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CBSE Notes
Class 9
Science
Chapter 4 - Describing Motion Around Us

Frequently Asked Questions

The notes cover motion, distance, displacement, speed, velocity, acceleration, and motion graphs.

The notes explain the difference between distance and displacement with clear concepts and examples.

Yes, the notes cover speed, velocity, their formulas, units, and differences.

Yes, the notes explain distance-time and velocity-time graphs for better understanding of motion.

Yes, these notes summarise important definitions, formulas, and concepts for quick revision.

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CBSE Class 9 Science Chapter Chapter  4: Describing motion around us

Important concepts become easier to study when they are explained in a clear and organised way. CBSE Notes for Class 9 Science present key definitions, explanations, and important points in a simple format, helping students revise topics and improve their understanding. Chapter 4, “Describing Motion Around Us”, explains how the motion of objects is described using distance, displacement, speed, velocity, and acceleration. The notes also cover types of motion, distance-time and velocity-time graphs, and the relationship between different motion parameters, helping students understand motion and relate it to real-life examples.

1.0Download  CBSE Notes in Class 9 Describing motion around us

Download CBSE Class 9 Science Chapter 4: Describing Motion Around Us Notes for quick learning of key motion concepts.

NCERT Solutions Class 9 Science Chapter 4 Notes

2.0Important Concepts of Class 9 Science Chapter  4 Describing motion around us

Describing Motion  

  • Motion: An object is in motion if it changes position with time
  • Rest and motion are relative terms — there's no absolute rest; motion is described with respect to a reference point/origin
  • Point object: has no linear dimensions but possesses mass (used when object's own size is negligible compared to distance travelled)

Scalar and Vector Quantities  

#

Scalar Quantities

Vector Quantities

1

Specified by magnitude only

Specified by magnitude and direction

2

Change only by change in magnitude

Change by magnitude, direction, or both

3

Added/subtracted by ordinary algebra

Added/subtracted by vector laws

  • Scalars: mass, time, distance, speed, work, energy, temperature
  • Vectors: velocity, acceleration, force, displacement, momentum, weight

Distance and Displacement  

#

Distance

Displacement

1

Length of actual path traversed

Shortest distance between initial & final position

2

Scalar quantity

Vector quantity

3

Always positive

Can be negative, positive, or zero

4

Distance ≥ magnitude of displacement

Displacement ≤ distance

5

Not a unique path

Unique path between two points

6

Never zero for a moving body

Can be zero (e.g., circular path returning to start)

Key formulas:

  • Displacement = xf − xi
  • Circular path (¼ revolution): distance = πr/2, displacement = r√2
  • Circular path (full revolution): distance = 2πr (circumference), displacement = 0

Uniform and Non-Uniform Motion  

  • Uniform motion: equal distances in equal time intervals (straight line, constant velocity)
  • Non-uniform motion: unequal distances in equal time intervals (curved path or changing speed)

Speed and Velocity  

#

Speed

Velocity

1

Rate of change of distance

Rate of change of displacement

2

Scalar quantity

Vector quantity

3

Always positive

Can be negative, positive, or zero

4

Speed = velocity without direction

Velocity = directed speed

Key formulas:

  • Speed: v = s/t
  • Average speed = Total distance / Total time
  • Average velocity = Total displacement / Total time
  • If equal distances at speeds u, v: Average speed = 2uv/(u+v)
  • If equal time intervals at speeds u, v: Average speed = (u+v)/2
  • Conversion: 1 km/h = 5/18 m/s; 1 m/s = 18/5 km/h

Acceleration  

  • Acceleration = rate of change of velocity = (v − u)/t
  • Accelerated motion: velocity increases (a is positive)
  • Retarded motion: velocity decreases (a is negative/deceleration)
  • Uniform acceleration: equal changes in velocity in equal time intervals

Equations of Uniformly Accelerated Motion  

Equation

Relation Given

v = u + at

velocity, time

s = ut + ½at²

displacement, time

v² = u² + 2as

displacement, final velocity

(u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement)

