Physics Basics — TNPSC Group II/IIA Science
Overview
Physics questions in TNPSC Group 2/2A cover mechanics, heat, optics, sound, and basic electricity. This chapter summarises the most frequently tested concepts. Focus on Newton's Laws, energy types, mirrors and lenses, and practical applications.
Mechanics — Motion
Types of Motion
- Translatory: Linear, circular, random (e.g., ball rolling)
- Rotatory: Spinning around an axis (e.g., top)
- Oscillatory: Back and forth (e.g., pendulum)
- Uniform motion: Equal distances in equal time intervals
- Non-uniform motion: Unequal distances in equal time
Key Quantities
| Quantity | Definition | SI Unit |
|---|---|---|
| Distance | Total path length | Metre (m) |
| Displacement | Shortest path from start to end | Metre (m) |
| Speed | Distance/Time | m/s |
| Velocity | Displacement/Time | m/s |
| Acceleration | Change in velocity/Time | m/s² |
- Scalar quantities: Distance, speed, mass, time, energy (magnitude only)
- Vector quantities: Displacement, velocity, acceleration, force (magnitude + direction)
Newton's Laws of Motion
First Law — Law of Inertia
"A body continues to be in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force."
- Inertia: Resistance to change in state of motion
- Example: Passengers lurch forward when bus brakes suddenly (inertia)
Second Law — Law of Acceleration
F = ma (Force = Mass × Acceleration)
- Force is directly proportional to acceleration
- Force is measured in Newtons (N); 1 N = 1 kg·m/s²
- Example: A heavier ball needs more force to accelerate
Third Law — Law of Action-Reaction
"For every action, there is an equal and opposite reaction."
- Forces always come in pairs; act on different objects
- Example: Rocket propulsion — gas ejected backward, rocket goes forward; swimming — push water backward, move forward
Gravitation
| Concept | Formula | Details |
|---|---|---|
| Newton's Law of Gravitation | F = Gm₁m₂/r² | G = 6.674 × 10⁻¹¹ N·m²/kg² |
| Acceleration due to gravity | g = 9.8 m/s² | At Earth's surface |
| Weight | W = mg | Changes with g |
| Moon's gravity | g_moon = g/6 | 1.63 m/s² |
- Mass: Constant everywhere; measures amount of matter
- Weight: Variable; = mg; what a weighing scale measures
Work, Energy, and Power
Work
- Work = Force × Displacement × cos θ
- Unit: Joule (J); 1 J = 1 N·m
- Work done is zero if force and displacement are perpendicular (θ = 90°)
Energy
| Type | Description | Example |
|---|---|---|
| Kinetic Energy | Energy of motion | KE = ½mv² |
| Potential Energy | Energy due to position | PE = mgh |
| Thermal Energy | Heat energy | Fire, friction |
| Chemical Energy | Stored in chemical bonds | Food, fuel, batteries |
| Nuclear Energy | Atomic nucleus | Fission, fusion |
| Sound Energy | Vibrations | Music, speech |
| Electromagnetic (Light) Energy | Photons | Sunlight |
Law of Conservation of Energy: Energy cannot be created or destroyed; only converted from one form to another.
