Physics Compendium — Units, Formulas & Exam Traps

Why This Chapter Matters

Open any SSC CGL, CHSL, MTS or RRB NTPC paper from the last five years and you will find at least 2-4 questions that are pure recall: "SI unit of pressure?", "Which pair is correctly matched?", "The value of the universal gas constant is?" These are the easiest marks in the entire General Science section, worth more per second of effort than any numerical problem, and yet thousands of aspirants drop them every year for one reason: they never sat down with the units, the confusing pairs, and the constants in one place until exam eve, when it was too late to actually absorb them.

This chapter is that one place. You have already met most of these units and constants scattered across Chapters 2 to 13. Here they are pulled together, cross-checked against each other, and organised so you can revise the entire "fact layer" of physics in one sitting the night before your exam. The single biggest mistake aspirants make with this material is treating it as a list to memorise once and forget. It is not. It is a revision tool meant to be read three or four times over your prep, each time a little faster, until the answers arrive before you finish reading the question. Pay special attention to the "confused pairs" section below — examiners love these precisely because two out of three students mix them up.

1. Master SI Units Table

Every physical quantity has one SI unit. Examiners frequently swap in a wrong unit as a distractor (writing "Watt" for power is correct, but "Watt-hour" for power is wrong because that is a unit of energy, not power). Read this table quantity by quantity, not unit by unit, and say each unit out loud once.

Quantity	SI Unit	Symbol	Type
Length	metre	m	Base
Mass	kilogram	kg	Base
Time	second	s	Base
Electric current	ampere	A	Base
Temperature	kelvin	K	Base
Amount of substance	mole	mol	Base
Luminous intensity	candela	cd	Base
Area	square metre	m²	Derived
Volume	cubic metre	m³	Derived
Density	kilogram per cubic metre	kg/m³	Derived
Speed / Velocity	metre per second	m/s	Derived
Acceleration	metre per second squared	m/s²	Derived
Force	newton	N	Derived
Pressure / Stress	pascal	Pa	Derived
Work / Energy / Heat	joule	J	Derived
Power	watt	W	Derived
Momentum	kilogram metre per second	kg·m/s	Derived
Frequency	hertz	Hz	Derived
Electric charge	coulomb	C	Derived
Electric potential (voltage)	volt	V	Derived
Electrical resistance	ohm	Ω	Derived
Electrical capacitance	farad	F	Derived
Magnetic flux	weber	Wb	Derived
Magnetic flux density	tesla	T	Derived
Inductance	henry	H	Derived
Specific heat capacity	joule per kilogram kelvin	J/(kg·K)	Derived
Thermal conductivity	watt per metre kelvin	W/(m·K)	Derived
Surface tension	newton per metre	N/m	Derived
Viscosity	pascal second	Pa·s	Derived
Radioactivity (activity)	becquerel	Bq	Derived
Absorbed radiation dose	gray	Gy	Derived
Luminous flux	lumen	lm	Derived
Illuminance	lux	lx	Derived
Plane angle	radian	rad	Supplementary
Solid angle	steradian	sr	Supplementary

Exam trap: Pressure and stress share the SI unit pascal, but weight (a force) is measured in newton, not kilogram. A common wrong option writes "weight in kg" — weight is a force, so its correct unit is newton. Mass is measured in kilogram; weight never is.

Memory hook: For the seven base units, remember the phrase "My Kid Sat Alone, Making Cool Ants" — Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela. Say it in that order and all seven base quantities line up automatically.

Think of base units as the seven raw ingredients in a kitchen (flour, water, salt, and so on) and derived units as the dishes you cook from them. Speed is just length divided by time, the way a chutney is just two raw ingredients ground together. Once you see force (newton) as mass times acceleration, and pressure (pascal) as force divided by area, you stop memorising thirty separate units and start seeing them as combinations of the same seven building blocks.

2. SI Prefixes — Quick Refresher

You met these in Chapter 2, but they resurface constantly in questions about wavelength, frequency, and electrical units, so a fast second pass earns marks cheaply.

