भौतिकी – दैनिक जीवन · पाठ — सामान्य ज्ञान · 10वीं स्तर

Physics in Everyday Life — the science hiding in your kitchen, your bus ride and your mobile phone.

◆ The hook: Why does a bus passenger fall forward when the driver brakes suddenly? Why does dal cook so slowly in the hills of Shimla, but quickly in a pressure cooker? Why is the sky blue, but the Sun red at sunset? You have seen all of this a thousand times. Today you will learn the one-line physics behind each — and that is exactly what SSC, Railway and State exams ask.

◆ A true story: On 28 February 1928, C.V. Raman in Kolkata discovered that light changes its colour slightly when it is scattered by a liquid. This "Raman Effect" won him the Nobel Prize in Physics in 1930, and India celebrates 28 February as National Science Day. Physics did not begin in a laboratory — it began with curious people asking "why?" about ordinary things.

What you'll master today (Learning Objectives)

SI units and the instruments that measure everyday quantities

Motion, Newton's three laws, momentum, friction and gravity in daily life

Pressure, floating, surface tension and capillarity

Work, energy and power — including the "unit" on your electricity bill

Heat, temperature scales, expansion and latent heat

Sound — echo, ultrasound, pitch and loudness

Light — mirrors, lenses, refraction, scattering, the human eye

Electricity and magnetism at home — fuse, earthing, wiring, kWh

Simple formulas you can solve in 45 seconds

1. Measurement: The Language of Physics

Every quantity needs a number and a unit. The world uses the SI system (Système International).

Seven SI base units:
Length — metre (m)
Mass — kilogram (kg)
Time — second (s)
Electric current — ampere (A)
Temperature — kelvin (K)
Amount of substance — mole (mol)
Luminous intensity — candela (cd)

Common derived units: force — newton (N); work/energy — joule (J); power — watt (W); pressure — pascal (Pa); frequency — hertz (Hz); charge — coulomb (C); potential difference — volt (V); resistance — ohm (Ω); power of a lens — dioptre (D).

Units that look like time but are not:
Light year — a DISTANCE (distance light travels in one year, about 9.46 × 10^15 m)
Parsec — also a distance (about 3.26 light years)
Astronomical Unit (AU) — mean Earth–Sun distance, about 1.5 × 10^11 m
Angstrom (Å) — 10^-10 m; nanometre (nm) — 10^-9 m
Nautical mile — 1852 m; knot — a speed (one nautical mile per hour)
Horsepower — a unit of POWER, about 746 W
Kilowatt-hour (kWh) — a unit of ENERGY = 3.6 × 10^6 J (the "unit" on your electricity bill)

Mnemonic for the seven base units: "My Kind Sister Always Keeps Many Candles" → metre, kilogram, second, ampere, kelvin, mole, candela.

🧠 Active Recall: Is a light year a unit of time or distance? (Distance.) What does one "unit" on an electricity bill mean? (1 kWh = 3.6 × 10^6 J.)

Instruments you must know:
Barometer — atmospheric pressure
Hygrometer — relative humidity
Hydrometer — relative density of liquids
Lactometer — purity of milk (by its density)
Anemometer — wind speed
Altimeter — altitude (used in aircraft)
Seismograph — earthquake waves
Sphygmomanometer — blood pressure
Pyrometer — very high temperatures (furnaces)
Odometer — distance travelled by a vehicle; Speedometer — its speed
Ammeter — current (connected in series); Voltmeter — potential difference (connected in parallel)
Periscope — seeing over obstacles (submarines), uses two plane mirrors
SONAR — uses ultrasound under water; RADAR — uses radio waves

Mnemonic: "Hygro = Humidity, Hydro = density of liquid, Lacto = milk." All three start with H/L — sort them by the second syllable.

2. Motion and Newton's Laws

Speed = distance ÷ time (scalar). Velocity = speed in a fixed direction (vector). Acceleration = change in velocity ÷ time (m/s²).

