class 9, Science: Measurement

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

Let’s study the fundamental units from the given table:

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

    

Different types of physical devices are used to measure the various fundamental quantities. For example a beam balance is used to measure mass, a watch is used to measure time, a ruler or measuring tapeis used to measure length, a thermometer to measure temperature, an ammeter is used to measure electric current and so on.

Derived Units

The unit of acceleration is m/s². It includes two fundamental units i.e. meter(m) and second (s). In the same way, the unit of speed is ms^-2. In this unit there are two fundamental units i.e. metre (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 units of measurement which are formed by the combination of two or more fundamental units are called derived units.

Some of the important physical quantities and their derived units from the

following table:

Some physical quantities, their derived units and symbols:

Quantities	Formulae	Derived Units	Fundamental Units Involved
Area	length × breadth	metre × metre	m2
Volume	length × breadth× height	metre × metre ×metre	m3
Density	mass/volume	kilogram×metre3	Kg/m3 = kgm-3
Velocity	Displacement/ time	Metre/second	m/s = ms-1
Acceleration	change in velocity/time	Metre/second × second	m/s2 = ms2
Force	mass ×acceleration	kilogram × metre/second2 or newton	kg m/s2 or N kg ms2 or N
Pressure	Force/ area	newton/metre2 or Pascal	Kg/ms2 or Pa =kg m-1s-2 or Pa
Work	force × distance	newton × metre or Joule	Kgm2/s2 or J kgm2s-2 or J
Power	Work/time	newton × metre/second or watt	Kgm2/s3 or W =kg m2s-3or W
Moment	force × distance	newton × metre	Kgm2/s2 =kgm2s-2
Frequency	1/second	1/second or Hertz	s-1 or Hz

 Thus,

the units which depend upon the fundamental units as well as can be expressed in terms of fundamental units are called derived units

Differences between fundamental and derived units

S.N.	Fundamental Unit	S.N.	Derived Unit
1.	They do not depend upon other units.	1.	They depend upon fundamental units.
2.	Till today, only seven fundamental units are used. Examples: kilogram(kg), metre (m), second(s), etc.	2.	Many derived units can be formed from these seven fundamental units. Examples: newton(N), joule (J), Pascal (P), watt (W), etc.

While expressing the physical quantity, we need to mention the units too. 

For example, 10 N force, 3 kg mass, etc.

SI System

The system of measurement introduced by the Paris Conference to bring uniformity in measurement all over the world is called 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.

These SI units are used by all countries of the world making the measurement system consistent worldwide. According to the SI system, different physical quantities are well defined and their models are made. These models are distributed to different countries of the world to make their prototype. On the basis of these models and their prototypes, different physical quantities are being measured all over the world. So, this system has played a vital role to bring uniformity in the measurement of all the countries of the world.

Why is SI unit needed? 

It is difficult to run trade and business at the international level by using different units in different countries. So, SI unit is needed to make the transaction easy.

A Answer the following questions. 

1. What is unit? Write down the units of mass, temperature, power, and density.

2. Write any three differences between fundamental units and derived units.

3. What is SI system? Why has SI system been developed? Give reasons.

4. What is the unit of pressure? Why is it called a derived unit? Give reasons.

5. Why is the unit of temperature called a fundamental unit? Give reasons.

6. Find the fundamental units involved in the following derived units.

(i) Newton(N) (ii) Watt (W) (iii) Joule (J)

(iv) Pascal (Pa) (v) Cubic metre

7. The standard weights (dhaks) and meter scales of the shops in the market are verified in every two years by the department of measurement of Nepal, Why?

More lessons will be added very soon. You can have your query in the comment box given below.