NEET Physics — Units, Dimensions and Measurement

System of Units – Complete Notes, Revision, Important Questions & Downloads

System of Units covers the four standardised measurement frameworks tested in NEET Physics: the CGS System (centimetre-gram-second, also called the Gaussian system), the MKS System (metre-kilogram-second, also called the Giorgi system), the FPS System (foot-pound-second), and the SI System — the internationally accepted extension of MKS with seven base quantities (length, mass, time, electric current, temperature, amount of substance, luminous intensity) plus two supplementary units (radian and steradian). NEET frequently tests unit conversions between these systems — for instance, converting 1 joule (SI) to ergs (CGS) requires applying 1 J = 1 kg·m²/s² = 10³ g × (10² cm)² / s² = 10⁷ erg. Questions also probe knowledge of which quantities are fundamental versus derived within each system, and whether the student can identify the seven SI base units and their symbols from a table.

⬇ Download Notes PDF View Important Questions →

6 SubtopicsFoundation TheoryNCERT Ch.1

Expected QuestionsQ

0–1

NEET rarely asks a standalone 'name the system' question, but unit-conversion MCQs and dimensional-analysis numericals require knowledge of CGS, MKS, and SI base units.

Time Required⏱

1–2 hours

One focused session to memorise the four systems, their base units, and the seven SI quantities — then drill 10 conversion problems.

Difficulty⚡

Easy

Pure recall and straightforward power-of-ten conversions. No derivation or complex calculation required.

NRI USA Curriculum GapUS

Low–Medium

US Physics courses use SI exclusively and rarely mention CGS or FPS. NRI students may not recognise erg, dyne, or poundal as unit names, and may be unfamiliar with the Gaussian naming convention.

6Subtopics

8+Practice Questions

4Free Downloads

1–2 hrsPrep Time

⬇ Get Free Downloads

NEET Weightage — System of Units

Units, Dimensions and Measurement (Chapter 1)

NEET Year	Questions from this Topic	Bar	Marks
2024	0	0 Q	0
2023	1	1 Q	4
2022	0	0 Q	0
2021	1	1 Q	4
2020	0	0 Q	0
2019	1	1 Q	4
6-Year Total (2019–2024)	2–3		8–12

CGS-to-SI conversion factors (1 dyne = 10⁻⁵ N, 1 erg = 10⁻⁷ J, 1 gauss = 10⁻⁴ T) are tested in unit-conversion MCQs — memorise the powers of 10 for force, energy, and magnetic field.
The SI redefinition of 2019 fixed the kilogram to the Planck constant and the mole to an exact Avogadro number — NEET may test awareness that the platinum-iridium prototype is no longer the standard.
Supplementary units (radian and steradian) are dimensionless — NEET occasionally asks whether plane angle has a dimension or not.

📊

~0.5

Avg Questions / Year

🎯

8–12

Total Marks (6 yrs)

📈

Indirect

Pattern

⚠️

Easy

Difficulty

Exam Strategy for System of Units in NEET Physics

Memorise the seven SI base quantities with exact unit names and symbols Build a mnemonic for the seven base quantities: Length (m), Mass (kg), Time (s), Electric Current (A), Temperature (K), Amount of Substance (mol), Luminous Intensity (cd). The trap: confusing the symbol for kelvin ('K', uppercase) with that for kilo ('k', lowercase), or writing 'Kg' instead of 'kg'.

Know the three-quantity basis of each classical system CGS uses cm-g-s, MKS uses m-kg-s, FPS uses ft-lb-s. When a conversion question appears, always express the derived unit in terms of these three base units first, then substitute powers of 10. The trap: forgetting that CGS force unit 'dyne' = g·cm/s², not g·m/s².

Drill power-of-ten conversion chains For any derived unit, write it as M^a L^b T^c, then substitute the CGS/SI base-unit ratios: 1 kg = 10³ g, 1 m = 10² cm, 1 s = 1 s. The answer is 10^(3a+2b). The trap: reversing the exponent sign when converting from SI to CGS versus CGS to SI.

