Physics Notes for UPSC Preparation 2026
Complete Basic Physics Concepts (Part 1)
PHYSICS: INTRODUCTION
Physics is the branch of science that deals with matter, energy, motion, force, and interactions of nature.
For UPSC Prelims, Physics questions are generally asked from:
- Basic concepts
- Daily life applications
- Space technology
- Energy sources
- Electronics
- Modern physics
1. Measurement and Units
Physical Quantity
A physical quantity is a measurable property expressed with a number and unit.
Examples:
- Length = 5 metre
- Mass = 10 kilogram
- Time = 20 seconds
SI Units (International System of Units)
| Quantity | SI Unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Temperature | Kelvin | K |
| Electric Current | Ampere | A |
| Amount of Substance | Mole | mol |
| Luminous Intensity | Candela | cd |
UPSC Important Fact:
The SI system is used worldwide for scientific measurements.
2. Motion (गति)
Motion means the change in position of an object with respect to time.
Types of Motion
1. Linear Motion
Movement in a straight line.
Example:
- Moving car on a straight road
2. Circular Motion
Movement along a circular path.
Example:
- Earth revolving around the Sun
3. Rotational Motion
Body rotates around its axis.
Example:
- Rotation of Earth
Distance and Displacement
Distance
- Total path travelled
- Scalar quantity
- Always positive
Example:
A person walks 5 km east and returns 5 km west.
Distance = 10 km
Displacement
- Shortest distance between initial and final position
- Vector quantity
Example:
If a person returns to starting point:
Displacement = 0
Speed
Speed = Distance / Time
Formula:
Speed = Distance ÷ Time
Unit:
metre/second (m/s)
Example:
A car travels 100 km in 2 hours.
Speed = 50 km/hour
Velocity
Velocity is speed with direction.
Example:
50 km/hour towards north
Velocity is a vector quantity.
Acceleration
Acceleration is the rate of change of velocity.
Formula:
Acceleration = Change in Velocity / Time
Unit:
m/s²
Example:
A car increasing its speed from 20 km/h to 60 km/h has acceleration.
3. Newton's Laws of Motion
Sir Isaac Newton gave three laws of motion.
First Law of Motion (Law of Inertia)
"An object remains at rest or continues in uniform motion unless an external force acts on it."
Examples:
- Passenger falls forward when bus suddenly stops.
- Dust comes out when carpet is beaten.
Types of Inertia:
1. Inertia of Rest
Object wants to remain at rest.
Example:
A book kept on table remains there.
2. Inertia of Motion
Object wants to continue moving.
Example:
A moving bicycle continues after stopping pedalling.
3. Inertia of Direction
Object resists change in direction.
Example:
Water drops leave a rotating umbrella tangentially.
Second Law of Motion
Newton's second law states:
Force = Mass × Acceleration
Formula:
F = m × a
Where:
- F = Force
- m = Mass
- a = Acceleration
Unit:
Newton (N)
Third Law of Motion
"For every action, there is an equal and opposite reaction."
Examples:
Rocket Propulsion
Gas moves downward → Rocket moves upward.
Walking
Foot pushes ground backward → Ground pushes person forward.
4. Force
Force is a push or pull that changes the state of motion.
Formula:
Force = Mass × Acceleration
Types of Force:
1. Gravitational Force
Force due to attraction between masses.
Example:
Earth attracts objects towards itself.
2. Frictional Force
Force that opposes motion between surfaces.
Advantages:
- Walking possible
- Vehicle movement
Disadvantages:
- Heat production
- Wear and tear
3. Magnetic Force
Force produced by magnets.
Examples:
- Electric motors
- Magnetic trains
5. Work, Energy and Power
Work
Work is done when force causes displacement.
Formula:
Work = Force × Displacement
Unit:
Joule (J)
Example:
Lifting a bag from ground.
Energy
Energy is the ability to do work.
Types:
1. Kinetic Energy
Energy due to motion.
Formula:
KE = ½ mv²
Example:
Moving car, running person
2. Potential Energy
Energy due to position.
Formula:
PE = mgh
Example:
Water stored in dam
Conservation of Energy
Energy can neither be created nor destroyed.
It can only change from one form to another.
Examples:
Hydroelectric power:
Potential Energy → Electrical Energy
6. Power
Power is the rate of doing work.
Formula:
Power = Work / Time
Unit:
Watt (W)
Example:
100 watt bulb consumes energy faster than 40 watt bulb.
UPSC Quick Revision Points
✅ Force unit = Newton
✅ Work unit = Joule
✅ Power unit = Watt
✅ Speed is scalar quantity
✅ Velocity is vector quantity
✅ Acceleration unit = m/s²
✅ Newton gave laws of motion
✅ Energy conservation is a fundamental law
Physics Notes for UPSC Preparation 2026
Part 2: Gravitation, Heat, Thermodynamics, Waves, Sound, Light & Optics
1. Gravitation (गुरुत्वाकर्षण)
Meaning of Gravitation
Gravitation is the force of attraction between any two objects having mass.
Example:
- Earth attracts objects towards itself.
- Moon revolves around Earth due to gravitational force.
Newton's Law of Gravitation
According to Newton:
Every object in the universe attracts every other object with a force.
