Introduction
Welcome to the free notes for Class 9 Science Chapter 4 notes Describing Motion Around Us Notes. This chapter introduces the concepts of motion, distance, displacement, speed, velocity, acceleration, and equations of motion. These notes provide the key ideas for quick revision and exam preparation.
Based on the Latest NCERT Textbook
These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and cover all important concepts in an easy-to-understand format.
Table of Contents
What is Motion?
Motion is the change in the position of an object with time relative to a reference point. If the position of an object changes with time, it is said to be in motion.
Types of Motion
Rectilinear Motion
Motion along a straight line.
Example: A train moving on a straight track.
Circular Motion
Motion along a circular path.
Example: Blades of a fan.
Rotational Motion
Motion around a fixed axis.
Example: Earth spinning on its axis.
Periodic Motion
Motion that repeats at regular intervals.
Example: A swinging pendulum.
Distance and Displacement
Distance
Distance is the total length of the actual path travelled by an object.
SI Unit: metre (m)
Displacement
Displacement is the shortest straight-line distance between the initial and final positions of an object.
SI Unit: metre (m)
Speed
Speed is the distance travelled by an object per unit time.
Formula
Speed = Distance ÷ Time
SI Unit: metre per second (m/s)
Velocity
Velocity is the displacement covered by an object per unit time in a particular direction.
Formula
Velocity = Displacement ÷ Time
SI Unit: metre per second (m/s)
Acceleration
Acceleration is the rate of change of velocity with time.
Formula
Acceleration = (Final Velocity − Initial Velocity) ÷ Time
SI Unit: metre per second squared (m/s²)
Equations of Motion
For uniformly accelerated motion:
- v = u + at
- s = ut + ½at²
- v² = u² + 2as
Where:
- u = Initial velocity
- v = Final velocity
- a = Acceleration
- t = Time
- s = Displacement
Graphical Representation of Motion
Distance–Time Graph
- Shows how distance changes with time.
- A straight line indicates uniform speed.
Velocity–Time Graph
- Shows how velocity changes with time.
- The slope of the graph gives acceleration.
Uniform Circular Motion
Uniform circular motion is the motion of an object along a circular path with constant speed.
Examples
- Earth revolving around the Sun.
- A stone tied to a string and rotated.
- Ceiling fan blades.
Important Terms
- Motion
- Distance
- Displacement
- Speed
- Velocity
- Acceleration
- Uniform Motion
- Non-uniform Motion
- Circular Motion
- Equations of Motion
Chapter Summary
✅ Motion is the change in position with time.
✅ Distance is the actual path travelled.
✅ Displacement is the shortest distance between two points.
✅ Speed is distance travelled per unit time.
✅ Velocity is displacement per unit time.
✅ Acceleration is the rate of change of velocity.
Complete Your Preparation
The notes above cover only the basic concepts of this chapter.
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FAQ
What is motion?
Motion is the change in the position of an object with time relative to a reference point.
What is the difference between distance and displacement?
Distance is the total path travelled, while displacement is the shortest straight-line distance between the starting and ending points.
What is the SI unit of speed?
The SI unit of speed is metre per second (m/s).
Are these notes enough for exams?
These notes are useful for quick revision. For complete preparation with NCERT solutions, MCQs, case-based questions, mind maps, and practice papers, the complete eBook is recommended.
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