Distinguish fundamental from derived within each system In NEET, a common MCQ asks: 'Which of the following is NOT a fundamental quantity in the SI system?' or 'How many fundamental units does the SI system have?' Recognise that force, pressure, and energy are derived in all systems. The trap: thinking ampere is derived because it involves charge per time — ampere is actually a defined base unit in SI.

Download Study Notes — System of Units

PDF · Cheat Sheet · MCQ Set · PYQ

📘

System of Units — Full Notes

Complete notes covering all four systems (CGS, MKS, FPS, SI), the seven SI base quantities and their 2019-redefined standards, supplementary units, and worked conversion examples.

6 subtopicsConversion tablesSI redefinition notes

Download PDF

📗

System of Units — Formula Sheet

One-page reference: CGS-SI conversion factors for force, energy, pressure, and magnetic field — key formulas, conditions, and one worked example per subtopic.

1 pageAll conversion factors

Download PDF

📙

System of Units — MCQ Practice

12 NEET-style MCQs testing identification of fundamental vs derived units, CGS-to-SI conversions, and recognition of SI base-unit symbols across the four systems.

12 MCQsDetailed solutions

Download PDF

📕

System of Units — NEET-Style PYQ Practice

Collection of NEET-style practice questions on unit conversion between CGS and SI, identifying base quantities, and applying SI prefix magnitudes.

NEET-styleAnswer key included

Download PDF

Subtopics in System of Units

2-Column Table

Column A	Column B
CGS System↗	MKS System↗
FPS System↗	SI System↗
System of units↗	S.I. system↗

Rapid Revision — System of Units

Concept → Trap → Example

1) CGS System

Gaussian System Basics

CGS (Gaussian) system uses centimetre (cm) for length, gram (g) for mass, and second (s) for time as fundamental units. Derived CGS units: dyne = g·cm/s² (force), erg = g·cm²/s² (energy).

The CGS system is still used in electromagnetism literature — NEET may reference gauss (magnetic field) or maxwell (magnetic flux) as CGS units.
Force in CGS is dyne: 1 dyne = 1 g·cm/s² = 10⁻⁵ N. Energy in CGS is erg: 1 erg = 1 g·cm²/s² = 10⁻⁷ J.
NEET trap: writing 'CGS unit of force is newton' — newton belongs to SI/MKS, not CGS.

Example (NEET-style)Convert 1 N to dyne: 1 N = 1 kg·m/s² = 10³ g × 10² cm/s² = 10⁵ g·cm/s² = 10⁵ dyne.

2) MKS System

Giorgi System Basics

MKS (Giorgi) system uses metre (m) for length, kilogram (kg) for mass, and second (s) for time as fundamental units. It is the mechanical subset of the SI system.

MKS derived units (newton, joule, watt, pascal) are identical to SI mechanical units — the systems differ only in that SI adds four more base quantities.
When converting MKS to CGS, multiply by the appropriate powers: 1 m = 100 cm, 1 kg = 1000 g, 1 s = 1 s.
NEET trap: assuming MKS and SI are identical — MKS has only three base quantities while SI has seven.

Example (NEET-style)Convert 1 J (MKS) to erg (CGS): 1 J = 1 kg·m²/s² = 10³ g × (10² cm)²/s² = 10³ × 10⁴ g·cm²/s² = 10⁷ erg.

3) FPS System

British Imperial Units

FPS (British) system uses foot (ft) for length, pound (lb) for mass, and second (s) for time. Force is derived as poundal = lb·ft/s².

NEET rarely asks FPS conversion numericals, but definition-based MCQs may ask students to identify the FPS base units or name the FPS force unit.
1 foot = 0.3048 m, 1 pound = 0.4536 kg — these conversion factors are not typically memorised for NEET but may appear in assertion-reason questions.
NEET trap: confusing pound-mass (lb, a unit of mass) with pound-force (lbf, a unit of force) — in the FPS system, the derived force unit is the poundal, not the pound.

