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
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 (m^{2}) |
5 |
Volume (V) |
Cubic metre (m^{3}) |
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:
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 |
m^{2} |
Volume |
Cubic metre |
m^{3} |
Force |
Newton |
N or km/s^{2} |
Energy |
Joule |
J or Nm |
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