Equations of motion (uniform acceleration):
v = u + at
s = ut + ½at²
v² = u² + 2as

Newton's First Law (Law of Inertia): A body stays at rest or in uniform motion unless an external force acts on it.
Daily life: Passengers fall forward when a moving bus stops suddenly (inertia of motion). Passengers fall backward when a bus starts suddenly (inertia of rest). Beating a carpet removes dust. A coin on a card falls into the glass when the card is flicked. Heavier objects have more inertia — mass is the measure of inertia.

Newton's Second Law: Force = mass × acceleration (F = ma). Force equals the rate of change of momentum.
Momentum p = mv (kg m/s).
Daily life: A cricketer pulls his hands back while catching a ball — this increases the time of impact, so the force on his hands becomes smaller. Sand pits for high jumpers and air bags in cars work the same way.

Newton's Third Law: To every action there is an equal and opposite reaction. The two forces act on DIFFERENT bodies.
Daily life: Recoil of a gun, a rocket going up, a swimmer pushing water back, walking (we push the ground backward), a boat moving back when you jump forward from it.

Law of conservation of momentum: In the absence of external force, total momentum stays constant. That is why a gun recoils: momentum of bullet forward = momentum of gun backward.

🧠 Active Recall: Why does a cricketer pull his hands back while catching? (More time → less force.) Do action and reaction cancel each other? (No — they act on different bodies.)

Mnemonic for the three laws: "I-F-A" → Inertia, F = ma, Action–reaction. "I Feel Alive."

Friction: the force that opposes motion between surfaces in contact.
Static friction > sliding friction > rolling friction. That is why wheels and ball bearings make motion easy.
Friction is reduced by lubricants (oil, grease), ball bearings and streamlined shapes. It is increased by treads on tyres and soles of shoes. Walking on a polished or icy floor is hard because friction is low.

Circular motion: A body moving in a circle needs a force towards the centre (centripetal force). Roads are banked on curves; a cyclist leans inwards while turning; a washing-machine dryer and a cream separator use rotation.

Centre of gravity and stability: A body is more stable when its centre of gravity is low and its base is wide. That is why buses should not carry heavy luggage on the roof, and racing cars are built low.

3. Gravitation

Every two bodies attract each other — Newton's law of gravitation. F = G m1 m2 / r², where G = 6.67 × 10^-11 N m²/kg² (a universal constant).

Acceleration due to gravity on Earth, g ≈ 9.8 m/s². In vacuum, a feather and a coin fall together — the rate of fall does not depend on mass.

Mass vs weight:
Mass — amount of matter, measured in kg, same everywhere.
Weight — force of gravity on the body, W = mg, measured in newton, changes from place to place.
g on the Moon is about one-sixth of Earth's. A 60 kg person has mass 60 kg on the Moon too, but weighs about one-sixth as much.
g is slightly more at the poles than at the equator (Earth is flattened at the poles), so a body weighs slightly more at the poles.
g decreases as we go up (altitude) and also as we go deep inside the Earth; at the centre of the Earth g is zero.

Escape velocity from Earth: about 11.2 km/s.

Weightlessness: Astronauts in an orbiting spacecraft feel weightless because they and the spacecraft are falling freely together — gravity is NOT zero there. In a lift falling freely, your apparent weight becomes zero.

Tides in the sea are caused mainly by the Moon's gravitational pull.

Simple pendulum: time period T = 2π√(L/g). It does not depend on the mass of the bob. A longer pendulum swings more slowly. A pendulum clock taken to the Moon (smaller g) runs slow.

🧠 Active Recall: Is gravity zero inside an orbiting space station? (No — they are in free fall.) Where does a body weigh more — pole or equator? (Pole.)

4. Pressure, Floating and Liquids

Pressure = Force ÷ Area (unit pascal, Pa = N/m²). Same force on a smaller area → more pressure.
Daily life: A sharp knife cuts better; nails and pins are pointed; school bags have broad straps; camels have broad feet for sand; heavy trucks have many wide tyres; a building has a wide foundation.

Atmospheric pressure: At sea level it is about 1.01 × 10^5 Pa, equal to about 76 cm of mercury column. It decreases with height.
Daily life: Drinking with a straw, a dropper, and a suction hook all work because of atmospheric pressure. A fountain pen may leak in an aeroplane at height because outside air pressure is lower. Nose bleeding can happen at high altitudes.