Example (NEET-style)In FPS: force = mass × acceleration = 1 lb × 1 ft/s² = 1 poundal. Converting: 1 poundal = 0.4536 kg × 0.3048 m/s² ≈ 0.1383 N.

4) SI System

International Standard — 7 Base Units

SI (International System) is a modification of MKS with seven fundamental quantities: length (m), mass (kg), time (s), electric current (A), temperature (K), amount of substance (mol), luminous intensity (cd), plus two supplementary units: radian (rad) and steradian (sr).

After the 2019 SI redefinition, all seven base units are defined through fixed numerical values of fundamental constants — the kilogram is now defined via the Planck constant, not the platinum-iridium prototype.
The supplementary units (radian and steradian) are dimensionless ratios — radian = arc length / radius, steradian = intercepted area / radius².
NEET trap: listing 'radian' as a fundamental SI unit — it is a supplementary unit, not one of the seven base units.

Example (NEET-style)SI defines the metre as the distance light travels in vacuum in 1/299,792,458 of a second. The candela is defined through the luminous efficacy of 540 × 10¹² Hz radiation fixed at 683 lm/W.

US Curriculum Gaps — System of Units for NEET Physics

NRI students from US high schools may find these specific gaps when preparing for NEET Physics.

CGS and FPS System Familiarity (not covered in AP Physics 1 / AP Physics C)

US AP Physics courses work exclusively in SI units. Students are never required to name the CGS system, identify the dyne or erg, or convert between CGS and SI using dimensional exponent substitution. NEET expects students to recognise unit names like dyne, erg, and poise and perform power-of-ten conversions.

AP Physics 1 and AP Physics C use only SI/metric units throughout all problem sets and exams.
NEET MCQs may ask 'The CGS unit of viscosity is poise — express 1 poise in SI units' — a question format absent from AP exams.
Practice identifying CGS equivalents for every SI derived unit you encounter: N→dyne, J→erg, Pa→barye, T→gauss.

Seven SI Base Quantities as a Testable List (implicit in US courses, explicit in NEET)

In US courses, SI base units are used but rarely listed as a memorisation exercise. NEET directly tests whether students can enumerate all seven base quantities, identify their exact symbols (lowercase 'kg' not 'Kg'), and distinguish them from derived quantities like force or pressure.

US College Board exams do not test 'How many fundamental quantities does SI have?' as a standalone MCQ.
NEET may ask 'Which of the following is NOT a base SI unit?' with options mixing base and derived units.
Memorise the seven quantities with their exact symbols: m, kg, s, A, K, mol, cd — note that ampere and kelvin are capitalised as symbols.

NEET-Style Practice Questions — System of Units

4 NEET-style practice questions

1The CGS unit of energy is the erg. How many ergs are equivalent to 1 joule?NEET-style practice

10⁵ erg

10⁷ erg

10⁹ erg

10³ erg

1 joule = 1 kg·m²/s². Converting each base unit to CGS: 1 kg = 10³ g, 1 m = 10² cm. Therefore 1 J = 10³ g × (10² cm)² / s² = 10³ × 10⁴ g·cm²/s² = 10⁷ g·cm²/s² = 10⁷ erg. Option (a) confuses the force conversion (1 N = 10⁵ dyne) with the energy conversion. Option (c) adds an extra factor of 10². Option (d) uses only the mass conversion factor without the length-squared factor.

2The SI system has seven fundamental (base) quantities. Which of the following is NOT a fundamental quantity in the SI system?NEET-style practice

Luminous intensity

Electric current

Force

Amount of substance

The seven SI base quantities are: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Force is a derived quantity defined as mass × acceleration (unit: newton = kg·m/s²). Option (a) luminous intensity (candela) is the seventh base quantity — students often forget this. Option (b) electric current (ampere) is a defined base quantity, not derived from charge/time. Option (d) amount of substance (mole) is the sixth base quantity. The trap is that students may assume current is derived because I = Q/t, but in SI, ampere is defined independently.

