Question

What is the SI unit of mass? How is it defined?

Answer 1

SI unit is the International System of Units for the system of measurement which is accepted by every country of the world. This system rests on the foundation of seven defining contents.

Complete step by step answer:
The SI unit system is commonly known as the metric system. It is an International standard for measurement signed by seventeen countries. Seven base units are composed in it, and these seven define the ${\text{22}}$ derived units with special symbols and names. Those seven base units are:
Length – It is measured in meters. Its symbol is ${\text{(m)}}$. It is defined by taking the numerical value of speed of light in vacuum.
Time – It is measured in seconds. Its symbol is ${\text{(s)}}$.
Amount of substance – It is measured in mole. Its symbol is ${\text{(mol)}}$.
Temperature – It is measured in Kelvin. Its symbol is ${\text{(K)}}$.
Electric current – It is measured in Ampere. Its symbol is ${\text{(A)}}$.
Luminous Intensity – It is measured in Candela. Its symbol is ${\text{(cd)}}$.
Mass – It is measured in kilograms. Its symbol is ${\text{(kg)}}$.
Thus, the Si unit of mass is Kilogram.
The kilogram having symbol ${\text{(kg)}}$ is defined by taking the constant numerical value of the Plank constant ${\text{h}}$, which is equal to $6 \cdot 62 \times {10^{ - 34}}$, having unit ${\text{Js}}$. In terms of mass, ${\text{Js}}$ is equal to ${\text{kg}}{{\text{m}}^2}{s^{ - 1}}$. Kilo is the prefix used for expressing ${\text{1000 or }}{10^3}$.

Note: Mass, and weight are two different things. So, mass should not be used in place of weight, and vice versa. Weight is defined in terms of force, and it has unit Newton. Its symbol is $\left( {\text{N}} \right)$.