Boiling point and pressure: Higher pressure → higher boiling point. In a pressure cooker the pressure is high, water boils above 100 °C, so food cooks faster. On mountains the pressure is low, water boils below 100 °C, so cooking takes longer.

Pascal's law: Pressure applied to an enclosed liquid is transmitted equally in all directions. Used in hydraulic lifts, hydraulic brakes and hydraulic presses.

Archimedes' principle: A body immersed in a liquid experiences an upward force (buoyant force) equal to the weight of the liquid it displaces.
A body floats if its density is less than that of the liquid. An iron nail sinks but an iron ship floats — the ship's hollow shape displaces a large volume of water.
It is easier to float in sea water than in river water because sea water is denser.
Ice floats on water because ice is less dense (about 0.92 g/cm³) than water.

Surface tension: The surface of a liquid behaves like a stretched membrane. Raindrops and mercury drops are spherical; insects can walk on water; soap and detergent reduce surface tension so they wet clothes better.

Capillarity: A liquid rises in a narrow tube. The wick of a lamp, blotting paper and a towel soaking water work by capillary action.

Bernoulli's principle (where fluid flows faster, pressure is lower): An aeroplane wing gets lift; roofs can be blown off in a storm; an atomiser/sprayer works.

🧠 Active Recall: Why does food cook faster in a pressure cooker? (Higher pressure → higher boiling point.) Which law runs a hydraulic lift? (Pascal's law.)

5. Work, Energy and Power

Work W = Force × displacement (in the direction of force). Unit joule (J). If there is no displacement, no work is done — a man holding a heavy box still does no work in physics.

Kinetic energy (energy of motion) KE = ½mv². If speed doubles, KE becomes four times.
Potential energy (energy of position) PE = mgh. Water stored in a dam, a stretched bow, a wound spring.
Law of conservation of energy: Energy can neither be created nor destroyed; it only changes form.

Power = Work ÷ time. Unit watt (W). 1 horsepower ≈ 746 W.

Energy conversions you see every day:
Electric bulb — electrical → light (and heat)
Electric motor/fan — electrical → mechanical
Generator/dynamo — mechanical → electrical
Battery/cell — chemical → electrical
Solar cell — light → electrical
Microphone — sound → electrical; Loudspeaker — electrical → sound

Mnemonic: "MoGe" — MOtor takes electricity Out (to motion), GEnerator GEts electricity from motion.

6. Heat and Temperature

Heat is energy; temperature tells how hot or cold a body is.

Temperature scales:
°F = (9/5) × °C + 32
K = °C + 273 (more exactly 273.15)
Water freezes at 0 °C = 32 °F = 273 K, boils at 100 °C = 212 °F = 373 K (at normal pressure).
Normal human body temperature: 37 °C = 98.6 °F.
−40° is the same on the Celsius and Fahrenheit scales.
Absolute zero: 0 K = −273.15 °C.
A clinical thermometer reads about 35 °C to 42 °C; it has a kink so the mercury does not fall back on its own.

Thermal expansion: Most things expand on heating. Gaps are left between railway rails; electric wires sag in summer; a tight metal lid opens after putting it in hot water.

Anomalous expansion of water: Water contracts when heated from 0 °C to 4 °C. Water has maximum density at 4 °C. That is why lakes freeze from the top, and fish survive in the water below the ice.

Specific heat: Water has a very high specific heat (about 4200 J/kg K). It heats and cools slowly. So the sea stays cooler than land in the day → sea breeze in the day; land cools faster at night → land breeze at night. Coastal places have moderate climate.

Latent heat: Heat absorbed or released during a change of state without change of temperature.
Ice at 0 °C cools a drink better than water at 0 °C — ice also absorbs latent heat of fusion (about 3.34 × 10^5 J/kg).
Burns from steam at 100 °C are more severe than burns from boiling water at 100 °C — steam gives out extra latent heat of vaporisation (about 22.5 × 10^5 J/kg).