3A physical quantity P has dimensions M¹ L⁻¹ T⁻². Its SI unit is Pa (pascal). What is the CGS unit and its conversion factor?NEET-style practice

1 Pa = 10 barye

1 Pa = 10⁻¹ barye

1 Pa = 1 barye

1 Pa = 10² barye

The dimensions of pressure are M¹ L⁻¹ T⁻². Converting: 1 Pa = 1 kg/(m·s²). Substituting CGS units: 1 kg = 10³ g, 1 m = 10² cm. So 1 Pa = 10³ g / (10² cm · s²) = 10³/10² g/(cm·s²) = 10¹ g/(cm·s²) = 10 barye (where 1 barye = 1 g·cm⁻¹·s⁻²). Option (a) is correct. Option (b) inverts the ratio. Option (c) ignores the conversion factors entirely. Option (d) uses only the length conversion without the mass factor. The key is to substitute M^a × L^b × T^c with the correct powers: 10^(3×1) × 10^(2×(−1)) × 10^0 = 10^(3−2) = 10¹.

4Which system of units uses foot, pound, and second as fundamental units for length, mass, and time respectively?NEET-style practice

CGS system

MKS system

FPS system

SI system

The FPS (foot-pound-second) system uses foot for length, pound for mass, and second for time. Option (a) CGS uses centimetre, gram, second. Option (b) MKS uses metre, kilogram, second. Option (d) SI extends MKS to seven base quantities. The FPS system is also called the British system. Although rarely used in modern scientific work, NEET expects students to identify which base units belong to which system. The derived force unit in FPS is the poundal (= 1 lb·ft/s²), not the newton.

Practice Problems — System of Units

Click "Reveal Answer" after attempting

1The surface tension of water is 0.072 N/m. Express this in CGS units (dyne/cm).

72 dyne/cm

7.2 dyne/cm

0.72 dyne/cm

720 dyne/cm

👁 Reveal Answer

Option (a): 72 dyne/cm. Surface tension has dimensions M T⁻². Convert: 1 N/m = 1 kg/s². In CGS: 1 kg = 10³ g, so 1 N/m = 10³ g/s² = 10³ dyne/cm. Alternatively: 1 N = 10⁵ dyne, 1 m = 10² cm, so 1 N/m = 10⁵/10² dyne/cm = 10³ dyne/cm. Therefore 0.072 N/m = 0.072 × 10³ dyne/cm = 72 dyne/cm.

2If the unit of force is 100 N, the unit of length is 10 m, and the unit of time is 100 s, what is the unit of mass in this new system?

10⁵ kg

10⁶ kg

10⁴ kg

10³ kg

👁 Reveal Answer

Option (a): 10⁵ kg. Force = mass × acceleration = mass × length/time². So mass = force × time²/length = 100 N × (100 s)²/(10 m) = 100 × 10⁴/10 kg = 10⁵ kg. The unit of mass in this new system corresponds to 10⁵ kg in SI.

3The density of mercury is 13,600 kg/m³. Convert this to g/cm³.

13.6 g/cm³

136 g/cm³

1.36 g/cm³

1360 g/cm³

👁 Reveal Answer

Option (a): 13.6 g/cm³. Density dimensions: M L⁻³. Convert: 1 kg/m³ = 10³ g/(10² cm)³ = 10³ g/10⁶ cm³ = 10⁻³ g/cm³. Therefore 13,600 kg/m³ = 13,600 × 10⁻³ g/cm³ = 13.6 g/cm³. This is a standard conversion: divide kg/m³ by 1000 to get g/cm³.

4In the CGS system, the unit of viscosity is poise ≡ g/(cm·s). Express 1 poise in SI units (Pa·s = kg/(m·s)).