Prefix	Symbol	Factor	Everyday reference
tera	T	10¹²	data storage (terabyte)
giga	G	10⁹	mobile network (4G/5G data speeds)
mega	M	10⁶	power plant capacity (megawatt)
kilo	k	10³	your body weight (kilogram)
centi	c	10⁻²	your height in centimetres
milli	m	10⁻³	a pinch of salt (milligram)
micro	μ	10⁻⁶	a bacterium's size (micrometre)
nano	n	10⁻⁹	a virus's size (nanometre)
pico	p	10⁻¹²	atomic-scale measurements

Exam trap: Do not confuse milli (10⁻³) with micro (10⁻⁶). A question asking "1000 micrometres equal how many millimetres" trips up students who assume milli is always the smaller one — here 1000 micrometres actually equal 1 millimetre, because milli is the larger unit of the two.

3. Commonly Confused Physics Pairs

This is where SSC and RRB examiners fish for easy marks against unprepared students. Each pair below sounds similar in everyday Hindi-English usage but means something distinct in physics. Read every row twice.

Mass vs Weight. Mass is the amount of matter in a body — it never changes, whether you are on Earth or the Moon. Weight is the force of gravity pulling on that mass, so it changes with location. An astronaut has the same mass on the Moon as on Earth, but their weight there is about one-sixth, because the Moon's gravity is weaker. Mass is measured in kilogram; weight, being a force, is measured in newton.

Speed vs Velocity. Speed tells you how fast something moves, with no regard for direction — it is a scalar. Velocity tells you how fast and in which direction — it is a vector. A car doing laps on a circular track at a constant 60 km/h has constant speed but constantly changing velocity, because its direction keeps changing.

Heat vs Temperature. Heat is a form of energy in transit, measured in joules. Temperature is a measure of how hot or cold something is, an indicator of the average kinetic energy of particles, measured in kelvin or celsius. A bucket of lukewarm water can hold more total heat than a lit matchstick, even though the matchstick has a far higher temperature — heat depends on quantity of matter, temperature does not.

Distance vs Displacement. Distance is the total path length covered, always positive, a scalar. Displacement is the shortest straight-line change in position from start to finish, a vector that can even be zero. If you walk one full round of a park and return to your starting bench, your distance covered is the park's perimeter, but your displacement is zero.

Work vs Power vs Energy. Energy is the capacity to do work. Work is energy actually transferred when a force moves an object through a distance. Power is how fast that work gets done. Two workers lifting the identical brick to the identical height do the same work, but the one who does it faster has delivered more power.

Reflection vs Refraction. Reflection is light bouncing back from a surface without entering a new medium — a mirror. Refraction is light bending as it passes from one medium into another of different density — a straw appearing bent in a glass of water.

Conduction vs Convection vs Radiation. Conduction transfers heat through direct contact within a solid (a metal spoon heating up in hot tea). Convection transfers heat through the movement of a fluid, liquid or gas (water boiling in a pot, room air warmed by a heater). Radiation transfers heat through electromagnetic waves needing no medium at all (sunlight reaching Earth across empty space).

Fission vs Fusion. Fission splits a heavy nucleus into lighter ones, releasing energy — this is how nuclear power plants and atom bombs work. Fusion joins light nuclei into a heavier one, releasing far more energy — this is how the Sun and hydrogen bombs work. Fission breaks apart; fusion brings together. Fusion needs extreme temperatures, which is why it is far harder to control on Earth.

Isotopes vs Isobars vs Isotones. Isotopes share the same atomic number (same protons) but different mass numbers — carbon-12 and carbon-14 are both carbon. Isobars share the same mass number but different atomic numbers — different elements that happen to weigh the same on the mass-number scale. Isotones share the same number of neutrons across different elements.

Alpha vs Beta vs Gamma radiation. Alpha particles are the heaviest and least penetrating, stopped by a sheet of paper. Beta particles are lighter, faster, and stopped only by a few millimetres of aluminium. Gamma rays carry no mass at all, penetrate deepest, and need thick lead or concrete to block them.

