics – Machines function precisely because of controlled motion.

5. Health & medicine – Heart rate, blood flow, and physical movement are examples of motion inside living beings.

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💡 Real-Life Examples to Understand Motion Better

• Throwing a ball upward: Non-uniform motion, acceleration due to gravity acts downward.

• Fan rotation: Circular motion with constant speed but changing direction.

• Clock pendulum: Periodic motion.

• Earth’s orbit: Uniform circular motion around the sun.

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📘 Summary

Concept	Formula	Type	Example
Speed	Distance ÷ Time	Scalar	60 km/h car
Velocity	Displacement ÷ Time	Vector	10 m/s north
Acceleration	(v - u) ÷ t	Vector	2 m/s² car
Displacement	Shortest path	Vector	3 km east
Distance	Total path	Scalar	5 km walk

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🧮 Practice Numericals

1. A car moves 60 km in 2 hours. Find speed.
   → 60/2 = 30 km/h

2. An object moves from rest with acceleration 2 m/s² for 4 s. Find velocity.
   → v = u + at = 0 + 2×4 = 8 m/s

3. A bike accelerates from 10 m/s to 30 m/s in 5 s. Find acceleration.
   → (30-10)/5 = 4 m/s²

4. A car moves 100 m north, then 100 m east. Find displacement.
   → √(100² + 100²) = 141.4 m (NE direction)

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🔍 Advanced Concept: Motion Under Gravity

All freely falling objects accelerate toward Earth at the same rate, g = 9.8 m/s².
Formulas remain same; replace ‘a’ with ‘g’.

1. v = u + gt

2. h = ut + ½ gt²

3. v² = u² + 2gh

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🧭 Summary for Quick Revision

• Motion: Change in position with time.

• Rest is relative.

• Speed: Scalar; Velocity: Vector.

• Acceleration = Rate of change of velocity.

• Uniform circular motion = constant speed + changing direction.

• Three equations of motion apply to uniformly accelerated motion.

• Graphs give visual understanding of movement.

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🎯 Fun Activity: “Find the Motion Around You”

Take a notebook and list five types of motion you observe in your surroundings.
Then classify them as:

• Linear

• Circular

• Periodic

Example:

Object	Type of Motion
Fan	Circular
Car	Linear
Pendulum	Periodic

This simple activity will make you see the world scientifically 🌍✨

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🧩 Mini Quiz – Test Your Motion Skills!

1️⃣ If a car travels equal distances in equal intervals of time, its motion is:
A. Non-uniform
B. Uniform
C. Circular
D. Random

2️⃣ The area under velocity-time graph represents:
A. Acceleration
B. Displacement
C. Speed
D. Force

3️⃣ Which of these is a vector quantity?
A. Distance
B. Speed
C. Velocity
D. Time

4️⃣ When an object is thrown upward, its velocity becomes zero at:
A. Start
B. Highest point
C. Ground
D. Never

5️⃣ Acceleration due to gravity on Earth is approximately:
A. 9.8 m/s²
B. 10.8 m/s²
C. 8.9 m/s²
D. 0

✅ Answers: 1-B, 2-B, 3-C, 4-B, 5-A