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It is comparison of an unknown physical quantity with the known or standard quantity.
For e.g: While measuring the length of a book, it is compared with the known length of scale.

Significance of Measurement

  • Essential in purchasing and selling activities.
  • Gives accurate amount of quantities present in physical quantity.
  • Helps in scientific experiments and research activities.
  • Construction of building, roads, infrastructures etc.

Physical Quantities
They are the one that can be measured directly or indirectly regarding their quantity and quality.
For example: Mass is the physical quantity, which can be measured using pan balance or beam balance.

Types of Physical Quantities

  1. Fundamental Quantities
    These are the physical quantities that do not depend on other quantities and are independent. Seven fundamental quantities recognized by International Committee:
    • Length
    • Mass
    • Time
    • Temperature
    • Current
    • Luminous Intensity
    • Amount of Substance
  2. Derived Quantities
    They are physical quantities that depend upon fundamental quantities. They are also called dependent quantities. For eg: Velocity that depends on length and time.
    Power that depends on length, mass and time.

The standard reference quantity with reference to other physical quantities of same kind are measured is called unit.
For eg: Unit of kilogram is Kg. If we measure mass to be 7 kg, it means that the measured mass is 7 times greater than the standard reference quantity.
They are of two types, Fundamental and Derived Unit

Difference between Fundamental Units and Derived Units.

Fundamental Units

Derived Units

They are the units that do not depend upon other units.

They are the units that depend upon other units.

Also called as basic or Independent units.

Also called as Dependent units.

There are seven fundamental units.

There are large numbers of derived units.

Reasoning Concepts
Velocity has its unit meter per second which is a derived unit that it is obtained from fundamental units meter (length) and second (time).
Unit of velocity = \(\frac{Unit \; of \; length}{Unit \; of \; time}\) = \(\frac{meter}{sec}\) or \(\frac{kilometer}{hour}\)


  1. Define measurement. Mention its significance in practical life.
  2. Give any two difference between:
    a) Fundamental and Derived Quantities
  3. Unit of acceleration is called derived unit, why?
  4. A substance is 10 meter in length, what does it mean?

System of Units
System of writing different units in different parts of the world makes the measurement activities quite difficult as the world is becoming a single global market.
To make uniformity in measurement, following systems have been developed:

  • FPS System:
    System in which length is measured in foot, mass in pound and time in second. This is actually a British system of measurement frequently used in Europe.
  • CGS System:
    System in which length is measured in centimeters, mass in gram and time in second. Also called a French system of measurement.
  • MKS System:
    System of units in which length is measured in meter, mass in kilogram and time in second is MKS system. It is also called metric system of measurement.

However, these three systems can only explain about the units of length, mass and time but not other fundamental physical quantities like Temperature, electric current, Amount of substance and frequency. So, the scientists have developed standard International system of units in which all seven physical quantities and their units are included.
SI system of units is the world – wide accepted system of units used in all forms of scientific activities that is the extended form of MKS system. Full Form of SI Units: System of International De – Units

Difference between M.K.S. and SI System of Units.

M.K.S. System

S.I System

It is a metric and simple system of measurement.

It is the extended and revised system of SI system.

It involves three fundamental quantities length, mass and time.

It involves all seven fundamental quantities.

Significance of SI system

  • brings uniformity and similarity in measurement all over the world
  • can be used to express different fundamental and derived units
  • helps to eliminate and solve the problems that arise during scientific works

Reasoning Concepts

  • It is difficult to measure volume of gas.
    \(\rightarrow\) It is so because air molecules are loosely packed and do not have fixed shape and size. Similarly, gases can easily be compressed and expanded.
  • Unit of force is a derived unit.
    \(\rightarrow\) Force is physically expressed as the product of mass and acceleration. So, its units is derived from fundamental units of mass and acceleration.
    F = m \(\times\) a
    or, F = m \(\times\) \(\frac{v}{t}\)
    So, the unit is expressed as,
    \(\Rightarrow\) kg \(\times\) \(\frac{m/sec}{sec}\)

Table Showing SI units of different physical quantities:

Physical Quantities

SI Units (Symbol)


Metre (m)


Kilogram (kg)


Second (sec)


Ampere (A)


Kelvin (K)


Hertz (Hz)

Intensity of Light

Candela (Cd)

Amount of Substance



Joule (J)


Joule (J)


Watt (W)


Pascal (Pa) or N/m2


Newton (N)


m/ sec



Potential Difference






  1. Define SI system of units with its two importance.
  2. Unit of pressure is called derived unit. Why?
  3. SI system us better than MKS system of measurement. Explain.
  4. What is meant by standard unit?
  5. Name the physical quantities to which following units are related:
    Ohm, Hertz, Joule, Watt, Pascal
  6. What is the full form of SI Unit?
  7. Mention the use of unit- 'Light Year'.


#Things To Remember