Exam trap: A frequent SSC question format is "arrange in increasing order of penetrating power" for alpha, beta, gamma radiation. The correct order is alpha, then beta, then gamma — but students who memorise the order by "danger" instead of "penetration" often flip alpha and gamma, since alpha is actually the most damaging to living tissue at close range despite penetrating the least.

Concave vs Convex. A concave surface curves inward, like the inside of a bowl — a concave mirror converges light and is used in torches, headlights, and shaving mirrors. A convex surface bulges outward, like the outside of a spoon — a convex mirror diverges light and is used in vehicle rear-view mirrors and shop security mirrors because it gives a wider field of view.

Electric current vs Electric charge. Charge is the fundamental quantity, measured in coulomb, representing "how much electricity" a body carries. Current is the rate at which that charge flows, measured in ampere — coulomb per second. A fully charged battery has stored charge sitting still; current only exists once that charge starts flowing through a circuit.

4. Physical Constants Every Aspirant Should Know

These constants appear as direct one-line recall questions ("value of Planck's constant is?") and occasionally as matching questions ("constant named after which scientist?"). You do not need to memorise every decimal place — know the name, the rounded value, and what it governs.

Constant	Symbol	Approximate Value	Governs
Speed of light in vacuum	c	3 × 10⁸ m/s	Fastest possible speed in the universe; basis of relativity
Universal gravitational constant	G	6.674 × 10⁻¹¹ N·m²/kg²	Newton's law of gravitation
Planck's constant	h	6.626 × 10⁻³⁴ J·s	Energy of a photon; quantum theory
Boltzmann constant	k	1.381 × 10⁻²³ J/K	Links particle energy to temperature
Avogadro's number	Nₐ	6.022 × 10²³ per mole	Number of particles in one mole of a substance
Universal gas constant	R	8.314 J/(mol·K)	Ideal gas equation
Elementary charge	e	1.602 × 10⁻¹⁹ C	Charge on one electron (or proton, opposite sign)
Electron rest mass	mₑ	9.109 × 10⁻³¹ kg	Mass of one electron
Standard acceleration due to gravity	g	9.8 m/s²	Free-fall acceleration near Earth's surface
Standard atmospheric pressure	atm	1.013 × 10⁵ Pa	Air pressure at sea level
Absolute zero	0 K	-273.15°C	Coldest theoretically possible temperature

Exam trap: Do not confuse G (universal gravitational constant, a fixed number that never changes anywhere in the universe) with g (acceleration due to gravity, which changes with altitude, depth, and which planet you stand on). SSC questions test exactly this capital-versus-small-letter distinction, sometimes in the same sentence.

Think of G as a universal rulebook printed once and never edited, while g is the local weather report — it depends on where you are standing. That is why g is roughly 9.8 m/s² on Earth's surface, about 1.6 m/s² on the Moon, and zero for an astronaut in free orbital fall, while G stays exactly the same number everywhere, always.

Memory hook: For the four most-asked constants in exams, remember "Please Give Bright Answers" — Planck's constant, Gravitational constant, Boltzmann constant, Avogadro's number. If a question names one of these four, you now already know which symbol and rough magnitude to expect.

5. Scientist-to-Constant Quick Cross-Reference

You covered scientist-discovery pairs in detail in Chapter 12; here is the narrower slice relevant to constants and named units, useful for last-mile revision.

