Everyday Physics — Why Things Work the Way They Do

Why This Chapter Matters

Every SSC and RRB paper carries two or three questions that are not really physics questions at all. They are "why does this happen" questions dressed up in exam language: why does a pressure cooker cook dal faster, why does the sky turn blue, why do you hear thunder after you see lightning. Across CGL, CHSL, MTS, CPO, and RRB NTPC papers, this "everyday why" category shows up almost every single attempt, because the exam setters know these questions cannot be mugged up from a formula sheet. You either understand the idea or you guess.

That is exactly why this chapter matters more than its word count suggests. The single biggest mistake aspirants make here is memorising the answer to a specific question ("ice floats because of density") without holding onto the underlying idea (density decides who floats), so the moment the question is rephrased with a new object — say, an iceberg, a coin, or a steel ship — the student freezes. This chapter is built the opposite way. Each idea is explained once, properly, with the everyday example built right into the explanation, so you can apply the same logic to a new scenario the exam throws at you. Read it slowly once. You will not need to reread it before the exam.

1. Kitchen Physics — Why Your Food Cooks the Way It Does

Why a pressure cooker cooks faster

Water boils when its vapour pressure equals the surrounding atmospheric pressure. At normal atmospheric pressure, that happens at 100°C. A pressure cooker is a sealed vessel. As steam builds up inside, the pressure inside the cooker rises well above normal atmospheric pressure. Higher pressure forces water to need a higher temperature before it can boil, so water inside a cooker reaches around 120°C before it boils, instead of stopping at 100°C. Food cooks faster at a higher temperature, so dal that takes forty minutes in an open pan is done in twelve.

Exam trap: students often answer "because of high pressure" and stop there. The exam wants the mechanism: higher pressure raises the boiling point, and it is the higher temperature of the water, not the pressure itself, that cooks the food faster.

Memory hook: think of the cooker whistle as water being pushed to work overtime. More pressure, more heat before it's allowed to "escape" as steam, more cooking done in that heat.

Why food cooks faster at sea level than on a mountain

This is the pressure cooker principle running in reverse. On a high mountain, atmospheric pressure is lower, so water boils at a lower temperature, sometimes below 90°C. That water is boiling, but it is not hot enough to cook rice or dal properly, however long you wait. This is a classic SSC question: why is it hard to cook rice at high altitude in Ladakh or Shimla using an open pan. The answer is low atmospheric pressure lowering the boiling point, not "the water refuses to boil" — it boils, just too early and too cool.

Why a thermos flask (vacuum flask) keeps tea hot

A thermos has a double wall with a vacuum gap between the two layers of glass or steel. Heat needs a medium to travel by conduction or convection, and vacuum has no medium, so almost no heat escapes those two routes. The inner wall is also silvered to reflect back radiant heat, cutting the third route too. Trap all three heat-transfer routes and the tea inside stays hot for hours. This one question ties directly back to the three modes of heat transfer from Chapter 6 — conduction, convection, and radiation — and the exam loves testing that link.

Why a pan handle is made of wood or plastic, not metal

Metals are good conductors of heat; wood and plastic are poor conductors, or insulators. If the handle were metal, heat from the flame would conduct straight up the handle into your palm. A wood or plastic handle blocks that conduction path, so you can hold a pan that is cooking food at 200°C without burning your hand.

Why food is cut into smaller pieces before frying or boiling

Smaller pieces have more surface area exposed to heat relative to their volume. More exposed surface means heat penetrates the piece faster, so smaller pieces cook quicker than one large piece of the same total quantity. This surface-area-to-volume idea reappears later in this chapter when we look at why a broken thermometer bulb of mercury forms many tiny droplets and why perfume evaporates faster from a shallow dish.

