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Physics notes #class 9th, Summaries of Physics

Physics

Physics cbsc class 9th notes of chapter 1st Very good 👍 👏 👌

Typology: Summaries

2024/2025

Available from 10/21/2025

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Class 9 Physics – Chapter 1: Motion (Detailed Notes)
1. Introduction
Everything around us is either at rest or in motion. When an object changes its position
with respect to time and a fixed reference point, it is said to be in motion. If it does not
change its position, it is said to be at rest. Example: A parked bus is at rest with respect to
the road but in motion with respect to a person in a moving car. Thus, rest and motion are
relative terms.
2. Types of Motion
(a) Linear or Translatory Motion – Object moves in a straight or curved path. Example:
moving train. (b) Circular Motion – Object moves along a circular path. Example: fan
blades, planets. (c) Oscillatory or Vibratory Motion – Back and forth motion about a mean
position. Example: pendulum. (d) Rotational Motion – Object spins around a fixed axis.
Example: spinning top.
3. Distance and Displacement
Distance is the total path covered by an object, while displacement is the shortest distance
between initial and final position. Distance is scalar, displacement is vector. Displacement
can be zero, positive, or negative. Displacement ≤ Distance always. Example: A person
walks 4 m east and then 3 m west: Distance = 7 m, Displacement = 1 m east.
4. Speed and Velocity
Speed is distance per unit time. Velocity is displacement per unit time in a given direction.
Speed is scalar, velocity is vector. Types of speed/velocity: uniform, non-uniform, and
average. Formulas: Speed = distance/time; Velocity = displacement/time.
5. Acceleration
Acceleration is the rate of change of velocity with time: a = (v - u)/t. Positive acceleration
means increasing speed, negative acceleration (retardation) means decreasing speed.
Units: m/s². Example: A car increases its velocity from 20 to 40 m/s in 4 s ⇒ a = 5 m/s².
6. Graphical Representation of Motion
Distance-time graph: Straight line → Uniform motion, Curved → Non-uniform, Horizontal
→ Rest. Velocity-time graph: Straight line → Uniform acceleration, Area under graph →
Distance covered, Slope → Acceleration.
7. Equations of Uniformly Accelerated Motion
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Class 9 Physics – Chapter 1: Motion (Detailed Notes)

1. Introduction

Everything around us is either at rest or in motion. When an object changes its position with respect to time and a fixed reference point, it is said to be in motion. If it does not change its position, it is said to be at rest. Example: A parked bus is at rest with respect to the road but in motion with respect to a person in a moving car. Thus, rest and motion are relative terms.

2. Types of Motion

(a) Linear or Translatory Motion – Object moves in a straight or curved path. Example: moving train. (b) Circular Motion – Object moves along a circular path. Example: fan blades, planets. (c) Oscillatory or Vibratory Motion – Back and forth motion about a mean position. Example: pendulum. (d) Rotational Motion – Object spins around a fixed axis. Example: spinning top.

3. Distance and Displacement

Distance is the total path covered by an object, while displacement is the shortest distance between initial and final position. Distance is scalar, displacement is vector. Displacement can be zero, positive, or negative. Displacement ≤ Distance always. Example: A person walks 4 m east and then 3 m west: Distance = 7 m, Displacement = 1 m east.

4. Speed and Velocity

Speed is distance per unit time. Velocity is displacement per unit time in a given direction. Speed is scalar, velocity is vector. Types of speed/velocity: uniform, non-uniform, and average. Formulas: Speed = distance/time; Velocity = displacement/time.

5. Acceleration

Acceleration is the rate of change of velocity with time: a = (v - u)/t. Positive acceleration means increasing speed, negative acceleration (retardation) means decreasing speed. Units: m/s². Example: A car increases its velocity from 20 to 40 m/s in 4 s ⇒ a = 5 m/s².

6. Graphical Representation of Motion

Distance-time graph: Straight line → Uniform motion, Curved → Non-uniform, Horizontal → Rest. Velocity-time graph: Straight line → Uniform acceleration, Area under graph → Distance covered, Slope → Acceleration.

7. Equations of Uniformly Accelerated Motion

For constant acceleration: (1) v = u + at (2) s = ut + ½at² (3) v² = u² + 2as Derived from the velocity–time graph where area under the line gives displacement.

8. Uniform Circular Motion

An object moving in a circle at constant speed undergoes uniform circular motion. Even though speed is constant, velocity changes due to changing direction. Centripetal acceleration acts toward the center: a = v²/r. Examples: motion of moon, electrons, or car turning on a curved track.

9. Difference Between Quantities

Distance – Scalar, unit: m. Displacement – Vector, unit: m. Speed – Scalar, unit: m/s. Velocity – Vector, unit: m/s. Acceleration – Vector, unit: m/s². Speed can’t be negative; velocity can.

10. Key Points to Remember

  • Motion is relative. • Displacement can be zero while distance ≠ 0. • For uniformly accelerated motion, v–t graph is a straight line. • Area under v–t graph = distance. • Slope of v–t graph = acceleration. • In uniform circular motion, speed constant but velocity changes.

11. Numerical Examples

Example 1: A car moves with initial velocity 5 m/s and final velocity 25 m/s in 4 s. Acceleration = (25 - 5)/4 = 5 m/s². Distance = ut + ½at² = 5×4 + ½×5×4² = 60 m. Example 2: A body is thrown upward with velocity 20 m/s. At top, v = 0, using v² = u² - 2gh ⇒ h = 20.4 m. Time to reach top = u/g = 2.04 s.

12. Summary

Speed = s/t (m/s) Velocity = ∆s/t (m/s) Acceleration = (v - u)/t (m/s²) v = u + at s = ut + ½at² v² = u² + 2as Centripetal acceleration = v²/r (m/s²)