0.1 Pa·s

10 Pa·s

0.01 Pa·s

1 Pa·s

👁 Reveal Answer

Option (a): 0.1 Pa·s. Viscosity dimensions: M L⁻¹ T⁻¹. 1 poise = 1 g/(cm·s). Convert to SI: 1 g = 10⁻³ kg, 1 cm = 10⁻² m. So 1 poise = 10⁻³ kg/(10⁻² m · s) = 10⁻³/10⁻² kg/(m·s) = 10⁻¹ kg/(m·s) = 0.1 Pa·s. Viscosity of water at 20°C is about 1 centipoise = 0.01 poise = 0.001 Pa·s.

Physics — System of Units Revision Checklist

Check off chapters as you revise

Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.

CGS System: Identify CGS base units and convert dyne to newtonWrite the three CGS base units (cm, g, s), then derive the CGS force unit: 1 dyne = 1 g·cm/s² = 10⁻⁵ N. Verify by substituting 1 g = 10⁻³ kg and 1 cm = 10⁻² m.

MKS System: State how MKS relates to SI and list three base unitsWrite that MKS uses m, kg, s as base units and is the mechanical subset of SI. Confirm that newton, joule, and watt are MKS derived units identical to their SI counterparts.

FPS System: Name the three FPS base units and the derived force unitWrite foot (length), pound (mass), second (time). Derive the force unit: 1 poundal = 1 lb·ft/s². Convert 1 poundal to SI: 0.4536 × 0.3048 ≈ 0.1383 N.

SI System: List all seven base quantities with their units and symbolsWrite from memory: 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). Verify capitalisation of symbols.

SI System: State the two supplementary units and explain why they are dimensionlessWrite: radian (plane angle = arc length / radius) and steradian (solid angle = intercepted area / radius²). Both are ratios of like quantities (length/length or area/area), making them dimensionless.

Frequently Asked Questions — System of Units

Notes · Downloads · Revision · Important Questions

What is a system of units?

A system of units is a complete set of units — both fundamental and derived — for measuring all physical quantities. The textbook defines it as 'a complete set of units, both fundamental and derived for all kinds of physical quantities.' Each system chooses a small number of base quantities (typically three in mechanics) and derives all other units from them.

Why does the SI system have seven base units instead of three?

Three base units (length, mass, time) suffice for mechanics alone. However, physics extends beyond mechanics into electromagnetism, thermodynamics, photometry, and chemistry. SI adds electric current (ampere), temperature (kelvin), amount of substance (mole), and luminous intensity (candela) to cover these domains without ambiguity. Each added base unit resolves a measurement dimension that cannot be expressed purely in terms of length, mass, and time.

What is the difference between MKS and SI?

MKS (metre-kilogram-second) uses only three base quantities — length, mass, and time. SI is a modification and extension of MKS that includes four additional base quantities: electric current, temperature, amount of substance, and luminous intensity. All MKS units (newton, joule, watt) are unchanged in SI, but SI adds ampere, kelvin, mole, and candela as independently defined base units.

Are radian and steradian base units or supplementary units?

In the original SI definition, radian and steradian were classified as supplementary units — not base units, not derived units. After the 1995 CGPM resolution, they were reclassified as dimensionless derived units. For NEET, remember that the SI system has seven base units plus two supplementary units (radian for plane angle, steradian for solid angle), and both are dimensionless ratios.

How do I convert a derived unit from SI to CGS?

Write the derived unit in terms of base dimensions M^a L^b T^c. Then substitute the CGS-to-SI ratios: 1 kg = 10³ g, 1 m = 10² cm, 1 s = 1 s. The conversion factor becomes 10^(3a+2b). For example, energy (M¹L²T⁻²): 1 J = 10^(3×1+2×2) erg = 10^7 erg. For pressure (M¹L⁻¹T⁻²): 1 Pa = 10^(3×1+2×(−1)) barye = 10^1 barye.