2. Water and Density — Why Some Things Float and Others Sink

Why ice floats on water

Water is one of the very few substances that is less dense in its solid state than in its liquid state. When water freezes, its molecules arrange into a hexagonal crystal lattice that actually takes up more space than the same molecules do as liquid water. So a fixed mass of ice occupies more volume than the same mass of liquid water, meaning ice is less dense than water. Anything less dense than the fluid it sits in floats on that fluid. That is why ice cubes float in your glass and icebergs float in the ocean instead of sinking to the bottom.

Exam trap: do not confuse this with the general rule that solids are denser than liquids of the same substance — that rule holds for almost every substance except water. Water breaking this rule is exactly why it is a favourite exam question.

Why a massive steel ship floats but a small steel needle sinks

This is pure Archimedes' Principle, and it is one of the most repeated questions in this chapter across all major exams. A floating object displaces a volume of water whose weight equals the object's own weight. A steel needle is solid and dense, and its small volume cannot displace enough water to equal its weight, so it sinks. A ship, however, is not solid steel — it is a hollow steel shell shaped like a bowl, enclosing a huge volume of air. That shape lets the ship displace a very large volume of water, and the weight of that displaced water equals the ship's total weight (steel body plus cargo plus air inside). It is the shape, not the material, that decides whether steel floats.

Analogy: picture a steel katori (bowl) placed gently on water versus a steel ball of the same weight dropped in. The katori's hollow shape spreads its weight over a large displaced volume of water and floats; the solid ball has almost no hollow volume to displace water with, so it sinks straight down.

Why a needle can float on water if placed carefully, but sinks the moment it is pushed under

This is a different principle from the ship: surface tension. Water molecules at the surface pull on each other more strongly than they are pulled by the air above, forming a thin elastic-like skin at the surface. If you lay a needle flat and gently on that surface, the skin's tension can support the needle's small weight, and it floats without actually being buoyed up by displaced water in the Archimedes sense. Break that surface skin (push the needle in, or add a drop of soap which weakens surface tension) and the needle sinks immediately, because its density is far higher than water's.

Exam trap: the "ship floats, needle sinks" question tests density and Archimedes' Principle. The "needle floats flat on water" question tests surface tension. These look similar but are answered with completely different physics — a very common examiner trick.

Why soap and detergent help clean oily dishes

Water alone cannot mix with oil because oil is non-polar and water is polar; they simply do not bond. Soap molecules have two ends: one end that bonds with water and one end that bonds with oil and grease. The soap molecule wraps around oil droplets with its oil-loving end and faces its water-loving end outward, letting the oil droplet be pulled away and rinsed off with water. This same "one end likes water, one end likes oil" idea is why soap also lowers water's surface tension, which is why soap bubbles form easily in soapy water but not in plain water.

Why hot water rises and cold water sinks (and why this matters for a matka/earthen pot)

Hot water is less dense than cold water of the same substance, because heating makes molecules move apart. Less dense fluid rises above denser fluid. This is convection, and it explains why the top layer of a heated pot of water gets hot before the bottom, and why lakes freeze at the surface first in cold climates while the denser, slightly warmer water stays at the bottom, letting fish survive winter. An earthen matka works on an entirely different principle worth contrasting here: water seeps slowly through the porous clay walls to the outside, evaporates there, and evaporation always absorbs heat from its surroundings, cooling the water left inside the pot. One is about density and rising fluid; the other is about evaporative cooling. Do not mix the two up in an exam answer.

3. Why the Sky Does What It Does

Why the sky is blue

Sunlight looks white but is actually a mix of all visible colours. When sunlight enters the atmosphere, it collides with gas molecules far smaller than its wavelength, and this scatters the light in different directions — a process called Rayleigh scattering. Rayleigh scattering is far stronger for shorter wavelengths. Blue light has a much shorter wavelength than red light, so blue light gets scattered across the sky many times more than red light does. Wherever you look up, away from the sun, you are seeing scattered blue light arriving from every direction, so the sky appears blue.

Exam trap: violet light actually has an even shorter wavelength than blue and scatters even more, but our eyes are far less sensitive to violet and sunlight itself has less violet in it to begin with, so the sky reads as blue to us, not violet. This exact reasoning is a favourite "why not violet" follow-up question.

