World Geography — Continents & Geopolitics

Why This Chapter Matters

Physics questions show up in almost every SSC MTS and CHSL paper, usually 3 to 5 marks in the General Awareness section, and the good news is they are the most predictable questions in the whole exam. Unlike history dates or geography names, physics facts repeat year after year: the same units, the same laws of motion, the same simple machines. Learn them once properly and you carry these marks for life, across MTS, CHSL, CGL, and even railway exams later.

The single biggest mistake aspirants make here is treating physics like a memorisation subject when it is really an observation subject. You already know that a fan takes a moment to stop after you switch it off, that ice floats on water, that sound travels slower than light. Physics just gives these everyday truths proper names — inertia, density, wave speed. This chapter connects what you already know to the terms SSC tests, using things from your own kitchen, street, and school days. No formulas beyond basic arithmetic, no heavy derivations. Just clear, testable facts explained the way a good teacher would explain them on a blackboard, not the way a textbook buries them in jargon.

1. Units and Measurement

Every measurement needs two things: a number and a unit. Saying "the rope is 5" means nothing until you add "metres." Physics fixed this confusion by creating one worldwide system called SI units (International System of Units), used by scientists and exams everywhere.

Exam trap: Students often mix up mass and weight. Mass is the amount of matter in a body, measured in kilogram (kg), and it stays the same everywhere, even on the moon. Weight is the force of gravity pulling on that mass, measured in newton (N), and it changes with location. A 60 kg person has the same mass on the moon but weighs about six times less there because the moon's gravity is weaker.

Here are the base SI units you must know cold:

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

Beyond these seven base units, SSC also likes derived units, built by combining base units. Speed is distance divided by time, so its unit is metre per second (m/s). Force is measured in newton, pressure in pascal, energy and work in joule, and power in watt. Power is simply how fast work gets done — a 100-watt bulb converts electrical energy to light and heat faster than a 40-watt bulb, which is why it burns brighter but also costs more on your electricity bill.

Memory hook: Remember the seven base units with the sentence "My Kid Studies Till Afternoon, Making Circles" — Metre, Kilogram, Second, Temperature (kelvin), Ampere, Mole, Candela. Silly, but you will not forget it during the exam.

One more measurement idea that trips up students: the difference between distance and displacement. Distance is the total path covered, always positive. Displacement is the shortest straight line from start to end point, and it can be zero. If you walk one full round of a circular park and return to your starting bench, your distance covered might be 400 metres, but your displacement is zero because you ended up exactly where you started. This exact question — "a person walks around a circular track and returns to start, what is the displacement" — has appeared in various forms across competitive exams for years.

2. Simple Machines

A simple machine does not create energy out of nothing. It only changes the direction of force or reduces the effort needed, at the cost of moving that effort over a longer distance. Think of a well with a pulley. Without the pulley, you would haul the bucket straight up with your full body weight working against gravity in one hard pull. With the pulley, you pull the rope downward instead, using your own body weight to help, and the effort feels lighter even though the total work done is nearly the same.

There are six classical simple machines, and SSC loves asking which everyday tool belongs to which category:

• Lever — a rigid bar that pivots on a fixed point called the fulcrum. A seesaw, a pair of scissors, and a crowbar are levers.
• Pulley — a wheel with a groove for a rope, used to lift loads, as in a well or a flagpole.
• Wheel and axle — a wheel fixed to a rod so both turn together, seen in a doorknob or a bicycle's steering.
• Inclined plane — a sloped surface that lets you move a load up gradually instead of lifting it straight up, like a ramp used to load furniture into a truck.
• Wedge — two inclined planes joined back to back, used for cutting or splitting, like an axe or a knife.
• Screw — an inclined plane wrapped around a cylinder, used in a bottle cap or a bolt.

Exam trap: A pair of scissors is often asked as an example, and students sometimes call it a wedge because it cuts. It is actually a lever — specifically two levers joined at a fulcrum (the screw holding the blades together), even though the sharp edges themselves work like tiny wedges. When the question says "identify the simple machine," go with lever for scissors.

Levers are classified into three classes based on where the fulcrum, effort, and load sit relative to each other:

Class	Arrangement	Example
First class	Fulcrum in the middle	Seesaw, scissors, crowbar
Second class	Load in the middle	Wheelbarrow, nutcracker, bottle opener
Third class	Effort in the middle	Fishing rod, tweezers, human forearm lifting a weight

Memory hook: Remember the order Fulcrum-Load-Effort for classes one-two-three using "Friends Love Eating" — First class has Fulcrum in middle, second class has Load in middle, third class has Effort in middle.

