Economics — Basics, Trade & Development

Why This Chapter Matters

RRB NTPC General Awareness papers routinely carry 4 to 6 physics questions, and SSC exams push that number higher when you count applied questions hidden inside "everyday science" sections. Physics is also the section where marks slip away fastest, not because the concepts are hard, but because units, formulas, and closely related terms get swapped under exam pressure. A candidate who knows Newton's laws cold can still lose a mark by writing the wrong SI unit for pressure or confusing mass with weight.

Here is the pattern across ten years of RRB and SSC papers: examiners love asking about units (SI unit of X), simple numerical relationships (speed, work, power), and "which scientist discovered/proposed" questions. The single biggest mistake aspirants make is treating physics as a memorise-the-formula subject and skipping the logic behind each law. That approach collapses the moment a question rephrases a familiar law in unfamiliar words. This chapter builds the logic first, so the facts stick on their own.

Units and Measurements

Physics begins with measurement, and measurement begins with a standard everyone agrees on. The International System of Units (SI) is that agreed standard, adopted globally so a kilogram in Delhi means the same as a kilogram in Tokyo.

There are seven base SI units, and exams love testing these directly:

Quantity	SI Unit	Symbol
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

Memory hook: Think of a "MASKATC" relay team — Metre, Ampere, Second, Kelvin, Ampere (current), mole, candela — actually easier as a sentence: "My Aunt Sells Kachori At The Corner" gives Mass, Ampere, Second, Kelvin, Ampere-current, Time, Candela in a jumbled but memorable string. Pick whichever ordering helps you; the point is anchoring seven odd words to one sentence you will not forget in an exam hall.

From these seven base units, every other unit is "derived." Speed is metre per second (m/s), because speed is just distance divided by time. Force is measured in newton (N), which itself breaks down to kg·m/s². Energy and work share the unit joule (J). Pressure is measured in pascal (Pa), and power in watt (W).

Exam trap: Students confuse mass and weight. Mass is the amount of matter in a body, measured in kilograms, and it never changes regardless of location. Weight is the force gravity exerts on that mass, measured in newtons, and it changes with location. Your mass on the Moon is identical to your mass on Earth; your weight on the Moon is roughly one-sixth, because the Moon's gravity is weaker. A question that asks "what remains constant when you go to the Moon" is testing exactly this distinction.

Two more units that show up often: temperature is measured in Celsius in daily life but in kelvin for scientific work (0°C equals 273.15 K, and absolute zero, the theoretical coldest possible temperature, is 0 K or -273.15°C). Density is mass per unit volume, measured in kg/m³, and it explains why oil floats on water: oil is less dense.

Laws of Motion

Sir Isaac Newton gave three laws that describe how objects move, published in 1687 in his work Principia Mathematica. These three laws are asked about more than any other physics topic in RRB and SSC exams.

Newton's First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless an external force acts on it. Think of standing in a bus that suddenly brakes. Your body keeps moving forward even though the bus has stopped, because your body's inertia resists the change in motion. That forward lurch is inertia in action, not some mysterious force pushing you.

Exam trap: "Inertia" and "momentum" sound related but test different things. Inertia is a body's resistance to a change in its state of motion, and it depends only on mass. Momentum is the quantity of motion a body has, calculated as mass times velocity (p = mv). A stationary truck has huge inertia but zero momentum; the moment it starts rolling, it gains momentum.

Newton's Second Law: Force equals mass times acceleration (F = ma). This is the workhorse formula of mechanics. It tells you that a heavier object needs more force to achieve the same acceleration as a lighter one, and that the same force produces less acceleration on a heavier object. This is why pushing an empty auto-rickshaw is easy but pushing one loaded with five passengers takes real effort for the same speed-up.

Newton's Third Law: For every action, there is an equal and opposite reaction. When you push against a wall, the wall pushes back on you with equal force; you don't move the wall, but you feel the resistance. A boat moves forward when a rower pushes water backward with the oar, because the water pushes the oar (and boat) forward with equal force. Rocket propulsion works on the exact same principle: burning fuel is expelled downward at high speed, and the rocket is pushed upward with equal force.

Memory hook: Remember the three laws as "Rest–Race–Return." Rest = first law (a body at rest resists change). Race = second law (force and acceleration determine how fast something speeds up, like a race). Return = third law (every action sends an equal reaction right back).

Work, Energy, and Power

These three terms are used loosely in daily conversation but have precise, different meanings in physics, and exams exploit that gap constantly.