Evaporation causes cooling: water in an earthen pot (matka) stays cool; we feel cool when sweat dries; dogs pant; a fan cools us by speeding up evaporation of sweat. A refrigerator cools by evaporating a refrigerant inside its pipes.

Transfer of heat:
Conduction — through solids (a metal spoon in hot tea gets hot)
Convection — through fluids by movement (sea breeze, boiling water, heating of a room)
Radiation — needs no medium (heat of the Sun reaching Earth)
A thermos flask reduces all three: vacuum stops conduction and convection, silvered walls reduce radiation.
Light-coloured clothes are comfortable in summer (they reflect heat); dark clothes absorb more heat. Woollen clothes keep us warm because they trap air, a poor conductor.

🧠 Active Recall: Why is a steam burn worse than a boiling-water burn? (Latent heat of vaporisation.) At what temperature is water densest? (4 °C.)

Mnemonic: "4 °C — Fish Free" → maximum density at 4 °C keeps fish alive below the ice.

7. Sound

Sound is a mechanical wave produced by vibration. It needs a medium — it CANNOT travel through vacuum (astronauts on the Moon use radio).
Speed of sound: Fastest in solids, then liquids, slowest in gases. Approximate values (NCERT, 25 °C): air 346 m/s, water about 1500 m/s, steel about 5960 m/s. Speed of sound in air increases with temperature.
Light travels much faster than sound, so we see lightning before we hear thunder.

Wave equation: speed = frequency × wavelength (v = fλ).

Frequency ranges:
Infrasound — below 20 Hz (earthquakes, whales, rhinoceroses, elephants)
Audible range — 20 Hz to 20,000 Hz (20 kHz) for humans
Ultrasound — above 20 kHz (bats, dolphins; used in SONAR, ultrasonography, cleaning, finding cracks in metal)

Characteristics:
Pitch — depends on frequency (a woman's or child's voice usually has higher pitch)
Loudness — depends on amplitude; measured in decibel (dB)
Quality (timbre) — lets us recognise a voice or instrument even at the same pitch and loudness

Echo: A reflected sound heard distinctly. The sensation of sound stays in our brain for about 0.1 s, so the reflecting surface must be at least about 17.2 m away (at 344 m/s). Distance of obstacle = (speed × time) ÷ 2.
Bats find their way by echoes of ultrasound (echolocation).

Doppler effect: The pitch of an ambulance siren seems higher as it comes towards you and lower as it goes away.

Supersonic: Faster than sound (Mach number more than 1). Supersonic aircraft create a sonic boom.

🧠 Active Recall: Can sound travel on the Moon's surface? (No — no air.) Which animal uses ultrasound to fly in the dark? (Bat.)

Mnemonic for speed of sound: "Solid Speeds, Liquid Lags, Gas Goes slow" — S > L > G.

8. Light

Light travels at 3 × 10^8 m/s in vacuum (fastest). Sunlight takes about 8 minutes 20 seconds (about 500 s) to reach Earth.

Reflection:
Plane mirror — image is virtual, erect, same size, as far behind the mirror as the object is in front, and laterally inverted (that is why "AMBULANCE" is written in mirror letters on the front of ambulances).
To see your full image, a plane mirror must be at least half your height.
Two plane mirrors at an angle θ form (360/θ − 1) images when 360/θ is even: at 90° → 3 images, at 60° → 5 images. Parallel mirrors → infinite images. A kaleidoscope uses inclined mirrors; a periscope uses two plane mirrors.
Concave mirror (converging) — shaving and make-up mirrors, dentist's mirror, reflectors of torches and car headlights, solar furnaces.
Convex mirror (diverging) — rear-view mirrors in vehicles (always gives an erect, smaller image and a wider field of view), mirrors at sharp road turns.

Refraction (bending of light when it passes from one medium to another):
A pencil in water looks bent; a pool looks shallower than it is; a lemon in a glass of water looks bigger.
Refractive index n = speed of light in vacuum ÷ speed in medium. Water ≈ 1.33, glass ≈ 1.5, diamond ≈ 2.42.
Apparent depth = real depth ÷ n.
Atmospheric refraction: Stars twinkle (planets usually do not, because they are closer and appear as extended sources). The Sun is seen about 2 minutes before actual sunrise and about 2 minutes after actual sunset.