Why is the FPS system still mentioned in NEET textbooks?

The FPS system is included for completeness and comparison. NEET expects students to know that different countries historically used different systems. While FPS is obsolete in scientific work, NEET MCQs may ask students to identify which system uses foot, pound, and second, or to distinguish the poundal from the newton. It also reinforces the concept that the choice of base units is arbitrary.

Can any three quantities serve as fundamental quantities in mechanics?

Yes. The textbook explicitly states that the choice of fundamental quantities is not unique — any three independent mechanical quantities can serve as the basis. For example, if speed and time are chosen as fundamental, length becomes derived (length = speed × time). If force and acceleration are fundamental, mass becomes derived (mass = force/acceleration). The standard choice of length, mass, and time is merely a convention.

What changed in the 2019 SI redefinition?

The 26th General Conference on Weights and Measures (2018, effective May 2019) redefined four SI base units by fixing exact numerical values of fundamental constants: the kilogram via the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s), the ampere via the elementary charge (e = 1.602176634 × 10⁻¹⁹ C), the kelvin via the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K), and the mole via the Avogadro constant (Nₐ = 6.02214076 × 10²³ mol⁻¹). The metre and second were already defined via fundamental constants. The platinum-iridium kilogram prototype is no longer the standard.

What are practical units and how do they relate to SI?

Practical units are non-SI units retained for convenience in specific contexts: light year and astronomical unit (distance), electron-volt (energy), atmosphere and bar (pressure), horse power (power). They may or may not belong to a formal system but can always be converted to SI. For example, 1 light year ≈ 9.461 × 10¹⁵ m, 1 eV = 1.6 × 10⁻¹⁹ J. NEET may ask for the SI equivalent of a practical unit.

For NRI / OCI / U.S.-Based Families

NEET NRI Counseling & Admission eBook Download

A practical guide covering sponsor rules, document checklist, verification traps, NRI quota reality, and step-by-step counselling flow. Designed to prevent last-minute rejections and wrong choice filling.

Sponsor + Proof ClarityDocuments ChecklistState-wise Traps

↓ Download eBook (PDF)→ See What's Inside

Tip: Keep this eBook open during verification + choice filling week for quick cross-checking.

NEET Prep (India + NRI-USA)

Schedule Trial Session For NEET Prep

Get a short diagnostic + study roadmap: syllabus gaps (NCERT vs U.S. curriculum), weak chapters, and the exact weekly plan needed to improve accuracy under time.

Gap MappingWeekly PlanAccuracy Fix

→ Book Trial Session → WhatsApp Us

Best for: Students in Grade 10–12 (U.S. / India) who want a clear NEET timeline and daily practice structure.

System of Units

System of Units – Complete Notes, Revision, Important Questions & Downloads

1) CGS System

2) MKS System

3) FPS System

4) SI System

CGS and FPS System Familiarity (not covered in AP Physics 1 / AP Physics C)

Seven SI Base Quantities as a Testable List (implicit in US courses, explicit in NEET)

Physics — System of Units Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

CGS System

MKS System

FPS System

SI System

System of units

S.I. system

Subtopics

CGS System

MKS System

FPS System

SI System

System of units

S.I. system

NEET > Physics > Physical World and Measurement Chapters

Comments

Leave a comment

Comments (0)

System of Units

Overview content

System of Units – Complete Notes, Revision, Important Questions & Downloads

1) CGS System

2) MKS System

3) FPS System

4) SI System

CGS and FPS System Familiarity (not covered in AP Physics 1 / AP Physics C)

Seven SI Base Quantities as a Testable List (implicit in US courses, explicit in NEET)

Physics — System of Units Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

CGS System

MKS System

FPS System

SI System

System of units

S.I. system

Subtopics

CGS System

MKS System

FPS System

SI System

System of units

S.I. system

NEET > Physics > Physical World and Measurement Chapters

Comments

Leave a comment

Comments (0)