Work is done only when a force causes displacement in the direction of the force. Its formula is Work = Force × Displacement, measured in joules. Here is the trap: if you push a wall with all your strength and the wall does not move, you have done zero work in the physics sense, even though you are exhausted. Displacement is the deciding factor, not effort.

Energy is the capacity to do work. It comes in many forms: kinetic energy (energy of motion, given by ½mv²), potential energy (stored energy due to position, given by mgh for gravitational potential energy), chemical energy, thermal energy, electrical energy, and so on. The Law of Conservation of Energy states that energy can neither be created nor destroyed, only converted from one form to another. A ball dropped from height has maximum potential energy at the top and maximum kinetic energy just before it hits the ground, the potential energy having converted into kinetic energy along the way.

Picture a temple bell being struck: the striker's kinetic energy converts into sound energy and a little heat at the point of contact. Nothing is lost; it just changes form. This single idea, energy conversion rather than creation or loss, answers a huge share of "which type of energy conversion happens in X" questions.

Power is the rate of doing work, or Power = Work / Time, measured in watts. This is where household electricity bills connect to physics: your electricity meter tracks energy consumption in kilowatt-hours (kWh), commonly called "units" on your bill. One unit equals one kilowatt of power used for one hour.

Exam trap: Horsepower (hp) is another unit of power, used for vehicle engines. 1 horsepower equals approximately 746 watts. A common wrong-option trick in MCQs swaps this figure with 750 or 764; memorise 746 specifically.

Electricity and Magnetism Basics

Electric current is the flow of electric charge, measured in amperes. It flows through a conductor when there is a difference in electric potential, called voltage, measured in volts. Resistance is what opposes this flow, measured in ohms.

Ohm's Law ties these three together: Voltage = Current × Resistance (V = IR). This is one of the most tested formulas in the entire GA syllabus. Think of a crowded railway platform: voltage is like the pushing crowd behind you, current is how many people actually squeeze through the gate per second, and resistance is how narrow that gate is. A narrower gate (higher resistance) means fewer people get through per second for the same push (voltage).

Materials are classified by how well they conduct electricity. Conductors (copper, silver, aluminium) allow current to flow easily, because they have free electrons. Insulators (rubber, glass, wood) resist current flow, which is why electrical wires are coated in rubber or plastic. Semiconductors (silicon, germanium) sit in between and form the backbone of all modern electronics, from your phone's processor to solar panels.

Magnetism is closely linked to electricity. Hans Christian Oersted discovered in 1820 that an electric current produces a magnetic field around it, the foundational discovery connecting the two forces. This link is exploited in two directions: an electromagnet uses current to create a magnetic field (used in electric bells, cranes lifting scrap metal, MRI machines), while a generator uses a moving magnetic field to produce electric current, based on the principle of electromagnetic induction discovered by Michael Faraday.

Exam trap: Do not confuse a generator with a motor. A generator converts mechanical energy into electrical energy (turning a turbine produces electricity). A motor converts electrical energy into mechanical energy (electricity makes a fan blade spin). They are, in a sense, mirror images of each other, and exams frequently swap the two in wrong options.

Fuses in your home wiring exploit resistance heating: a fuse wire is deliberately made of a low-melting-point metal so that if current exceeds a safe limit, the wire heats up and melts, breaking the circuit before your appliances or house wiring catch fire.

Light and Optics

Light is a form of energy that travels in straight lines and enables vision. It shows two key behaviours tested repeatedly: reflection (light bouncing off a surface) and refraction (light bending as it passes from one medium to another).

Reflection follows a simple rule: the angle of incidence equals the angle of reflection. This is why a plane mirror gives you an image that looks like you but reversed left-to-right, technically called a virtual, erect image of the same size.

Refraction happens because light travels at different speeds in different media. It travels fastest in vacuum, slower in air, slower still in water, and slowest in glass among common examples. This speed change bends the light ray. It is why a straight pencil dipped in a glass of water appears bent at the water's surface, and why a swimming pool always looks shallower than it actually is.

Two kinds of lenses matter for exams. A convex lens (thicker in the middle) converges light rays to a point and is used to correct hypermetropia (long-sightedness, difficulty seeing near objects) and is the working lens in a magnifying glass. A concave lens (thinner in the middle) diverges light rays and corrects myopia (short-sightedness, difficulty seeing distant objects).