Total internal reflection: When light goes from a denser to a rarer medium at an angle greater than the critical angle, it is fully reflected. Examples: sparkling of diamond, optical fibres (used in internet cables and endoscopy), mirage in deserts and on hot roads.

Dispersion: White light splits into seven colours by a prism — VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). Red bends least, violet bends most. A rainbow forms by dispersion, refraction and internal reflection of sunlight in raindrops; it is seen in the sky opposite to the Sun.

Scattering of light:
The sky looks blue because air molecules scatter blue light (shorter wavelength) much more than red.
The Sun looks red at sunrise and sunset because light travels a longer path through air, and most blue is scattered away.
Danger signals are red because red light is scattered least and can be seen from far away.
To an astronaut at great height the sky looks dark — there is no air to scatter light.
Clouds look white because large water droplets scatter all colours nearly equally.
The path of a beam of light seen in a dusty room or forest is the Tyndall effect.

Lenses:
Convex lens (converging) — magnifying glass, camera, human eye lens, spectacles for long-sightedness.
Concave lens (diverging) — spectacles for short-sightedness.
Power of a lens P = 1/f (f in metres), unit dioptre (D). Convex lens has positive power; concave lens negative.

The human eye:
The eye lens is convex; the image on the retina is real and inverted; the brain makes it upright.
Near point of a normal eye is 25 cm; far point is infinity.
Persistence of vision: an image stays on the retina for about 1/16 of a second — that is why a movie looks continuous.
Myopia (short-sightedness: cannot see far) — corrected by concave lens.
Hypermetropia (long-sightedness: cannot see near) — corrected by convex lens.
Presbyopia (old age) — bifocal lens. Astigmatism — cylindrical lens. Cataract — cloudy eye lens, treated by surgery.

Electromagnetic spectrum (decreasing wavelength): Radio waves → Microwaves → Infrared → Visible → Ultraviolet → X-rays → Gamma rays.
Uses: radio/TV and RADAR (radio waves and microwaves), microwave oven, TV remote and night vision (infrared), sterilisation (ultraviolet), bone imaging (X-rays), cancer treatment (gamma rays). The ozone layer absorbs most harmful ultraviolet rays.

Mnemonics:
"MyopiA → concAve" (both have A); "HypermetropiA → conveX" — remember "Hyper eX-tra power".
Spectrum order: "Raju Met In Vizag Uncle X-ray Gupta" → Radio, Micro, Infrared, Visible, UV, X-ray, Gamma.

🧠 Active Recall: Why are danger lights red? (Least scattered.) Which mirror is used as a rear-view mirror? (Convex.) Which lens corrects myopia? (Concave.)

9. Electricity and Magnetism at Home

Current I = charge ÷ time (ampere). Ohm's law: V = IR (at constant temperature). Resistance unit: ohm (Ω).
Resistance of a wire increases with length and decreases with thickness (area of cross-section).
Series: R = R1 + R2 + … (same current). Parallel: 1/R = 1/R1 + 1/R2 + … (same voltage).
Household appliances are connected in PARALLEL, so each gets the full voltage and can be switched on or off separately.

Electric power P = VI = I²R = V²/R. Energy = Power × time. 1 kWh = 1 unit = 3.6 × 10^6 J.
Example: A 100 W bulb used for 10 hours uses 100 × 10 = 1000 Wh = 1 kWh = 1 unit.

Heating effect of current:
Electric iron, heater, toaster — heating element made of nichrome (an alloy with high resistivity that does not oxidise easily).
Filament bulb — tungsten filament (very high melting point); bulbs are filled with inert gases such as nitrogen or argon to make the filament last longer.
Fuse — a wire of low melting point connected in SERIES with the live wire; it melts and breaks the circuit when too much current flows (overloading or short circuit). An MCB (miniature circuit breaker) does the same job and can be reset.

