To know the time spent while travelling, we need some reference.

A similar thing happens with the distance, how much distance we have covered in our journey.

To express the quantities such as distance, time, units have been developed.

The measurement of a quantity is expressed with a number followed by the unit.

There are some units from which the unit of other quantity is developed. Those units are called the fundamental units.

There are total seven fundamental quantities; length, mass, time, electric current, thermodynamic temperature, amount of substance and the luminous intensity.

The units of these fundamental quantities are known as the fundamental units.

Since measurement is very important in our daily life and units are used to describe the measure of physical quantities. So, units become necessary to us.

Let’s study the SI unit of the fundamental quantities.

The full form of the $SI$ is ‘systeme internatinale d’unites’ in French.

The $SI$ unit of length is metre and it is measured on the meter scale.

Meter is the length of the path traveled by the light in vacuum during a time interval of $1/299792458$ of a second.

The second fundamental quantity is mass and the $SI$ unit of the mass is Kilogram represented by $Kg$.

The $Kg$ is equal to the mass of international prototype of the kilogram kept at the international bureau of weights and measures, France.

While further moving down in the list, the next fundamental unit is time and its $SI$ unit is second denoted by $Sorsec$.

One second is the time that elapses during $9192631770$ cycles of radiation produced by the transition between two levels of $Cesium−133$ atom.

There are different types of clocks used to measure time.

The next fundamental unit is electric current. The $SI$ unit of the electric current is Ampere abbreviated as $A$.

Current is measured using the ammeter.

Further discussion is about the next fundamental unit which is the thermodynamic temperature.

The $SI$ unit of thermodynamic temperature is Kelvin denoted as $K$. It is measured by thermometer.

The next fundamental quantity is the amount of substance. Its $SI$ unit is mole expressed as $mol$.

The mole is the amount of substance of a system which contains as many entities as there are atoms in $0.012kg$ of $carbon−12$.

The last fundamental quantity in the list is luminous intensity, its $SI$ unit is Candela and it is denoted by $cd$.

Revision

There are some units from which the unit of every other quantity is developed, called the fundamental units.

There are seven fundamental quantities; length, mass, time, electric current, thermodynamic temperature, amount of substance and the luminous intensity.

The units derived from the fundamental units are called the derived units.