3. Laws of Motion in Everyday Language

Sir Isaac Newton gave three laws that explain nearly every motion you see around you, from a bus jerking forward to a rocket launching into space.

First law — the law of inertia. An object at rest stays at rest, and an object in motion stays in motion at the same speed and direction, unless an external force acts on it. You feel this every single day on a bus. When the bus suddenly stops, your body keeps moving forward because your body's inertia wants to continue the motion it already had — that is why you jerk forward and grab the seat in front. When the bus suddenly starts, you get pushed backward into your seat because your body was at rest and resists starting to move. Inertia is simply the tendency of matter to resist a change in its state of motion, and it depends only on mass — heavier objects have more inertia and are harder to start or stop.

Second law — force equals mass times acceleration. The harder you push something, the faster it speeds up, but heavier objects need more push for the same speeding-up effect. This is why pushing an empty handcart is easy but pushing the same handcart loaded with bricks needs real effort for the same acceleration. In formula form it is written F = ma, where F is force, m is mass, and a is acceleration. You will not need to solve complex numericals for MTS or CHSL, but you must recognise this relationship when a question describes it in words.

Third law — action and reaction. For every action, there is an equal and opposite reaction. When you jump off a small boat onto a riverbank, the boat pushes back and drifts away from the shore. Your feet pushed down and back against the boat (action), and the boat pushed you up and forward (reaction), while pushing itself in the opposite direction. This is exactly how a rocket works too — burning fuel shoots hot gas downward out of the engine, and the reaction pushes the rocket upward, even in the vacuum of space where there is no air to "push against." That last point is a classic exam trap: rockets do not need air to move, because they work on gas pushing against the rocket itself, not against air.

Exam trap: Students often confuse the first law (inertia) with the third law (action-reaction). If a question describes something continuing to move or resisting a change on its own, that is the first law. If it describes two objects pushing each other in opposite directions, that is the third law.

Related to these laws is the idea of momentum, which is mass multiplied by velocity. A slow-moving truck can be harder to stop than a fast-moving bicycle because the truck's much larger mass gives it far more momentum. This is why trucks need a longer braking distance — it is not just about speed, it is about the total momentum that needs to be brought to zero.

Gravity deserves a special mention here. Every object with mass pulls every other object with mass toward it, and Earth's gravity pulls everything toward its centre with an acceleration of roughly 9.8 metres per second squared, often rounded to 9.8 m/s² in exam questions. This is why a dropped stone and a dropped feather would hit the ground at the same time in a vacuum (no air resistance), even though in real air the feather floats down slower because air resistance affects it far more than the stone.

4. Heat, Light, and Sound Basics

Heat is a form of energy that flows from a hotter object to a colder one until both reach the same temperature. This flow happens in three ways. Conduction is heat moving through direct contact, like a steel spoon getting hot when left in hot tea. Convection is heat moving through the actual movement of a fluid like air or water, like a room warming up as hot air rises from a heater and circulates. Radiation is heat travelling through empty space as waves, needing no medium at all, exactly how the Sun's heat reaches Earth across 150 million kilometres of empty space.

Exam trap: A thermos flask (vacuum flask) keeps tea hot by blocking all three methods of heat transfer at once — the vacuum between its double walls stops conduction and convection, and its shiny silvered inner surface reflects back radiation. This is a favourite SSC question: "how does a thermos flask work" tests whether you understand all three heat transfer methods together.

Temperature is commonly measured on two scales that exams love to test conversions between: Celsius (°C), where water freezes at 0° and boils at 100°, and Kelvin (K), the SI unit, where the same points are 273 K and 373 K. To convert Celsius to Kelvin, simply add 273.

Light travels in straight lines and is the fastest thing known, moving at roughly 3 lakh kilometres per second in a vacuum — nothing in the universe travels faster. When light passes from one medium into another, like from air into water, it bends. This bending is called refraction, and it is why a straight pencil dipped into a glass of water looks bent at the surface. This happens because light slows down when it enters the denser medium (water), and that change in speed bends its path.

When light bounces off a surface instead of passing through it, that is reflection. A plane mirror reflects light in a very predictable way, forming an image that is the same size as the object, upright, and exactly as far behind the mirror as the object is in front of it — but left and right appear swapped, which is why text held up to a mirror reads backward.