Memory hook: Remember "Convex Corrects Close-up problems" is wrong; flip it: a person who cannot see close objects (hypermetropia) needs a convex lens to converge the rays more strongly onto the retina. Anchor it instead as "Concave for Close (myopia, seeing near is fine, far is blurry), Convex for Far-sight problems (hypermetropia)." If that still confuses you, remember concave lenses are used in cameras and telescopes for spreading focus outward, while convex lenses are used in magnifying glasses, projectors, and to fix long-sightedness.

Dispersion is the splitting of white light into its seven constituent colours when passed through a prism, producing the familiar VIBGYOR sequence: Violet, Indigo, Blue, Green, Yellow, Orange, Red. A rainbow forms through the same principle, with raindrops acting as tiny natural prisms that refract, disperse, and reflect sunlight back to your eye.

Exam trap: The human eye works like a camera, with the cornea and lens focusing light onto the retina, which acts like a screen. The image formed on the retina is actually inverted (upside down), and the brain flips it back for us. Exams sometimes ask which structure acts as the "screen" (retina) versus the "lens" (crystalline lens behind the cornea); do not swap them.

Sound

Sound is a form of energy produced by vibration, and unlike light, it needs a medium to travel; sound cannot travel through vacuum. This is why astronauts in space cannot hear sound directly through empty space and must rely on radio communication instead.

Sound travels fastest through solids, slower through liquids, and slowest through gases, because particles are packed tightest in solids, allowing vibrations to pass along quickest. This is the opposite of what many students assume, expecting air to carry sound fastest simply because that is the medium we experience daily.

Frequency determines pitch: higher frequency means a higher-pitched sound, measured in hertz (Hz). The human ear typically hears frequencies between 20 Hz and 20,000 Hz. Sound above 20,000 Hz is called ultrasonic (used in ultrasound imaging during pregnancy scans, and by bats for echolocation), and sound below 20 Hz is called infrasonic (produced by earthquakes and some large animals like elephants for long-distance communication).

Amplitude determines loudness, measured in decibels (dB). A whisper is around 20-30 dB, normal conversation around 60 dB, and prolonged exposure above 85 dB can damage hearing, which is the science behind hearing-protection warnings at construction sites and loud concerts.

Echo is the reflection of sound, and the human ear needs a minimum gap of about 0.1 seconds between the original sound and its reflection to perceive them as separate, which requires the reflecting surface to be at least around 17 metres away, explaining why you hear echoes in large empty halls or canyons but not in a small room.

Modern Physics Basics

Atomic structure underpins several exam-favourite questions. An atom consists of a dense central nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) orbiting in shells. J.J. Thomson discovered the electron in 1897. Ernest Rutherford, through his famous gold foil experiment, discovered the nucleus in 1911, proving that an atom is mostly empty space with a tiny, dense, positively charged centre. Niels Bohr later proposed that electrons orbit the nucleus in fixed energy levels or shells, refining the atomic model further. James Chadwick discovered the neutron in 1932.

Think of an atom like a school campus: the nucleus is the small, heavily guarded administrative block at the centre holding the important "positive" authority (protons) alongside neutral staff (neutrons), while electrons are students constantly circling the campus grounds at a distance, in defined pathways (shells), never crashing into the administrative block under normal conditions.

Exam trap: Students confuse atomic number and mass number. Atomic number equals the number of protons in the nucleus, and it defines which element an atom is (this is what the periodic table is ordered by). Mass number equals the total number of protons plus neutrons. Two atoms of the same element with different mass numbers, because they have different numbers of neutrons, are called isotopes.

Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei, discovered by Henri Becquerel in 1896 while working with uranium salts. Marie Curie and Pierre Curie extended this research, discovering the elements polonium and radium, and Marie Curie became the first person to win Nobel Prizes in two different sciences (Physics and Chemistry).

Radioactive decay releases three types of radiation, tested as a set:

• Alpha particles: heavy, positively charged, least penetrating (stopped by paper or skin)
• Beta particles: lighter, negatively charged, more penetrating (stopped by a thin sheet of aluminium)
• Gamma rays: pure energy, no charge, most penetrating (needs thick lead or concrete to stop)

Memory hook: Remember penetrating power in order using "Alpha Barely Goes far" (Alpha least penetrating, Beta moderate, Gamma greatest), which also conveniently keeps alphabetical order matching increasing penetration power.

Radioactivity has real applications tested in exams: carbon dating uses the decay of radioactive carbon-14 to determine the age of ancient fossils and archaeological remains. Cobalt-60 is used in cancer treatment (radiotherapy) to destroy tumour cells. Nuclear power plants use controlled radioactive decay (nuclear fission) of uranium to generate electricity.