Formula:
F = G × (M₁M₂ / r²)
Where:
- F = Gravitational force
- G = Universal gravitational constant
- M₁ and M₂ = Mass of objects
- r = Distance between objects
Acceleration Due to Gravity (g)
The acceleration produced due to Earth's gravitational force is called acceleration due to gravity.
Value:
g = 9.8 m/s²
It means the velocity of a falling object increases by 9.8 m/s every second.
Weight and Mass
Mass
- Amount of matter present in an object
- Constant everywhere
- Unit: kilogram
Weight
Weight is the force with which Earth attracts an object.
Formula:
Weight = Mass × Gravity
W = mg
Unit:
Newton
UPSC Fact:
An astronaut's mass remains the same on Moon, but weight decreases because Moon's gravity is weaker.
Escape Velocity
Escape velocity is the minimum velocity required by an object to escape Earth's gravitational field.
Earth's escape velocity:
11.2 km/s
Example:
Rockets must achieve escape velocity to leave Earth.
Satellites
A satellite is an object that revolves around a planet.
Types:
Natural Satellite
Example:
Moon
Artificial Satellite
Example:
INSAT, IRS satellites
Geostationary Satellite
A satellite that appears fixed relative to Earth.
Features:
- Revolves at same speed as Earth's rotation
- Height: approximately 36,000 km
- Used for communication and weather forecasting
2. Heat and Temperature
Heat
Heat is a form of energy that flows from a hotter body to a colder body.
Unit:
Joule (J)
Temperature
Temperature measures the degree of hotness or coldness of an object.
Units:
- Celsius (°C)
- Kelvin (K)
- Fahrenheit (°F)
Conversion:
0°C = 273 K
Modes of Heat Transfer
1. Conduction
Transfer of heat through direct contact.
Example:
Metal spoon becomes hot in tea.
Good conductors:
- Copper
- Aluminium
Poor conductors:
- Wood
- Plastic
2. Convection
Transfer of heat through movement of fluids.
Example:
- Sea breeze
- Land breeze
3. Radiation
Transfer of heat without any medium.
Example:
Heat from Sun reaches Earth.
3. Thermodynamics
Thermodynamics deals with heat, temperature and energy conversion.
First Law of Thermodynamics
Energy can neither be created nor destroyed.
It can only change from one form to another.
Example:
Heat energy → Mechanical energy in engines
Second Law of Thermodynamics
Heat always flows from a hotter object to a colder object naturally.
Example:
Ice melts when kept outside.
Applications of Thermodynamics
- Refrigerators
- Air conditioners
- Engines
- Power plants
4. Waves
A wave is a disturbance that transfers energy from one place to another.
Important terms:
Amplitude
Maximum displacement of particles from mean position.
Higher amplitude = Higher energy
Frequency
Number of vibrations per second.
Unit:
Hertz (Hz)
Wavelength
Distance between two consecutive points of a wave.
Symbol:
λ (Lambda)
Types of Waves
1. Mechanical Waves
Require a medium.
Examples:
- Sound waves
- Water waves
2. Electromagnetic Waves
Do not require a medium.
Examples:
- Light
- Radio waves
- X-rays
5. Sound
Sound is a mechanical wave produced by vibrations.
Important Properties:
Frequency
Determines pitch.
High frequency = high pitch
Example:
Child's voice has higher frequency than adult male voice.
Amplitude
Determines loudness.
Greater amplitude = louder sound
Speed of Sound
Speed depends on medium:
- Solids: Highest
- Liquids: Medium
- Gases: Lowest
Speed in air:
Approximately 343 m/s
Human Hearing Range
Humans can hear:
20 Hz to 20,000 Hz
Infrasonic Sound
Frequency below 20 Hz
Example:
Earthquakes
Ultrasonic Sound
Frequency above 20,000 Hz
Uses:
- Medical imaging
- SONAR
- Cleaning instruments
6. Light and Optics
Light is a form of electromagnetic radiation.
Speed of light:
3 × 10⁸ m/s
Reflection of Light
When light falls on a surface and returns back.
Laws of Reflection:
- Angle of incidence = Angle of reflection
- Incident ray, reflected ray and normal lie in same plane
Mirrors
Plane Mirror
Properties:
- Image is virtual
- Same size
- Laterally inverted
Example:
Normal mirror
Concave Mirror
Uses:
- Shaving mirror
- Dentist mirror
- Headlights
Convex Mirror
Uses:
- Rear-view mirror in vehicles
Reason:
Provides wider field of view.
Refraction of Light
Bending of light when it passes from one medium to another.
Examples:
- Pencil appears bent in water
- Rainbow formation
Lens
Convex Lens
- Converging lens
- Used in magnifying glass and cameras
Concave Lens
- Diverging lens
- Used in spectacles for myopia
Important Optical Phenomena
Rainbow
Caused by:
- Refraction
- Reflection
- Dispersion of sunlight
Mirage
Caused by:
- Refraction of light due to temperature differences
Blue Colour of Sky
Due to scattering of shorter wavelength light.
UPSC Quick Revision Points
✅ g = 9.8 m/s²
✅ Escape velocity of Earth = 11.2 km/s
✅ Speed of light = 3 × 10⁸ m/s
✅ Human hearing range = 20 Hz–20 kHz
✅ Sound cannot travel in vacuum
✅ Heat transfer through vacuum occurs by radiation
✅ Convex mirror is used as rear-view mirror
✅ Convex lens is a converging lens
✅ Light travels fastest in vacuum
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