Exam trap: Do not confuse a rainbow's cause with plain reflection. A rainbow forms because sunlight passing through tiny raindrops gets refracted (bent), split into its seven colours (dispersion), and then reflected inside the drop before exiting toward your eye. All three — refraction, dispersion, and internal reflection — happen together, and this combination question appears often.

The seven colours of white light, in order, are Violet, Indigo, Blue, Green, Yellow, Orange, Red, remembered by the classic mnemonic VIBGYOR.

Sound is different from light in one crucial way: sound needs a medium to travel and cannot pass through a vacuum, while light can. This is exactly why astronauts in space cannot hear each other by shouting and must use radio communication instead — there is no air in space to carry sound waves. Sound also travels at very different speeds depending on the medium: roughly 343 metres per second in air, much faster in water, and fastest of all in solids like steel. This is why you can sometimes hear an approaching train by placing your ear near the rail before you can hear it through the air.

Exam trap: A very commonly tested comparison — sound travels fastest in solids, slower in liquids, and slowest in gases, which is the exact opposite of what many students guess. Denser, more tightly packed particles in solids pass on vibrations faster than the loosely spaced particles in a gas.

5. Electricity Basics

Electricity is the flow of electric charge, usually carried by electrons moving through a conductor like a copper wire. Three quantities matter here for exams. Current, measured in ampere (A), tells you how much charge flows per second. Voltage or potential difference, measured in volt (V), is what pushes the current through a circuit — think of it like water pressure pushing water through a pipe. Resistance, measured in ohm (Ω), is how much a material opposes the flow of current.

These three are linked by Ohm's Law: Voltage equals Current multiplied by Resistance, written V = IR. If resistance goes up while voltage stays the same, current must go down. Think of resistance like a narrow section in a water pipe — the narrower it gets, the less water flows through even if the pressure pushing it stays constant.

Materials that let electric current pass easily are called conductors — most metals, especially copper and silver, are excellent conductors, which is why household wiring uses copper. Materials that block current are called insulators — rubber, plastic, glass, and dry wood are common insulators, which is why electrical wires are coated in rubber or plastic and electricians wear rubber gloves and stand on rubber mats.

Exam trap: A very common SSC question asks why birds sitting on a single high-voltage wire do not get electrocuted. The answer is that electric current always takes the path of least resistance, flowing from high potential to low potential. A bird sitting on one single wire is at the same potential all along its body since both its feet touch the same wire, so there is no potential difference across its body and no current flows through it. If that same bird touched two different wires at once, or touched the wire and an earthed pole simultaneously, current would flow through its body and it would be shocked or killed.

Household circuits use fuses as safety devices. A fuse contains a thin wire that melts and breaks the circuit if too much current flows, protecting your home's wiring and appliances from damage or fire during a short circuit. This is a small, cheap piece of wire doing an important job, much like a security guard who deliberately faints (breaking the circuit) rather than let a dangerous surge pass through into the building.

Static electricity is different from current electricity. It is charge that builds up on a surface without flowing, and you experience it when you rub a plastic comb on dry hair and the comb then attracts small bits of paper, or when you get a mild shock touching a metal door handle after walking on a synthetic carpet in dry weather.

6. Everyday Physics Phenomena Explained Simply

Several physics phenomena that seem like small daily curiosities are actually favourite SSC questions once you know the correct term for them.

Why a spinning top does not fall over immediately relates to a property called angular momentum, which keeps a spinning object stable along its axis until friction slows it down enough for it to wobble and fall.

Why a ship made of heavy steel floats while a small steel pin sinks comes down to density and the principle of flotation. An object floats if it can displace a weight of water equal to its own weight before it is fully submerged. A ship's hollow shape spreads its steel over a huge volume, most of which is air-filled space, making its overall density lower than water. A solid steel pin has no such hollow shape, so it is denser than water and sinks. This principle is called Archimedes' Principle — an object submerged in a fluid experiences an upward force (buoyant force) equal to the weight of the fluid it displaces.

Exam trap: Archimedes' Principle is often confused with the general idea of "floating." Remember it precisely — the upward buoyant force equals the weight of fluid displaced, not the weight of the object itself. Whether the object floats or sinks depends on comparing this buoyant force with the object's own weight.