Everyday Physics Applications

Exams frequently test physics through the lens of daily life, expecting you to connect abstract laws to ordinary experiences.

A pressure cooker cooks food faster because increased pressure raises the boiling point of water above 100°C, letting food cook at a higher temperature than an open pot allows, which is basic thermodynamics at work in every Indian kitchen.

A thermos flask keeps liquids hot or cold by minimising heat transfer through all three modes: conduction, convection, and radiation, using a vacuum layer between double walls (blocking conduction and convection) and a silvered inner surface (reflecting radiant heat back inward).

Friction, often blamed as a nuisance, is actually essential. Without friction between your shoes and the ground, walking would be impossible; you would simply slide. Friction between tyres and road allows vehicles to grip and stop. But friction also causes wear and wastes energy as heat, which is why engineers use lubricants (oil, grease) to reduce it in machine parts, and why ball bearings replace sliding friction with the much lower rolling friction.

A kite flying in the sky demonstrates several forces simultaneously: the wind provides lift by flowing over the angled kite surface (similar to how an aeroplane wing generates lift), gravity pulls the kite down, and tension in the string balances the horizontal pull, letting a skilled flier hold a kite steady against the sky.

Why does a ceiling fan cool a room without lowering the actual air temperature? It does not cool the air at all; it simply speeds up air movement across your skin, increasing the rate of evaporation of sweat, and evaporation absorbs heat from your body, producing the sensation of cooling. This is a favourite "trick" fact in GA papers, phrased as "does a fan lower room temperature."

Refrigerators work on the opposite principle of evaporative cooling combined with a refrigerant gas cycle: a refrigerant absorbs heat from inside the fridge as it evaporates (turning liquid to gas), and this heat is then released outside as the gas is compressed back into a liquid, repeating continuously. This is why the back of a refrigerator always feels warm.

Understanding these applied examples matters because RRB and SSC papers often disguise a pure-physics question as a "why does this happen" everyday scenario, and candidates who only memorised formulas without understanding the underlying law get caught out.

Quick Revision — One-Line Facts

• SI unit of force is the newton; SI unit of energy and work is the joule.
• Mass is constant everywhere; weight changes with gravity, and weight is measured in newtons.
• Newton's First Law is also called the Law of Inertia.
• Momentum equals mass multiplied by velocity (p = mv).
• Force equals mass multiplied by acceleration (F = ma), Newton's Second Law.
• Newton's Third Law: every action has an equal and opposite reaction.
• Work is done only when force produces displacement in the force's direction.
• Energy can never be created or destroyed, only converted between forms (Law of Conservation of Energy).
• Power is the rate of doing work, measured in watts; 1 horsepower equals about 746 watts.
• One "unit" on an electricity bill equals one kilowatt-hour.
• Ohm's Law: Voltage = Current × Resistance (V = IR).
• Oersted discovered that current produces a magnetic field (1820).
• A generator converts mechanical energy to electrical; a motor does the reverse.
• Angle of incidence equals angle of reflection in reflection of light.
• Refraction is the bending of light when it passes between media of different densities.
• Convex lens corrects hypermetropia (long-sightedness); concave lens corrects myopia (short-sightedness).
• White light splits into seven colours (VIBGYOR) through a prism, called dispersion.
• Sound needs a medium to travel and cannot pass through vacuum.
• Sound travels fastest in solids, slowest in gases.
• Human hearing range is roughly 20 Hz to 20,000 Hz.
• Frequencies above 20,000 Hz are ultrasonic; below 20 Hz are infrasonic.
• J.J. Thomson discovered the electron; Rutherford discovered the nucleus; Chadwick discovered the neutron.
• Atomic number = number of protons; mass number = protons + neutrons.
• Isotopes are atoms of the same element with different numbers of neutrons.
• Henri Becquerel discovered radioactivity in 1896; Marie Curie won two Nobel Prizes in different sciences.
• Alpha particles are least penetrating, gamma rays are most penetrating.
• A pressure cooker raises the boiling point of water to cook food faster.
• A ceiling fan cools people by speeding up sweat evaporation, not by lowering air temperature.
• Absolute zero is 0 K, equal to -273.15°C.
• Density equals mass divided by volume; this is why less dense substances float.

