Physics — Class 9 (CBSE)
Board: CBSE | Class: 9 | Subject: Science (Physics Chapters) In Class 9, Physics is taught as part of integrated Science. These are the Physics chapters from the NCERT Science textbook.
Overview
Class 9 Physics introduces foundational concepts of motion, forces, gravitation, energy, and sound. These topics form the base for Class 11 and 12 Physics and are essential for competitive exams like NTSE and Olympiads.
Motion
What is Motion?
An object is said to be in motion when its position changes with respect to a reference point over time.
Key Concepts
Distance vs Displacement | Term | Definition | Nature | |---|---|---| | Distance | Total path length covered | Scalar | | Displacement | Shortest path from start to end point | Vector |
Speed and Velocity - Speed = Distance / Time (scalar) — SI unit: m/s - Velocity = Displacement / Time (vector) — SI unit: m/s - Average Speed = Total distance / Total time - Uniform motion — equal distances in equal time intervals - Non-uniform motion — unequal distances in equal time intervals
Acceleration
- Rate of change of velocity: a = (v - u) / t
- SI unit: m/s²
- Positive acceleration → speeding up; Negative acceleration (retardation) → slowing down
Equations of Motion (Uniform Acceleration)
v = u + at
s = ut + ½at²
v² = u² + 2as
Where: u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement
Graphical Representation
- Distance-Time graph: Slope = speed; straight line = uniform motion; curve = non-uniform
- Velocity-Time graph: Slope = acceleration; area under curve = displacement
- Uniform Circular Motion: Speed constant, but velocity changes direction continuously
Force and Laws of Motion
Types of Forces
- Balanced forces — net force = 0; no change in state of motion
- Unbalanced forces — net force ≠ 0; causes acceleration or change in motion
Newton's Laws of Motion
First Law (Law of Inertia)
An object remains at rest or in uniform motion unless acted upon by an external unbalanced force.
- Inertia — resistance to change in state of motion
- Greater mass → greater inertia
Second Law
Force = mass × acceleration: F = ma
- SI unit of force: Newton (N) = kg·m/s²
- Momentum (p) = mass × velocity → F = Δp/Δt
Third Law
Every action has an equal and opposite reaction. Forces always act in pairs on different objects.
Example: Rocket propulsion — hot gases expelled backward push rocket forward
Law of Conservation of Momentum
Total momentum of an isolated system remains constant if no external force acts on it.
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Gravitation
Universal Law of Gravitation
Every object in the universe attracts every other object with a force:
F = G × (m₁ × m₂) / r²
- G = 6.674 × 10⁻¹¹ N·m²/kg² (Universal Gravitational Constant)
- Force is directly proportional to product of masses
- Force is inversely proportional to square of distance between them
Free Fall
- An object falling under gravity alone (no air resistance)
- Acceleration due to gravity: g = 9.8 m/s² (on Earth's surface)
- Equations of motion apply with a = g
Mass vs Weight
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter | Gravitational force on object |
| SI Unit | Kilogram (kg) | Newton (N) |
| Nature | Scalar; constant | Vector; varies with location |
| Formula | — | W = mg |
Thrust and Pressure
- Thrust — force acting perpendicular to a surface
- Pressure = Force / Area; SI unit: Pascal (Pa)
- Higher pressure with same force → smaller area (knife, needle)
Archimedes' Principle
A body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced.
- Object floats when buoyant force ≥ weight
- Relative Density = Density of substance / Density of water
Work, Energy and Power
Work
W = F × s × cos θ
- Work done when force causes displacement in its direction
- SI unit: Joule (J)
- No displacement → No work done (even if force applied)
- Work is zero when force ⊥ displacement (θ = 90°)
Energy
The capacity to do work. SI unit: Joule (J)
Kinetic Energy (KE)
KE = ½mv²
Energy possessed by a body due to its motion.
Potential Energy (PE)
PE = mgh
Energy possessed by a body due to its position or configuration.
Law of Conservation of Energy Energy cannot be created or destroyed; it can only be transformed from one form to another. Total mechanical energy (KE + PE) remains constant in the absence of friction.
Power
Power = Work done / Time = W/t
- SI unit: Watt (W) = J/s
- 1 horsepower = 746 W
- Commercial unit of energy: 1 kilowatt-hour (kWh) = 3.6 × 10⁶ J
Sound
Nature of Sound
- Sound is a mechanical longitudinal wave — requires a medium to travel
- Cannot travel through vacuum
- Produced by vibrating objects
- Travels as compressions and rarefactions
Speed of Sound
| Medium | Speed (approx.) |
|---|---|
| Air (0°C) | 331 m/s |
| Air (25°C) | 346 m/s |
| Water | 1500 m/s |
| Steel | 5000 m/s |
Speed increases: solid > liquid > gas
Characteristics of Sound
- Frequency (f) — number of oscillations per second; SI unit: Hertz (Hz); determines pitch
- Amplitude — maximum displacement; determines loudness
- Wavelength (λ) — distance between two consecutive compressions
- Wave equation:
v = f × λ
Reflection of Sound
- Echo — reflected sound heard after 0.1 s delay (minimum distance: ~17 m)
- Reverberation — persistence of sound due to multiple reflections
- Uses: SONAR (underwater depth measurement), medical ultrasound
Range of Hearing
| Type | Frequency Range |
|---|---|
| Infrasound | < 20 Hz |
| Audible sound | 20 Hz – 20,000 Hz |
| Ultrasound | > 20,000 Hz |
Structure of Human Ear
Pinna → Ear canal → Eardrum (tympanic membrane) → Ossicles (malleus, incus, stapes) → Cochlea → Auditory nerve → Brain
Key Formulas — Quick Reference
| Concept | Formula | Unit |
|---|---|---|
| Speed | v = d/t | m/s |
| Acceleration | a = (v-u)/t | m/s² |
| Force | F = ma | Newton (N) |
| Momentum | p = mv | kg·m/s |
| Gravitational force | F = Gm₁m₂/r² | N |
| Weight | W = mg | N |
| Work | W = Fs cosθ | Joule (J) |
| Kinetic Energy | KE = ½mv² | Joule (J) |
| Potential Energy | PE = mgh | Joule (J) |
| Power | P = W/t | Watt (W) |
| Pressure | P = F/A | Pascal (Pa) |
Exam Tips
- In motion problems, always identify u, v, a, t, s before choosing the equation
- Newton's Third Law forces act on different objects — they never cancel each other
- Weight varies on different planets; mass does not
- Echo requires a minimum distance of ~17 m between source and reflector
- 1 kWh is the commercial unit of electrical energy (used in electricity bills)