Why a pressure cooker cooks food faster ties back to the relationship between pressure and boiling point. Normally water boils at 100°C at sea-level atmospheric pressure. Inside a sealed pressure cooker, trapped steam raises the internal pressure above normal, and this higher pressure raises water's boiling point above 100°C. Food cooks in hotter water and hotter steam, so it cooks faster, saving both time and fuel.

Why the sky looks blue during the day but red or orange at sunset is due to a phenomenon called scattering of light. Sunlight contains all seven colours, and as it passes through Earth's atmosphere, the gas molecules scatter blue light (a shorter wavelength) far more than red light (a longer wavelength) in all directions, so the sky appears blue overhead through most of the day. At sunset, sunlight travels through a much longer stretch of atmosphere to reach your eyes, scattering away almost all the blue light along the way and leaving mostly red and orange tones visible, which is why sunsets glow warm colours.

Why a fan keeps spinning briefly after you switch it off is a direct real-life demonstration of the first law of motion (inertia) discussed earlier — the blades were in motion and continue moving until friction and air resistance gradually bring them to rest.

Why you cannot easily push a bicycle uphill in high gear relates to the mechanical advantage of gears, which work like a system of wheel-and-axle simple machines, trading effort for distance exactly the way a lever or inclined plane does.

Understanding physics this way, as everyday events with formal names attached, is far more durable under exam pressure than memorising isolated facts. When you read an SSC question in the exam hall, picture the actual scene — a bus stopping, a pencil in water, a bird on a wire — and the correct physics term will come to you faster than trying to recall a rule in the abstract.

Quick Revision — One-Line Facts

• SI unit of mass is kilogram (kg); SI unit of length is metre (m); SI unit of time is second (s).
• Mass stays constant everywhere; weight changes with gravity and is measured in newton.
• Distance is total path covered; displacement is the shortest straight-line change in position.
• A lever, pulley, wheel and axle, inclined plane, wedge, and screw are the six simple machines.
• Scissors and a seesaw are examples of a first-class lever, with the fulcrum in the middle.
• A wheelbarrow is a second-class lever, with the load in the middle.
• A fishing rod and the human forearm act as third-class levers, with the effort in the middle.
• Newton's first law is the law of inertia — objects resist changes to their state of motion.
• Newton's second law links force, mass, and acceleration as F = ma.
• Newton's third law states every action has an equal and opposite reaction.
• Momentum is mass multiplied by velocity.
• Earth's gravitational acceleration is approximately 9.8 m/s².
• Heat transfers by conduction (contact), convection (fluid movement), and radiation (waves, no medium needed).
• Water freezes at 0°C and boils at 100°C at normal atmospheric pressure.
• To convert Celsius to Kelvin, add 273.
• Light travels at about 3 lakh km per second, faster than anything else known.
• Refraction is the bending of light as it passes between different media.
• Reflection is light bouncing off a surface, as in a mirror.
• A rainbow forms through refraction, dispersion, and internal reflection inside water droplets.
• VIBGYOR gives the order of colours in white light: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
• Sound cannot travel through a vacuum; light can.
• Sound travels fastest in solids, slower in liquids, slowest in gases.
• The speed of sound in air is roughly 343 metres per second.
• Ohm's Law states V = IR, linking voltage, current, and resistance.
• Copper and silver are good conductors; rubber, glass, and dry wood are good insulators.
• A bird is safe on a single high-voltage wire because there is no potential difference across its body.
• A fuse breaks a circuit automatically when current exceeds a safe limit.
• Archimedes' Principle says the buoyant force equals the weight of fluid displaced.
• A pressure cooker cooks faster because higher internal pressure raises water's boiling point.
• The sky appears blue due to greater scattering of blue light by atmospheric particles.
• A spinning fan continues moving briefly after switching off due to inertia.

Memory Tables

Table 1: Simple Machines and Their Everyday Examples

Simple Machine	Working Principle	Everyday Example
Lever	Pivots on a fulcrum to multiply force	Seesaw, scissors, crowbar
Pulley	Wheel and rope change direction of force	Well bucket, flagpole
Wheel and axle	Wheel fixed to rod turns together	Doorknob, steering wheel
Inclined plane	Slope spreads effort over distance	Loading ramp
Wedge	Two inclined planes joined back to back	Axe, knife
Screw	Inclined plane wrapped on a cylinder	Bottle cap, bolt

Table 2: Newton's Three Laws at a Glance

Law	Core Idea	Everyday Example
First law	Objects resist change in motion (inertia)	Jerking forward when a bus stops suddenly
Second law	Force = mass × acceleration	Pushing a loaded cart needs more force than an empty one
Third law	Every action has an equal, opposite reaction	Jumping off a boat pushes the boat backward