Why sunsets and sunrises look red or orange

At sunrise and sunset, sunlight has to travel through a much longer stretch of atmosphere to reach your eyes than it does at noon, because the sun is low on the horizon. Over that longer path, almost all the blue light gets scattered away before it ever reaches you, leaving mostly the longer wavelengths — red and orange — to come through directly. That is why the same sun that looks white-yellow at noon looks deep red or orange at sunset.

Memory hook: SKY — Short wavelengths Keep scattering, Yellow-to-red is left for sunset. Short wavelength (blue) scatters most at noon giving a blue sky; at sunset the long path filters blue away, leaving red.

Why we see lightning before we hear thunder

Lightning and the thunderclap it causes happen at essentially the same instant. But light travels at roughly 3,00,000 kilometres per second, while sound travels through air at only about 346 metres per second. Light from a bolt several kilometres away reaches your eye almost instantly; the sound of thunder from the same distance takes several seconds to arrive because sound is thousands of times slower than light. That gap is exactly why you can count "one-Mississippi, two-Mississippi" between the flash and the rumble to estimate how far away a storm is — roughly 3 seconds of delay for every kilometre of distance.

Why a mirage appears on a hot road

On a very hot day, the layer of air right above the road surface is much hotter, and therefore much less dense, than the cooler air higher up. Light from the sky travelling toward the road bends, or refracts, as it passes through these layers of changing density, and can bend so much that it curves back upward before reaching the road — a phenomenon called total internal reflection at the boundary between hot and cool air layers. Your brain, used to light always travelling in straight lines, interprets that bent ray as if it came from a reflective puddle of water on the road ahead. There is no water; it is bent sky light.

Why a rainbow forms after rain

Sunlight entering a raindrop bends (refracts) as it enters, reflects once off the inside back wall of the drop, and bends again as it exits. Because each colour in white light has a slightly different wavelength, each colour bends by a slightly different amount — this is dispersion, the same effect a glass prism uses to split white light into colours. Millions of raindrops each split sunlight this way at once, and your eye catches the arc of colours arranged in order: violet, indigo, blue, green, yellow, orange, red.

Memory hook: VIBGYOR is the standard order from the inner to outer band of a rainbow, and the same VIBGYOR order applies to a glass prism splitting white light — both are dispersion in action, just one uses raindrops and the other uses glass.

4. Electricity in Everyday Life — Why the Wiring Is Built the Way It Is

Why AC (alternating current), not DC, is used for long-distance power transmission

Power lost in a wire as heat depends on the square of the current flowing through it. To send the same amount of power, you can either use high current at low voltage or low current at high voltage; low current at high voltage loses far less energy as heat over long transmission lines. A transformer can step AC voltage up or down very efficiently, letting power companies transmit electricity at very high voltage (and correspondingly low current, minimising loss) over hundreds of kilometres, then step the voltage back down to a safe level before it reaches your home. Transformers work only with alternating current, because they rely on a constantly changing magnetic field, which direct current cannot produce. This single fact — transformers need AC to work — is the entire reason the whole national grid runs on AC.

Exam trap: students sometimes think AC is used because "it is safer." It is not chosen for safety; it is chosen because it can be transformed to high voltage for efficient transmission and then stepped back down. Safety at the household end comes from fuses, earthing, and insulation, which we cover next.

Why birds can sit safely on a high-voltage wire but a person touching the same wire and the ground gets electrocuted

Current flows only when there is a difference in electric potential across two points that a conductor connects, and it takes the path of least resistance. A bird sitting on a single wire has both its feet on that same wire, at nearly the same electric potential, so there is no significant voltage difference across its body and virtually no current flows through it. A person touching that live wire while also touching the ground (or a grounded pole) creates a large potential difference between the wire and the earth, and the person's body becomes the path connecting them, so current flows through the body. It is the potential difference across the body, not contact with electricity itself, that causes a shock.