Named after	Constant / Unit	Field
Isaac Newton	newton (force), gravitational constant G	Mechanics, gravitation
Max Planck	Planck's constant h	Quantum physics
Ludwig Boltzmann	Boltzmann constant k	Thermodynamics, statistical mechanics
Amedeo Avogadro	Avogadro's number	Chemistry-physics interface, mole concept
James Prescott Joule	joule (energy)	Work and energy
James Watt	watt (power)	Power
Blaise Pascal	pascal (pressure)	Fluid mechanics
Andre-Marie Ampere	ampere (current)	Electricity
Alessandro Volta	volt (potential)	Electricity
Georg Simon Ohm	ohm (resistance)	Electricity
Michael Faraday	farad (capacitance)	Electromagnetism
Heinrich Hertz	hertz (frequency)	Waves
Nikola Tesla	tesla (magnetic flux density)	Magnetism
Wilhelm Weber	weber (magnetic flux)	Magnetism
Anders Celsius	celsius scale	Thermometry
William Thomson (Lord Kelvin)	kelvin scale	Thermometry

Exam trap: The SI temperature unit is named "kelvin", not "Kelvin's scale" and not "degree kelvin" — modern usage drops the degree symbol for kelvin (write 300 K, not 300°K), unlike celsius or fahrenheit which do keep the degree symbol. Question setters occasionally test this exact formatting rule.

6. Formula Compendium — Simplest Form Only

You do not need to derive these for SSC/RRB, only recognise them and know which symbol stands for what. Every formula here is in its simplest plain-text form.

Quantity	Formula	Notes
Speed	distance / time	Scalar
Velocity	displacement / time	Vector
Acceleration	change in velocity / time	Rate of change of velocity
Force	mass × acceleration	Newton's second law
Momentum	mass × velocity	Conserved in a closed system
Work	force × distance moved in direction of force	Zero if no displacement
Kinetic energy	half × mass × velocity squared	Energy of motion
Potential energy	mass × g × height	Energy of position
Power	work / time	Rate of doing work
Pressure	force / area	Same unit as stress
Density	mass / volume	Determines floating or sinking
Ohm's law	voltage = current × resistance	Basis of all circuit problems
Wave equation	speed = frequency × wavelength	Applies to sound and light alike
Ideal gas equation	pressure × volume = number of moles × R × temperature	Links four gas properties

Exam trap: In the wave equation, students often forget that "speed" here refers to the wave's own propagation speed through the medium (such as the fixed 340 m/s for sound in air), not the speed of any object producing the wave. Frequency and wavelength are inversely related for a wave of fixed speed — raise one, the other must fall.

7. Instruments and What They Measure

A last table pulling together every measuring instrument scattered across earlier chapters, because "instrument to quantity" matching is one of the most repeated SSC question formats.

Instrument	Measures	Chapter reference
Vernier caliper	Small lengths, internal/external diameter	Ch 2
Screw gauge	Very small lengths, wire thickness	Ch 2
Thermometer	Temperature	Ch 6
Barometer	Atmospheric pressure	Ch 6
Manometer	Gas pressure	Ch 6
Ammeter	Electric current	Ch 9
Voltmeter	Electric potential difference	Ch 9
Galvanometer	Small electric current	Ch 9
Ohmmeter	Electrical resistance	Ch 9
Hydrometer	Density/specific gravity of liquids	Ch 4
Anemometer	Wind speed	—
Odometer	Distance travelled by a vehicle	—
Tachometer	Rotational speed (RPM) of an engine	—
Seismograph	Intensity of earthquakes	—
Sonometer	Frequency of a vibrating string	Ch 8
Sextant	Angular distance between two objects, used in navigation	—

You have now seen the physics of an entire year's syllabus reduced to seven tables. Treat this chapter the way a cricket team treats its final net practice before a match: not the place to learn a new stroke, but the place to make every stroke you already know completely automatic.

