Published by: onep_admin

Published Date: 04 Jul 2021

Measurement in Class 9 Science


Measurement is defined as the comparison of unknown quantities with known qualities with the known or standard quantity. Have you ever bought daily used materials like rice, sugar, salt, clothes, etc. from the market? Can they sell these things without measurement? Absolutely No. They sell it by measuring in units (for example rice in kilograms i.e. kg). They sell just after measuring all items and give you as required, this is how measurement is deeply applicable in human life every single day. 

In almost every school, the bell rings after a fixed period of time (45 minutes) according to the daily class routine. Likewise, the earthquake is measured on the Richter scale. The different physical quantities like mass, volume, length, time, the magnitude of an earthquake, etc. are measured by comparing with some standard known quantities called units. Kilogram (kg), liter (l) meter (m), second (s), Richter scale, etc. are some of the units of physical quantities. 

The standard known quantity that is used to measure a physical quantity is called a unit.  

Types of units

The units of the physical quantities have been categorized into two groups. They are:

  1. Fundamental Units
  2. Derived Units

1. Fundamental Units

The unit of mass is the kilogram, the unit of time is second and the unit of length is meter. The above mentioned physical quantities like time, mass, length, etc. have their own existence. Similarly, their units also do not depend upon other units as well as other units of the measurement do not affect them. Such types of units are called fundamental units. 

 In brief, the fundamental units are defined as: 

Those units of measurement which have their own independent existence and do not depend upon other units are called fundamental units.

 The fundamental units are shown from the given table:

S.N. Physical quantities Units  Symbols
1 Length  meter  m
2 Mass kilogram kg 
3 Time  second  s
4 Temperature  kelvin  K
5   Luminous intensity  candela  cd
6   Electric current  ampere  
7 Amount of substance  mole  mol


2. Derived Units

The unit of density is kg/m³. It includes two fundamental units i.e. kilogram (kg) and meter (m). In the same way, the unit of force is the newton (N) or kgm/s². In this unit, there are three fundamental units i.e. kilogram (kg), meter (m), and second (s). The above units of density and force are derived by the combination of two or more fundamental units. These units fully depend on the fundamental units. Such types of dependent units are called derived units. 

We can define derived units in the following way. 

Those dependent units which are formed by the combination of two or more fundamental units are called derived units.

Some physical quantities, their derived units, and symbols:

Quantities Formulae Derived Units Fundamental Units Involved 
Area  length × breadth  meter × meter
Volume  length × breadth × height meter × meter × meter 
Density mass/volume kilogram/meter³ kg/m²=kgm¯³
Velocity  displacement/time meter/second m/s=ms¯¹
Acceleration Change in velocity with time meter/second*second m/s²
Force mass*acceleration kilogram*meter*second¯²or newton N or kgm/s²
Pressure  force/area Newton/meter²or Pascal Kg/ms2 or Pa
Work force*distance newton*meter or Joule kgm²/s²or J
Power  work/time newton*meter/second or Watt kgm²/s³or W
Moment  force*distance newton*meter kgm²/s²
Frequency  1/second 1/second or Hertz s¯¹or hertz

In this way, the units which depend upon the fundamental units as well as can be expressed in terms of fundamental units are called derived units.

A standard system of units

In Nepal, some people still use local units like haat and mana for measurement of lengths and mass. These units may vary from place to place. So, in order to maintain uniformity in the measurement of physical quantities, the following standard units are used:

  • MKS system: In this system, mass, time, and length are measured in a kilogram, second, and meter respectively. It is also known as the metric system.
  • CGS system: In this system, the length is measured in centimeter, mass in gram, and time in second. 
  • FPS system: In this system, the length is measured in the foot, mass in the pound, and time in second. 
  • SI system: It is described below in brief.

 SI System 

To bring uniformity in the measurement all over the world, a conference of scientists was held in 1960 AD in Paris, the capital of France. The conference introduced the different standard units in the measurement. Now, these standard units are being used all over the world. The system of measurement introduced by the Paris Conference to bring uniformity in measurement all over the world is called the SI system.  Similarly, those standard units of measurement which are introduced by the Paris conference of the scientists held in 1960 AD are called SI units. Its full form is System International de’units.

Why is the SI unit needed? (Importance)

  • The international system is a rational system of units. That’s why the system makes use of only one unit for physical quantity.
  • It is a coherent system of units. That is, in this system, all the derived units can be easily obtained from fundamental units.
  • It is a metric system that multiplies and sub-multiplies can be expressed as powers of 10.