Why a fuse wire protects household appliances

A fuse is a short length of wire made of a metal (usually a tin-lead alloy) with a low melting point and it is deliberately the weakest link in a circuit's current path. If current in a circuit rises far beyond the safe limit — a short circuit or overload — the fuse wire heats up faster than any other component, melts, and breaks the circuit before the excess current can damage appliances or start a fire. A fuse must always have a lower melting point and higher resistance per unit length than the copper wiring around it, or it would not be the first thing to fail.

Exam trap: a common wrong answer is that a fuse "regulates" current. It does not regulate anything — it works exactly once, breaking the circuit completely when current exceeds a safe threshold, and then has to be replaced.

Why electric wires are covered in rubber or PVC insulation

Rubber and PVC (Polyvinyl Chloride) are excellent electrical insulators, meaning they do not allow current to pass through them easily. Wrapping a current-carrying metal wire in an insulating layer stops current from leaking out into anything that touches the wire's outer surface, including a human hand, preventing shock. This is the same insulator property that makes plastic and wood safe handle materials in Section 1 — one property, many everyday applications.

Why a step-down transformer sits near your locality but electricity leaves the power plant at very high voltage

This directly follows from the AC transmission point above. Power plants generate electricity and immediately step it up to very high voltage (hundreds of kilovolts) for efficient transmission with minimal loss. Near residential areas, a step-down transformer reduces that dangerously high voltage down to the safe 220–240 volts used in Indian homes. Two transformers, one job each — step up for transmission efficiency, step down for household safety — both possible only because the current is alternating.

5. Everyday Devices, Decoded

Why a fan makes you feel cool without actually lowering room temperature

A fan does not cool the air; a thermometer placed in front of a running fan shows no drop in air temperature. What the fan does is speed up the evaporation of sweat from your skin. Evaporation is a cooling process because the sweat molecules that turn into vapour carry away heat energy from your skin as they leave, which is exactly why evaporation always cools whatever it evaporates from. Moving air also constantly sweeps away the thin layer of warm, moisture-laden air right next to your skin, replacing it with drier air that can absorb more sweat faster. Faster evaporation means faster cooling of your skin, so you feel cooler even though the room's air temperature has not changed at all.

Exam trap: this is one of the most misworded questions in SSC papers. If asked "does a fan lower room temperature," the correct answer is no — it only speeds evaporative cooling of your body.

Why a refrigerator keeps food cold

A refrigerator uses a special fluid called a refrigerant that is made to repeatedly evaporate and condense in a closed loop. Inside the fridge, the refrigerant is allowed to expand and evaporate, and evaporation absorbs heat from its surroundings — exactly the same principle as sweat cooling your skin — pulling heat out of the food compartment. A compressor then squeezes that vapour back into a liquid outside the fridge, at the coils you can feel warm at the back, releasing the absorbed heat into the kitchen. The cycle repeats continuously, constantly moving heat from inside the fridge to outside it.

Why sunglasses with polarised lenses cut glare but ordinary tinted glasses do not

Ordinary light waves vibrate in every direction as they travel. Light reflected off a flat, shiny surface like water, a car bonnet, or a road becomes strongly polarised, meaning it vibrates mostly in one direction, usually horizontal, and this concentrated horizontal glare is what strains your eyes. A polarised lens has a microscopic grid structure that only lets light vibrating in the vertical direction pass through, blocking most of that horizontal glare while still letting normal scene light through. Ordinary tinted glasses simply reduce the brightness of all light equally and do nothing special to glare, which is why polarised sunglasses feel different from simply "darker" ones.

Why a periscope lets a submarine see above the water surface

A periscope uses two mirrors (or prisms), each angled at 45 degrees, fixed parallel to each other inside a straight tube. Light from above the surface hits the upper mirror, reflects downward through the tube, and hits the lower mirror, which reflects it again straight into the observer's eye at the bottom. Two reflections at matching angles is all it takes to bend a line of sight around a corner, letting a submerged submarine crew see ships and aircraft on the surface without exposing the vessel itself. This device-principle pair — periscope and two 45-degree mirrors — is asked almost every cycle in some form.