Memory Tables

Table 1: Physical Quantities and Their SI Units

Physical Quantity	SI Unit	Symbol
Force	newton	N
Work / Energy	joule	J
Power	watt	W
Pressure	pascal	Pa
Electric current	ampere	A
Voltage	volt	V
Resistance	ohm	Ω
Frequency	hertz	Hz
Temperature	kelvin	K

Table 2: Key Scientists and Their Discoveries

Scientist	Discovery/Contribution
Isaac Newton	Three Laws of Motion, Law of Gravitation
J.J. Thomson	Discovered the electron (1897)
Ernest Rutherford	Discovered the atomic nucleus (1911)
James Chadwick	Discovered the neutron (1932)
Niels Bohr	Proposed electron shell/energy level model
Hans Christian Oersted	Linked electric current to magnetism (1820)
Michael Faraday	Principle of electromagnetic induction
Henri Becquerel	Discovered radioactivity (1896)
Marie Curie	Discovered polonium and radium; two Nobel Prizes

Practice MCQs

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

Q2. A person's weight on the Moon compared to Earth is: (a) Same (b) About six times more (c) About one-sixth (d) Exactly double

Q3. Which law is also known as the Law of Inertia? (a) Newton's Second Law (b) Newton's Third Law (c) Newton's First Law (d) Law of Conservation of Energy

Q4. Work done is zero when: (a) Force causes large displacement (b) Force is applied but no displacement occurs (c) Force and displacement are in the same direction (d) Mass increases

Q5. One horsepower is approximately equal to: (a) 640 watts (b) 746 watts (c) 1000 watts (d) 550 watts

Q6. Which device converts mechanical energy into electrical energy? (a) Motor (b) Generator (c) Transformer (d) Battery

Q7. A pencil dipped in water appears bent due to: (a) Reflection (b) Dispersion (c) Refraction (d) Diffraction

Q8. Which lens is used to correct myopia (short-sightedness)? (a) Convex lens (b) Concave lens (c) Cylindrical lens (d) Bifocal lens only

Q9. Sound travels fastest through: (a) Vacuum (b) Gases (c) Liquids (d) Solids

Q10. The human ear can normally hear sound frequencies in the range: (a) 2 Hz to 2,000 Hz (b) 20 Hz to 20,000 Hz (c) 200 Hz to 200,000 Hz (d) 2,000 Hz to 40,000 Hz

Q11. Who discovered the atomic nucleus through the gold foil experiment? (a) J.J. Thomson (b) Niels Bohr (c) Ernest Rutherford (d) James Chadwick

Q12. Atoms of the same element with different numbers of neutrons are called: (a) Ions (b) Isotopes (c) Isobars (d) Isomers

Q13. Among alpha, beta, and gamma radiation, which has the greatest penetrating power? (a) Alpha particles (b) Beta particles (c) Gamma rays (d) All are equal

Q14. A ceiling fan cools a person primarily by: (a) Lowering the room's air temperature (b) Increasing air pressure (c) Speeding up evaporation of sweat (d) Absorbing body heat directly

Q15. According to Ohm's Law, if resistance increases while voltage stays constant, current will: (a) Increase (b) Decrease (c) Remain unchanged (d) Become zero always

Answer Key

Q	Answer	Reason
1	(c) Newton	Force = mass × acceleration; unit is the newton, named after Isaac Newton.
2	(c) About one-sixth	Moon's gravity is weaker, so weight (a force) reduces even though mass stays the same.
3	(c) Newton's First Law	It describes a body's tendency to resist change in motion, called inertia.
4	(b) Force applied but no displacement	Work in physics requires actual displacement in the direction of force, not just effort.
5	(b) 746 watts	This exact conversion figure is a frequently tested numerical fact.
6	(b) Generator	A generator converts mechanical energy (turbine motion) into electrical energy; a motor does the reverse.
7	(c) Refraction	Light bends when passing from air to water because its speed changes between media.
8	(b) Concave lens	Concave lenses diverge light rays, correcting the eye's tendency to focus images before the retina in myopia.
9	(d) Solids	Tightly packed particles in solids transmit vibrations fastest; sound cannot travel through vacuum at all.
10	(b) 20 Hz to 20,000 Hz	This is the standard tested range for normal human hearing.
11	(c) Ernest Rutherford	His 1911 gold foil experiment proved atoms have a small, dense, positively charged nucleus.
12	(b) Isotopes	Same atomic number (protons) but different mass number (due to differing neutrons) defines isotopes.
13	(c) Gamma rays	Gamma rays carry no charge and the highest energy, requiring thick lead or concrete to stop.
14	(c) Speeding up evaporation of sweat	The fan does not change air temperature; it enhances heat loss through faster evaporation.
15	(b) Decrease	From V = IR, if voltage is constant and resistance rises, current must fall proportionally.