Power
- Power = Work/Time
- Unit: Watt (W); 1 W = 1 J/s
- 1 horsepower (HP) = 746 watts
Heat and Thermodynamics
Temperature Scales
| Scale | Freezing Point of Water | Boiling Point of Water | Absolute Zero |
|---|---|---|---|
| Celsius (°C) | 0°C | 100°C | −273.15°C |
| Fahrenheit (°F) | 32°F | 212°F | −459.67°F |
| Kelvin (K) | 273.15 K | 373.15 K | 0 K |
Conversions:
- °C = (°F − 32) × 5/9
- K = °C + 273.15
Heat Transfer Methods
| Method | Description | Example |
|---|---|---|
| Conduction | Heat transfer through direct contact; no movement of matter | Metal rod in fire |
| Convection | Heat transfer through movement of fluid (liquid/gas) | Hot air rises; water boiling |
| Radiation | Heat transfer without medium; through electromagnetic waves | Sun heating Earth |
Thermal Expansion
- Solids, liquids, gases expand when heated
- Thermometer: Uses expansion of mercury or alcohol
- Railway tracks: Expansion gaps left to prevent buckling
Optics — Light
Properties of Light
- Speed in vacuum: 3 × 10⁸ m/s
- Travels in straight lines (rectilinear propagation)
- Can undergo reflection, refraction, diffraction, interference
- Visible spectrum: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red)
- Red light: Longest wavelength (750 nm); Violet: Shortest (400 nm)
Reflection
- Law of Reflection: Angle of incidence = Angle of reflection
- Regular reflection: Smooth surface → clear image
- Diffuse reflection: Rough surface → no clear image
Mirrors
| Mirror | Image Properties | Uses |
|---|---|---|
| Plane mirror | Virtual, erect, same size, laterally inverted | Common mirror, periscope |
| Concave mirror | Real inverted (far object); virtual erect magnified (near object) | Shaving mirror, headlights, solar cookers, doctor's mirror (ENT) |
| Convex mirror | Virtual, erect, diminished | Rear-view mirror, security mirrors in shops |
Refraction
- Light bends when passing from one medium to another
- Snell's Law: n₁ sin θ₁ = n₂ sin θ₂
- Total Internal Reflection: When light goes from denser to rarer medium at critical angle → used in optical fibres
Lenses
| Lens | Type | Image | Uses |
|---|---|---|---|
| Convex (converging) | Thicker in middle | Real inverted (beyond f); virtual erect magnified (inside f) | Reading glasses, camera, microscope, magnifying glass |
| Concave (diverging) | Thinner in middle | Virtual, erect, diminished | Spectacles for myopia |
Defects of Vision
| Defect | Problem | Correction |
|---|---|---|
| Myopia (Short-sightedness) | Near objects clear; far objects blurred | Concave lens |
| Hypermetropia (Long-sightedness) | Far objects clear; near objects blurred | Convex lens |
| Astigmatism | Blurred image due to irregular cornea | Cylindrical lens |
| Presbyopia | Age-related loss of near vision | Bifocal lens |
Sound
Properties
- Speed in air at 25°C: ~346 m/s
- Needs a material medium (cannot travel in vacuum)
- Travels faster in solids > liquids > gases
- Echo: Reflected sound heard after 1/17th of a second (sound travels 17m from wall)
- Ultrasound: Frequency > 20,000 Hz (above human hearing); used in SONAR, medical imaging
Sound Terminology
| Term | Meaning |
|---|---|
| Frequency | Number of vibrations per second; Hz |
| Amplitude | Maximum displacement; determines loudness |
| Pitch | High frequency = high pitch; musical quality |
| Loudness | Measured in Decibels (dB) |
| Resonance | Object vibrates at its natural frequency when external vibration matches |
Human hearing range: 20 Hz to 20,000 Hz
- Below 20 Hz: Infrasound (earthquakes, whales)
- Above 20,000 Hz: Ultrasound (dogs, bats, medical imaging)
Key Facts for TNPSC Group II/IIA
- Newton's First Law: Inertia — body continues in its state unless external force acts
- F = ma: Force = Mass × Acceleration; Force unit is Newton
- Newton's Third Law: Action-Reaction; equal and opposite; example: rocket propulsion
- Weight = mg: Changes with gravity; Mass stays constant
- Speed of light: 3 × 10⁸ m/s; Speed of sound in air: ~346 m/s
- Convex mirror: Rear-view mirror — virtual, erect, diminished image
- Concave mirror: Shaving mirror, headlight reflector — can magnify
- Myopia corrected by concave lens; Hypermetropia corrected by convex lens
- Ultrasound (> 20,000 Hz): Used in SONAR, prenatal scans
- Law of Conservation of Energy: Total energy in a closed system remains constant
Previous Year MCQ Patterns
Q: The rear-view mirror used in vehicles is: a) Plane mirror b) Concave mirror c) Convex mirror d) Parabolic mirror
Q: Newton's Second Law of Motion is given by: a) Every action has an equal and opposite reaction b) A body in motion stays in motion unless acted on c) F = ma d) Energy cannot be created or destroyed
Q: Ultrasound has frequency: a) Below 20 Hz b) Between 20 Hz and 20,000 Hz c) Above 20,000 Hz d) Exactly 20,000 Hz
Frequently Asked Questions
Newton's First Law (Inertia): A body at rest stays at rest; a body in motion stays in uniform motion unless acted upon by an external force. Newton's Second Law: Force = Mass × Acceleration (F = ma). Newton's Third Law: For every action, there is an equal and opposite reaction.