Why milk is boiled and stored in a wide-mouthed container to cool it faster, but stays hot longer in a thermos

Cooling by evaporation and radiation both happen fastest from a large exposed surface. A wide-mouthed pan exposes far more milk surface to the air than a narrow-mouthed one, so heat escapes faster and the milk cools quicker — the same surface-area logic as small food pieces cooking faster. A thermos, in complete contrast, is deliberately designed with a vacuum layer and a narrow, sealed mouth specifically to minimise exposed surface and cut off all three heat transfer routes, keeping the same milk hot for hours. Same physics of heat loss, opposite engineering goals.

Why a knife cuts better when sharp, and why sharp objects hurt more with the same force

Pressure equals force divided by area. A sharp blade concentrates the same applied force onto an extremely small area at its edge, producing enormous pressure at that point, enough to cut through fibres that a blunt edge, spreading the same force over a larger area, cannot penetrate. This is the identical reasoning behind why camel feet are broad (spreading body weight over a large area to reduce pressure on soft sand) and why a sharp nail goes into wood with a light hammer tap while a blunt one does not — small area, same force, higher pressure.

Quick Revision — One-Line Facts

• A pressure cooker raises water's boiling point above 100°C by trapping steam and raising internal pressure, cooking food faster.
• At high altitude, lower atmospheric pressure makes water boil below 100°C, so open-pan cooking takes longer.
• A thermos flask blocks conduction and convection with a vacuum gap and blocks radiation with a silvered wall.
• Pan handles are made of insulators like wood or plastic to stop heat conducting into your hand.
• Smaller food pieces cook faster because they have more surface area relative to their volume.
• Ice floats because water is one of the few substances that is less dense as a solid than as a liquid.
• A steel ship floats because its hollow shape displaces enough water to equal its total weight (Archimedes' Principle).
• A steel needle sinks because its solid, dense shape cannot displace enough water to equal its weight.
• A needle can float flat on water only due to surface tension, a different principle from a ship's buoyancy.
• Soap molecules have a water-loving end and an oil-loving end, letting them lift grease off dishes.
• Hot water is less dense than cold water and rises above it, driving convection currents.
• An earthen matka cools water through evaporation from its porous walls, not through density-driven convection.
• The sky looks blue because shorter (blue) wavelengths scatter more strongly than longer ones — Rayleigh scattering.
• Sunsets look red because sunlight travels a longer atmospheric path, scattering away almost all the blue light.
• Lightning is seen before thunder is heard because light (about 3,00,000 km/s) travels far faster than sound (about 346 m/s in air).
• A mirage forms when light bends through layers of unevenly heated air near a hot road surface.
• A rainbow forms through dispersion of sunlight inside raindrops, splitting white light into VIBGYOR.
• AC is used for long-distance transmission because transformers, which only work with AC, allow efficient high-voltage transmission with low current loss.
• Birds are safe on a live wire because both their feet touch the same wire, creating no potential difference across their body.
• A fuse wire has a low melting point so it melts and breaks the circuit during a current overload, protecting appliances.
• Wires are insulated with rubber or PVC to prevent current from leaking into anything touching the wire.
• A step-up transformer raises voltage for efficient transmission; a step-down transformer lowers it for safe household use.
• A fan cools you by speeding up the evaporation of sweat, not by lowering room air temperature.
• A refrigerator cools food by evaporating a refrigerant inside the unit, absorbing heat, and releasing it outside via a compressor.
• Polarised sunglasses cut glare by blocking the strongly horizontal-vibrating light reflected off shiny surfaces.
• A periscope uses two parallel 45-degree mirrors to let a submerged submarine see the surface.
• Milk cools faster in a wide-mouthed vessel because more surface area is exposed to the air.
• A sharp blade cuts more easily than a blunt one because the same force concentrated on a smaller area creates far higher pressure.
• Camel feet are broad to spread body weight over a larger area, reducing pressure on soft desert sand.
• Evaporation always cools the surface it happens on, because escaping molecules carry away heat energy.
• Total internal reflection, not literal water, is what the brain misreads as a puddle in a mirage.