Table 3: Modes of Heat Transfer

Mode	How It Works	Example
Conduction	Heat passes through direct contact	Steel spoon heating up in hot tea
Convection	Heat moves with a flowing fluid	Warm air rising from a room heater
Radiation	Heat travels as waves, no medium needed	Sun's heat reaching Earth through space

Practice MCQs

Q1. What is the SI unit of mass? (a) Newton (b) Kilogram (c) Joule (d) Watt

Q2. A person walks one full round of a circular park and returns to the starting point. What is the person's displacement? (a) Equal to the distance covered (b) Zero (c) Half the distance covered (d) Cannot be determined

Q3. Which of the following is an example of a first-class lever? (a) Wheelbarrow (b) Nutcracker (c) Seesaw (d) Fishing rod

Q4. Newton's first law of motion is also known as the law of: (a) Gravitation (b) Inertia (c) Conservation of energy (d) Reflection

Q5. Which colour is at the top of the VIBGYOR sequence, bending the most when light is dispersed? (a) Red (b) Violet (c) Green (d) Orange

Q6. Sound travels fastest in which medium? (a) Vacuum (b) Gas (c) Liquid (d) Solid

Q7. What does Ohm's Law state? (a) V = I/R (b) V = IR (c) I = VR (d) R = VI

Q8. A bird sitting on a single high-voltage electric wire is safe because: (a) Birds are naturally insulated (b) There is no potential difference across its body (c) The wire is not carrying current (d) Birds have very low body weight

Q9. Which simple machine is a knife an example of? (a) Lever (b) Wedge (c) Pulley (d) Screw

Q10. Water boils at what temperature in Celsius at normal atmospheric pressure? (a) 0°C (b) 50°C (c) 100°C (d) 273°C

Q11. According to Newton's third law, when you jump off a small stationary boat onto the shore, the boat: (a) Stays exactly still (b) Moves in the same direction as you (c) Moves in the opposite direction (d) Sinks immediately

Q12. Why does a pencil appear bent when partly dipped in a glass of water? (a) Reflection (b) Refraction (c) Dispersion (d) Scattering

Q13. A pressure cooker cooks food faster mainly because: (a) It uses more fuel (b) Higher pressure raises the boiling point of water (c) It reduces the water content (d) It increases oxygen supply

Q14. Which of these best explains why astronauts cannot talk to each other directly by shouting in open space? (a) Space is too cold (b) Sound needs a medium to travel and space is a vacuum (c) Their helmets block all sound (d) Light travels faster than sound

Q15. According to Archimedes' Principle, the buoyant force on a submerged object equals: (a) The weight of the object itself (b) The volume of the object (c) The weight of the fluid displaced by the object (d) The density of the fluid

Answer Key

Q	Answer	Reason
1	(b) Kilogram	Kilogram is the SI base unit for mass; newton measures weight/force, not mass.
2	(b) Zero	Displacement is the straight-line distance between start and end points, which are identical here.
3	(c) Seesaw	In a first-class lever the fulcrum sits between the load and the effort, as in a seesaw.
4	(b) Inertia	The first law describes a body's resistance to any change in its state of motion.
5	(b) Violet	Violet has the shortest wavelength among visible colours and bends the most during dispersion.
6	(d) Solid	Tightly packed particles in solids transmit vibrations fastest, faster than liquids or gases.
7	(b) V = IR	Ohm's Law links voltage, current, and resistance in this exact form.
8	(b) There is no potential difference across its body	Current needs a potential difference to flow; both the bird's feet touch the same wire.
9	(b) Wedge	A knife's two sloped cutting edges joined back to back form a wedge.
10	(c) 100°C	This is the standard boiling point of water at sea-level atmospheric pressure.
11	(c) Moves in the opposite direction	Your push against the boat (action) causes the boat to push back (reaction) and drift away.
12	(b) Refraction	Light bends when passing from air into the denser medium of water, creating the illusion of a bend.
13	(b) Higher pressure raises the boiling point of water	Trapped steam raises internal pressure, so water boils hotter and cooks food faster.
14	(b) Sound needs a medium to travel and space is a vacuum	Unlike light, sound waves cannot travel through the vacuum of space.
15	(c) The weight of the fluid displaced by the object	This is the precise statement of Archimedes' Principle, distinct from the object's own weight.