Detail Notes on Units and Measurement for Competitive Exams in 2025

Detail Notes on Units and Measurement for Competitive Exams in 2025

Units and Measurements

UNIT

A unit is a standard or fixed quantity used to measure other quantities of the same kind. Units are broadly classified into two categories: fundamental units and derived units. Fundamental units are associated with fundamental quantities, which cannot be expressed in terms of any other physical quantities. Examples of such fundamental quantities include length, mass, time, and temperature. The units used to measure these quantities are known as fundamental units. On the other hand, derived units are those that are derived from the fundamental units. They are used to measure quantities like area, volume, pressure, and force, and they maintain a constant relationship with the fundamental units.

International System of Units

 In earlier times, scientists from different countries used various systems of units for measurement. Among these, the most commonly used systems were the CGS, FPS, and MKS systems. Each system had its own set of base units for measuring length, mass, and time:

• FPS System:
  • Length – Foot
  • Mass – Pound
  • Time – Second
• CGS System:
  • Length – Centimetre
  • Mass – Gram
  • Time – Second
• MKS System:
  • Length – Metre
  • Mass – Kilogram
  • Time – Second

Eventually, to unify measurement systems globally, scientists adopted the S.I. units (Systeme International d’Unités), which is now the internationally accepted system for standard measurement.

Fundament Units of FPS, CGS, MKS and SI Units

SI Derived Units

S.No             Quantity                                     SI Unit

1                   Area                                            Square metre

2                   Volume                                       Cubic metre

3                   Velocity                                       metres/second

4                   Acceleration                                metres/second square         

5                   Density                                       Kilogram/metre Cube

6                   Work                                           Joule

7                   Energy                                        Joule

8                   Force                                          Newton

9                   Weight                                        Newton

10                 Pressure                                     Pascal 

11                 Frequency                                   Hertz

12                 Power                                         Watt

13                 Impulse                                       Newton-second

14                 Angular velocity                           Radian /second

15                 Electric charge                            Coulomb

16                 Electric potential(voltage)             Volt

17                 Capacitance                                Farad

18                 Inductance                                  Henry

19                 Resistance                                  Ohm

20                 Impedance                                  Ohm

21                 Reactance                                   Ohm

22                 Electrical conductance                 siemens 

23                 Magnetic flux                               Weber

24                 Magnetic flux density                   Tesla

25                 Heat                                            Joule

26                 Angle                                          Radian

27                 Radioactivity                                Becquerel

28                 Luminous flux                              Lumen

29                 Momentum                                  kilogram meter per second

30                 Torque                                         Newton metre

31                 Specific heat                     Joule per kilogram kelvin

 SI derived Unit

SI Base Quantities And Units – There are seven fundamental units in the SI system of units. They are also known as base units

Important Points

• 1 fermi = 10–15 m
• 1 angstrom = 1 Å = 10–10 m
• Speed of the light in vacuum is 3.00 × 108 m/s
• Astronomical unit (AU): It is the mean distance of the centre of the Sun from the centre of the Earth.
• 1 astronomical unit = 1.496 × 10^11 m
• Light year is a unit of length used to express astronomical distances.
• Light year=9.46 × 1015 m
• Parsec: Parsec is the unit of distance used to measure astronomical objects outside the solar system.
• 1 Parsec = 3.08 × 1016 m
• 1 Parsec = 3.26 light year.
• Time is a measure of duration of events and the intervals between them. The SI unit of time is second.
• Heat is a form of energy. Temperature is the degree of hotness or coldness of a body. The relationship for conversion from one temperature scale to the others is 

          R/80=C/100=K-273/100=F-32/180

• Mass: Mass is the quantity of matter contained in a body. The SI unit of mass is kilogram (kg).
• 1 g = 1/1000 × 1 kg = 0.001 kg 
• 1 mg = 1/1000000 × 1 kg = 0.000001 kg 
• 1 quintal = 100 × 1 kg = 100 kg 
• 1 metric tonne = 1000 × 1 kg = 10 quintal
• Mass of 1 ml of water = 1g 
• Mass of 1l of water = 1kg
• Mass of the other liquids vary with their density.
• Atomic mass unit: Mass of a proton, neutron and electron can be determined using atomic mass unit (amu).
• 1 amu = (1/12)th of the mass of C12 atom

Measurement of length

• Metre scale is used for lengths from 10–3 m to 102 m
• Verniercalliper is used for lengths to an accuracy of 10–4 m
• Screw gauge and a spherometer can be used to measure lengths as less as to 10–5m

List of Scientific Instruments And Their Uses