Memory Tables

Table 1: Device or Phenomenon → Core Principle

Everyday phenomenon	Core physics principle	One-line reason
Pressure cooker cooks faster	Boiling point rises with pressure	Trapped steam raises pressure, water boils above 100°C
Ice floats on water	Density anomaly of water	Solid ice is less dense than liquid water
Ship floats, needle sinks	Archimedes' Principle	Hollow shape displaces enough water to match weight
Needle floats flat on water	Surface tension	Elastic-like water surface skin supports light weight
Sky is blue	Rayleigh scattering	Shorter (blue) wavelengths scatter far more than red
Lightning seen before thunder heard	Speed of light vs speed of sound	Light (3,00,000 km/s) far outpaces sound (346 m/s)
Rainbow after rain	Dispersion in raindrops	White light splits into VIBGYOR by wavelength
Mirage on hot road	Refraction and total internal reflection	Light bends through unevenly heated air layers
AC used for power transmission	Transformer action	Transformers need AC to step voltage up or down
Bird safe on live wire	No potential difference	Both feet touch the same wire, so no current flows through it
Fuse protects appliances	Low melting point of fuse wire	Wire melts first, breaking circuit during overload
Fan feels cool	Evaporative cooling	Faster sweat evaporation removes heat from skin
Refrigerator cools food	Evaporation of refrigerant	Evaporating refrigerant absorbs heat from inside the fridge
Periscope shows surface view	Two 45-degree mirror reflections	Light path bends around a corner via two reflections
Sharp knife cuts easily	Pressure = force / area	Small edge area concentrates force into high pressure

Table 2: Look-Alike Pairs the Exam Loves to Confuse

Concept A	Concept B	The key difference
Ship floating	Needle sinking	Shape decides displaced water volume, not the material
Ship floating (Archimedes)	Needle floating flat (surface tension)	Two different mechanisms, easily confused
Fan cooling you	Air conditioner cooling a room	Fan speeds sweat evaporation only; AC actually lowers air temperature
Fuse wire	Circuit breaker	Fuse melts and must be replaced; a breaker trips and can be reset
AC chosen for efficiency	AC assumed to be "safer"	AC is chosen because transformers need it, not for inherent safety
Matka cooling (evaporation)	Convection currents in heated water	Evaporative cooling versus density-driven rising and sinking
Rayleigh scattering (sky is blue)	Dispersion (rainbow, prism)	Scattering by tiny air molecules versus bending and splitting by a medium
Mirage (refraction/total internal reflection)	Rainbow (refraction and dispersion)	Mirage bends light around hot air layers; rainbow splits light by colour inside drops

Practice MCQs

Q1. A pressure cooker cooks food faster mainly because it: (a) increases the amount of water used (b) raises the boiling point of water above 100°C (c) reduces cooking time by using more flame (d) removes air completely from the vessel

Q2. Ice floats on water because: (a) ice is a poor conductor of heat (b) ice contains trapped air bubbles (c) ice is less dense than liquid water (d) ice has a lower surface tension than water

Q3. A ship made of steel floats on water mainly due to: (a) the low density of steel (b) surface tension of water (c) Archimedes' Principle and the ship's hollow shape (d) the high salinity of seawater

Q4. We see lightning before we hear thunder because: (a) sound is produced later than light in a storm (b) light travels much faster than sound (c) thunder originates from clouds farther away than lightning (d) our eyes react faster than our ears

Q5. The sky appears blue mainly due to: (a) reflection of the ocean's colour (b) absorption of blue light by the atmosphere (c) scattering of shorter wavelengths of light by air molecules (d) presence of blue gases in the upper atmosphere