Quick Revision — One-Line Facts

1. There are exactly 7 SI base units: metre, kilogram, second, ampere, kelvin, mole, candela.
2. Newton (N) is the SI unit of force; pascal (Pa) is the SI unit of pressure.
3. Weight is measured in newton, never in kilogram — kilogram measures mass only.
4. Speed is a scalar; velocity is a vector with the same units, m/s.
5. Heat is energy in transit (joule); temperature is a measure of hotness (kelvin/celsius).
6. Distance is always positive; displacement can be zero or negative.
7. Power equals work divided by time, measured in watt.
8. Speed of light in vacuum is approximately 3 × 10⁸ m/s, the fastest speed possible.
9. G, the universal gravitational constant, is fixed everywhere; g, acceleration due to gravity, varies by location.
10. Standard value of g near Earth's surface is 9.8 m/s².
11. Absolute zero is -273.15°C, or 0 kelvin, the coldest temperature theoretically possible.
12. Avogadro's number, about 6.022 × 10²³, gives the number of particles in one mole.
13. Planck's constant governs the energy of a single photon in quantum physics.
14. 1 kilo = 10³; 1 milli = 10⁻³; 1 micro = 10⁻⁶; 1 nano = 10⁻⁹.
15. Conduction needs a solid medium; convection needs a moving fluid; radiation needs no medium at all.
16. Fission splits a heavy nucleus; fusion joins light nuclei; fusion releases more energy per reaction.
17. Isotopes of an element share the same number of protons but different numbers of neutrons.
18. Alpha particles are the least penetrating form of radiation; gamma rays are the most penetrating.
19. A concave mirror converges light rays; a convex mirror diverges them.
20. Reflection bounces light off a surface; refraction bends light passing between media.
21. Ohm's law states voltage equals current multiplied by resistance.
22. The wave equation states speed equals frequency multiplied by wavelength.
23. Electric charge is measured in coulomb; electric current is the rate of flow of charge, measured in ampere.
24. Kinetic energy depends on both mass and the square of velocity.
25. Density equals mass divided by volume, and determines whether an object floats or sinks.
26. A vernier caliper and a screw gauge both measure small lengths, but a screw gauge is more precise.
27. A barometer measures atmospheric pressure; a manometer measures gas pressure in a closed system.
28. Kelvin is written without a degree symbol; celsius and fahrenheit keep the degree symbol.
29. The SI unit tesla is named after Nikola Tesla and measures magnetic flux density.
30. Standard atmospheric pressure at sea level is approximately 1.013 × 10⁵ pascal.

Memory Tables

Table A — Confused Pairs at a Glance

Pair	First term	Second term	Key difference
Mass vs Weight	Amount of matter, kg	Force of gravity on mass, N	Mass is constant; weight changes with location
Speed vs Velocity	Scalar, magnitude only	Vector, magnitude and direction	Velocity changes if direction changes even at constant speed
Heat vs Temperature	Energy in transit, joule	Degree of hotness, kelvin	A large cool body can hold more heat than a small hot one
Distance vs Displacement	Total path length	Shortest start-to-end change	Displacement can be zero when distance is not
Fission vs Fusion	Splitting heavy nucleus	Joining light nuclei	Fusion releases more energy but needs extreme heat
Conduction vs Convection vs Radiation	Solid, direct contact	Fluid movement	Radiation alone needs no medium
Concave vs Convex mirror	Curves inward, converges light	Curves outward, diverges light	Convex gives a wider field of view

Table B — Constants, Units, and Instruments Master Sheet

Category	Item	Value / Symbol	Where used
Constant	Speed of light	c, 3 × 10⁸ m/s	Relativity, optics
Constant	Gravitational constant	G, 6.674 × 10⁻¹¹ N·m²/kg²	Newton's gravitation law
Constant	Planck's constant	h, 6.626 × 10⁻³⁴ J·s	Quantum physics
Constant	Avogadro's number	Nₐ, 6.022 × 10²³/mol	Mole concept
Base unit	Length	metre (m)	All spatial measurement
Base unit	Mass	kilogram (kg)	All matter measurement
Derived unit	Force	newton (N)	Mechanics
Derived unit	Pressure	pascal (Pa)	Fluids, atmosphere
Derived unit	Power	watt (W)	Electrical and mechanical rating
Instrument	Vernier caliper	measures small length	Workshops, labs
Instrument	Ammeter	measures current	Circuits
Instrument	Barometer	measures atmospheric pressure	Weather stations
Instrument	Seismograph	measures earthquake intensity	Geology