Graphical Representation of Motion  

Distance-Time Graph

  • Slope = speed of the body
  • Straight line parallel to x-axis → body at rest
  • Straight line inclined to x-axis → uniform motion
  • Curved line → non-uniform motion

Velocity-Time Graph

  • Slope = acceleration of the body
  • Area under v-t graph = distance travelled
  • Straight line parallel to x-axis → uniform velocity (a = 0)
  • Straight line inclined → uniform acceleration

Distance-Time Graph

Velocity-Time Graph

Slope → speed/velocity

Slope → acceleration

—

Area under curve → distance

Circular Motion  

  • Motion along a circular path; velocity is always tangential to the path
  • Uniform circular motion: constant speed, but direction changes continuously → always accelerated motion (non-uniform velocity)
  • Speed formula: v = 2πr/t

Important Formula Table  

Quantity

Formula

Unit

Type

Distance

—

m

Scalar

Displacement

xf − xi

m

Vector

Speed/Velocity

v = s/t

m/s

Scalar/Vector

Average Speed

Total distance/Total time

m/s

Scalar

Average Speed (equal distances)

2v₁v₂/(v₁+v₂)

m/s

Scalar

Average Speed (equal time)

(v₁+v₂)/2

m/s

Scalar

Acceleration

a = (v−u)/t

m/s²

Vector 

3.0Detailed CBSE Notes Class 9 Science  Describing motion around us

Other Study Materials for Class 9 Science    

Students can use additional study materials along with these notes, such as:

Class 9 Science NCERT textbook

Class 9 Science NCERT Solutions

Class 9 Science Syllabus

Class 9 Science Sample Papers

4.0 Why CBSE Notes for Class 9 Science Chapter  4: Describing motion around us

  • Understand the Basics of Motion: These notes help students learn how the motion of objects is described using reference points, position, and time.
  • Learn Distance and Displacement Concepts: The notes explain the difference between distance and displacement with clear examples to strengthen conceptual understanding.
  • Understand Speed and Velocity: Students can easily revise the meaning, formulas, units, and differences between speed and velocity used to describe motion.
  • Master the Concept of Acceleration: The notes simplify acceleration and explain how changes in velocity affect the motion of objects.
  • Interpret Motion Through Graphs: These notes explain distance-time and velocity-time graphs, helping students understand graphical representation of motion.
  • Support Effective Revision: Important definitions, formulas, and concepts from the chapter are organised in a concise format for better exam preparation.


Chapter-wise CBSE Notes for Class 9 Science:

Class 9 Science Chapter 1 - Exploration: Entering the world of secondary science Notes

Class 9 Science Chapter 2 - Cell:The building blocks of life Notes

Class 9 Science Chapter 3 - Tissues in action Notes

Class 9 Science Chapter 4 - Describing motion around us Notes

Class 9 Science Chapter 5 -Exploring mixtures and their separation Notes

Class 9 Science Chapter 6 - How forces affect motion Notes

Class 9 Science Chapter 7 - Work,Energy and simple machines Notes

Class 9 Science Chapter 8 - Journey inside the atom Notes

Class 9 Science Chapter 9 - Atomic foundations of Matter Notes

Class 9 Science Chapter 10 - Sound Waves:Characteristics Applications Notes

Class 9 Science Chapter 11 - Reproduction:How Life Continues Notes

Class 9 Science Chapter 12 - Patterns in life:Diversity and Classification Notes

Class 9 Science Chapter 13 - Earth as a system:Energy,Matter and Life Notes


Chapter-wise NCERT Solutions for Class 9 Science:

Chapter 1 - Exploration: Entering the world of secondary science

Chapter 2 - Cell: The building blocks of life

Chapter 3 - Tissues in action

Chapter 4 - Describing motion around us

Chapter 5 - Exploring mixtures and their separation

Chapter 6 - How forces affect motion

Chapter 7 - Work,Energy and simple machines

Chapter 8 - Journey inside the atom

Chapter 9- Atomic foundations of Matter

Chapter 10- Sound Waves: Characteristics Applications

Chapter 11- Reproduction:How Life Continues

Chapter 12 - Patterns in life:Diversity andClassification

Chapter 13 - Earth as a system:Energy,Matter and Life