Q6. A fan makes a person feel cool chiefly by: (a) lowering the temperature of the room air (b) increasing the humidity of the room (c) speeding up the evaporation of sweat from the skin (d) reducing the oxygen content of the air

Q7. Which device is essential for using alternating current for long-distance electricity transmission? (a) fuse (b) transformer (c) capacitor (d) rectifier

Q8. A fuse wire used in household circuits is generally made of a metal with: (a) high melting point and low resistance (b) low melting point and high resistance (c) high melting point and high resistance (d) low melting point and low resistance

Q9. A bird sitting on a single high-voltage wire does not get an electric shock because: (a) birds have natural insulation on their feet (b) there is no potential difference between its two feet (c) the current in the wire is too weak (d) the bird's body resistance is infinite

Q10. A needle can be made to float on the surface of still water mainly due to: (a) Archimedes' Principle (b) surface tension of water (c) low density of the needle (d) atmospheric pressure

Q11. Panhandles of cooking vessels are usually made of wood or plastic because these materials are: (a) good conductors of heat, spreading it evenly (b) poor conductors of heat, preventing burns (c) lighter than metal, making the pan easier to lift (d) cheaper than metal alternatives

Q12. A mirage seen on a hot road is caused by: (a) reflection of sunlight off water on the road (b) refraction and total internal reflection of light through unevenly heated air layers (c) dispersion of light by dust particles (d) scattering of light by water vapour

Q13. Sunsets appear red or orange chiefly because: (a) the sun emits more red light in the evening (b) sunlight travels a longer path through the atmosphere, scattering away most of the blue light (c) red light is absorbed less by clouds (d) the eye's sensitivity to blue light decreases at dusk

Q14. A refrigerator keeps food cold by using a refrigerant that: (a) freezes instantly on contact with food (b) evaporates inside the fridge, absorbing heat, and condenses outside, releasing it (c) reflects heat away from the food compartment (d) chemically reacts with the air to lower its temperature

Q15. A knife with a sharper edge cuts more easily than a blunt one primarily because, for the same applied force, a sharper edge: (a) reduces the total force needed (b) increases the surface area of contact (c) reduces the area of contact, increasing pressure (d) reduces friction between the blade and the object

Answer Key

Q	Answer	One-line reason
Q1	(b)	Trapped steam raises internal pressure, which raises the boiling point above 100°C, so food cooks at a higher temperature.
Q2	(c)	Water's solid form (ice) is one of the rare cases where the solid is less dense than the liquid, so it floats.
Q3	(c)	The ship's hollow shape displaces a large volume of water whose weight equals the ship's total weight.
Q4	(b)	Light travels at about 3,00,000 km/s, while sound travels at only about 346 m/s in air, so light always arrives first.
Q5	(c)	Rayleigh scattering scatters shorter (blue) wavelengths of sunlight far more than longer (red) wavelengths.
Q6	(c)	A fan does not change air temperature; it speeds up evaporation of sweat, which cools the skin.
Q7	(b)	Transformers step voltage up for efficient transmission and down for safe use, and they work only with AC.
Q8	(b)	A fuse wire must melt first during an overload, so it needs a low melting point and correspondingly higher resistance than the main wiring.
Q9	(b)	Both of the bird's feet touch the same wire at nearly the same potential, so no significant current flows through its body.
Q10	(b)	The elastic-like surface tension skin of still water can support the small weight of a carefully placed needle.
Q11	(b)	Wood and plastic are poor conductors (insulators), so heat from the pan does not travel up into your hand.
Q12	(b)	Light bends through layers of hot, low-density air near the ground and curves back up, mimicking a reflective puddle.
Q13	(b)	At sunset, sunlight crosses a much longer stretch of atmosphere, scattering away nearly all the blue light before it reaches your eyes.
Q14	(b)	The refrigerant evaporates inside the fridge (absorbing heat from food) and is condensed back to liquid outside, releasing that heat.
Q15	(c)	Pressure equals force divided by area; a sharp edge concentrates the same force onto a tiny area, producing very high pressure.