Practice MCQs

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

Q2. Which of the following is a base SI unit? (a) Newton (b) Joule (c) Kelvin (d) Pascal

Q3. Weight is correctly measured in which unit? (a) Kilogram (b) Newton (c) Litre (d) Joule

Q4. Which quantity is a vector, unlike its close cousin? (a) Speed (b) Distance (c) Velocity (d) Mass

Q5. The value of standard acceleration due to gravity near Earth's surface is approximately: (a) 6.6 m/s² (b) 8.3 m/s² (c) 9.8 m/s² (d) 11.2 m/s²

Q6. Which mode of heat transfer can occur through empty space with no medium? (a) Conduction (b) Convection (c) Radiation (d) Diffusion

Q7. A convex mirror is commonly used in vehicle rear-view mirrors because it: (a) Converges light rays to a point (b) Gives a wider field of view (c) Magnifies the image greatly (d) Produces no image at all

Q8. Which nuclear process powers the Sun? (a) Fission (b) Fusion (c) Radioactive decay only (d) Chemical combustion

Q9. Which form of radioactive radiation has the least penetrating power? (a) Alpha (b) Beta (c) Gamma (d) X-rays

Q10. Ohm's law relates voltage, current, and: (a) Power (b) Frequency (c) Resistance (d) Capacitance

Q11. The SI unit "tesla" is used to measure: (a) Electric current (b) Magnetic flux density (c) Capacitance (d) Frequency

Q12. Absolute zero on the kelvin scale corresponds to which celsius value? (a) 0°C (b) -100°C (c) -273.15°C (d) -373.15°C

Q13. Which physical constant remains exactly the same value everywhere in the universe, unlike its lower-case counterpart which varies by location? (a) g (b) G (c) c (d) h

Q14. A body has stored charge that has not yet started to flow. Which quantity does this describe? (a) Current (b) Charge (c) Resistance (d) Power

Q15. In the ideal gas equation, which term represents the universal gas constant? (a) P (b) V (c) R (d) T

Answer Key

Q	Answer	One-line reason
Q1	(b) Newton	Force is mass times acceleration; its SI unit is the newton, while pascal is pressure and watt is power.
Q2	(c) Kelvin	Kelvin is one of the 7 SI base units for temperature; newton, joule, and pascal are all derived units.
Q3	(b) Newton	Weight is a force due to gravity acting on mass, so it takes the same unit as force, the newton, not kilogram.
Q4	(c) Velocity	Velocity includes direction and is a vector, unlike speed, distance, and mass, which are all scalars.
Q5	(c) 9.8 m/s²	This is the standard textbook value for g at Earth's surface, distinct from G which is a universal constant.
Q6	(c) Radiation	Radiation transfers heat via electromagnetic waves and needs no medium, which is how sunlight reaches Earth.
Q7	(b) Gives a wider field of view	A convex mirror diverges light rays, producing a smaller image but covering a larger area, ideal for spotting traffic.
Q8	(b) Fusion	The Sun fuses lighter hydrogen nuclei into heavier helium, releasing enormous energy, unlike fission which splits heavy nuclei.
Q9	(a) Alpha	Alpha particles are heaviest and slowest, stopped by a sheet of paper, making them the least penetrating of the three.
Q10	(c) Resistance	Ohm's law states voltage equals current multiplied by resistance, the foundational equation for all circuit calculations.
Q11	(b) Magnetic flux density	Tesla, named after Nikola Tesla, is the SI unit for how concentrated a magnetic field is at a given point.
Q12	(c) -273.15°C	Absolute zero is the coldest temperature theoretically possible, defined as 0 K, which converts to -273.15°C.
Q13	(b) G	G, the universal gravitational constant, never changes anywhere, while g, acceleration due to gravity, varies with altitude and planet.
Q14	(b) Charge	Charge is the stored quantity of electricity, measured in coulomb; current only exists once that charge starts flowing.
Q15	(c) R	R is the universal gas constant in the ideal gas equation, linking pressure, volume, moles, and temperature together.