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# Measurement

Measurement is the comparison of an unknown quantity with known standard quantity. For e.g. watch to measure time, measuring tape with standard lengths to measure length and a beam balance with standard weights to measure mass.

Necessity of measurement in our daily life

• Measurement is useful for the exchange of goods.
• It is important for making and preparing food.
• It is important for conducting scientific experiments.
• It is useful to sell and buy things.
• It is useful to measure raw materials in industries.

Physical Quantity
Those quantities which can be measured directly or indirectly are called physical quantities.

Unit
A unit is a reference standard quantity with respect to which we measure other quantities.
Units of some quantities are as follows:

 S.N. Physical Quantity Unit 1 Length (L) Metre (m) 2 Mass (M) Kilogram (kg) 3 Time (T) Seconds (s) 4 Area (A) Square metre (m2) 5 Volume (V) Cubic metre (m3)

Standard Units
The units of measurement which is used by majority of people in a region as a basic unit is called a standard unit. E.g. metre, kilogram, second, litre etc.

Systems of units
There are various systems of units to measure physical quantities. Out of them, the main systems of unit are as follows:

• MKS System
In the MKS system, length is measured in metre, mass is measured in kilogram and time is measured in second. It is also called metric system of units.

• CGS System
It is also known as French system of units. In this system, length is measured in centimeter, mass is measured in gram and time is measured in second.

• FPS System
It is the British system of units. In this system, length is measured in foot, mass is measured in pound and time is measured in second.

• SI System
The SI system is the improved and extended version of the MKS system of units. In 1960, the scientists in the "Eleventh General Conference on Weights and Measures" introduced this system. This system includes the MKS system of units as well as the units related to heat, light, electricity and magnetism.

Fundamental Units
Those units which are independent of any other units are called fundamental unit. The units of fundamental quantities are called fundamental units. The following are the seven fundamental quantities in the SI system.

 S.N. Physical Quantity Name of Unit Symbol 1 Length Meter m 2 Mass Kilogram kg 3 Time Second s 4 Temperature Kelvin K 5 Electric Current Ampere A 6 Luminous Intensity Candela Cd 7 Amount of substance Mole mol

Derived Unit
Those units, which are dependent upon fundamental units, are called derived units. Or, the unit which is expressed in terms of two or more fundamental units is called derived units.
Derive units which are dependent upon fundamental units are division and multiplication of seven fundamental units. E.g.

 Derived Quantity Name of Unit Symbol Area Square metre m2 Volume Cubic metre m3 Force Newton N or km/s2 Energy Joule J or Nm

### Measurement of Some Physical Quantities

1. Measurement of Length
The length is defined as the distance between any two points. The SI unit of length is metre. The length of a room, height of a building and length of your playground, etc. are measured in metre.

Sub-multiples and multiples of metre
1 millimetres (mm) = 1 centimetre (cm)
10 centimetres (cm) = 1 decimetre (dm)
10 decimetres (dm) = 1 metre (m)
10 metres (m) = 1 decametre (dam)
10 decametres (dam) = 1 hectometre (hm)
10 hectometres (hm) = 1 kilometre (km)

2. Measurement of Mass
The amount of matter contained in a body is called its mass. It is fixed for a given body wherever it may be. The SI unit of mass is kg.
Sub-multiples and multiples of kilogram:
1 milligrams (mm) = 1 centigram (cm)
10 centigrams (cm) = 1 decigram (dm)
10 decigrams (dm) = 1 gram (m)
10 grams (m) = 1 decagram (dam)
10 decagrams (dam) = 1 hectogram (hm)
10 hectograms (hm) = 1 kilogram (km)

3. Measurement of Time
Time is the interval between two events. In ancient time, people didn't have a watch. By studying regular event such as sunset, sunrise, change in seasons, etc. they used to calculate time.
Multiples of second
60 seconds (s) = 1 minute (min)
60 minutes = 1 hour
24 hours = 1 day
7 days = 1 week
52 weeks = 1 year
365 days = 1 year
12 months = 1 year

We know that, the earth rotates about its own axis as well as it revolves around the sun. The time taken by the earth to rotate once about its own axis is called a day. If a day is equally divided into 24 equal parts, each parts is called an hour.
If one hour is sub-divided into 60 equal parts, each part is called a minute. If one minute is further sub-divided into 60 equal parts, each part is called a second. One second may be defined as the $\frac{1}{86400}$th part of a solar day.
The time taken by the earth to revolve once around the sun is called one year. The earth takes about 365 days to revolve once around the sun. Hence, one year equal 365 days.

Reasoning

• SI units are used for the scientific works.
$\rightarrow$ SI units are used for the scientific works because it is the most advanced system of measurement and it is accepted in all over the world.