Household wiring in India (NCERT): Live wire — red insulation; neutral — black; earth — green. Potential difference between live and neutral is 220 V; AC frequency is 50 Hz. Earthing protects us from shocks from metal-bodied appliances.

A bird sitting on a single high-voltage wire is safe because there is no potential difference across its body; touching two wires at once would complete a circuit.

Magnetic effect:
Oersted showed that a current-carrying wire deflects a compass needle — electricity produces magnetism.
Electromagnets — used in cranes, electric bells, MRI machines.
Faraday discovered electromagnetic induction — a changing magnetic field produces current. Generators work on this. A transformer changes AC voltage; it does not work on DC.
Electric motor: electrical energy → mechanical energy. Generator: mechanical → electrical.
Earth behaves like a giant magnet; a freely suspended magnet points north–south. The Earth's geographic north has a magnetic south pole near it, so the north pole of a compass needle is attracted there.

🧠 Active Recall: Why are household appliances in parallel? (Each gets full voltage, independent switching.) In which wire is the fuse connected? (Live wire, in series.)

Mnemonic for wire colours: "Red Runs, Black Back, Green Ground" → Live red, neutral black (return), earth green.

⚠️ Caution: Examiner's Trap

Trap 1 — "Light year is a unit of time." Trap Type: Similar Name. The word "year" fools students. A light year is a unit of DISTANCE.

Trap 2 — "Astronauts float because there is no gravity in space." Trap Type: Almost True Statement. Gravity is present in orbit; astronauts feel weightless because they are in free fall along with the spacecraft.

Trap 3 — "A convex lens corrects myopia." Trap Type: Almost True Statement (lens type swapped). Myopia → concave lens; hypermetropia → convex lens.

Trap 4 — "The fuse is connected in parallel / in the neutral wire." Trap Type: Almost True Statement. The fuse is connected in series in the live wire.

Trap 5 — "The sky is blue because the sea reflects blue light." Trap Type: Reverse Causation. The sky is blue due to scattering of blue light by air molecules; the sea partly looks blue because it reflects the sky.

🔁 Linguistic Bridge: In Hindi we say this, in English we say that

जड़त्व — Inertia
संवेग — Momentum
घर्षण — Friction
गुरुत्वाकर्षण — Gravitation
उत्प्लावन बल — Buoyant force
पृष्ठ तनाव — Surface tension
केशिकत्व — Capillarity
विशिष्ट ऊष्मा — Specific heat
गुप्त ऊष्मा — Latent heat
प्रतिध्वनि — Echo
पराश्रव्य — Ultrasound
अपवर्तन — Refraction
पूर्ण आंतरिक परावर्तन — Total internal reflection
प्रकीर्णन — Scattering
निकट-दृष्टि दोष — Myopia
दूर-दृष्टि दोष — Hypermetropia
विद्युत चुंबकीय प्रेरण — Electromagnetic induction
भू-संपर्क तार — Earth wire

🔗 Static–Dynamic Link

Chandrayaan-3 (ISRO) soft-landed near the Moon's south pole on 23 August 2023; the landing site was named "Shiv Shakti Point", and 23 August is now observed as National Space Day in India. A soft landing on the Moon is a direct use of physics from this chapter: the Moon's gravity is about one-sixth of Earth's, and with no air there is no air resistance and no sound — the lander had to slow down using its own engines (Newton's third law).
National Science Day (28 February) marks C.V. Raman's discovery of the Raman Effect (1928) — scattering of light, the same topic that explains the blue sky.

📊 High-Yield Facts and Figures (Static)

📋 Summary Table

📅 Revision Planner

Review after 2 days: Close the book and write the seven SI base units, the three laws of Newton with one example each, and the formulas P = F/A, KE = ½mv², P = VI.
Weekly review: Do a 10-minute blurting session on Light and Sound — mirrors, lenses, eye defects, scattering, echo, frequency ranges. Then solve one practice set.
Monthly blurting: Write everything you remember about the whole chapter on one page, check it against the Summary Table, and mark the gaps in red. Revise only the red parts.