Question

The SI unit of density is:
[A] $g\text{ }{{\text{m}}^{-3}}$
[B] $Kg\text{ }{{\text{m}}^{-3}}$
[C] $g\text{ c}{{\text{m}}^{-3}}$
[D] $Kg\text{ c}{{\text{m}}^{-3}}$

Answer 1

HINT: To solve this firstly you have to know that the definition of density is mass per unit volume. Then you have to know the SI units of mass and volume. Simply put the unit of mass and volume and you will get the SI unit of density.

COMPLETE STEP BY STEP SOLUTION: To answer this question, firstly let us discuss what SI unit is. The International Systems of Unit, also known as SI is a system of measurement and it comprises a system of units of measurements starting with the seven basic units. These seven basic units are for properties like time, mass, current, temperature etc.
The seven basic units and their symbols are –

Property	Unit	Symbol
Time	second	s
Length	metre	m
Mass	kilogram	Kg
Electric current	ampere	K
Thermodynamic temperature	kelvin	A
Amount of substance	mole	mol
Luminous intensity	candela	cd

Now, let us see the definition of density and try to find its unit according to the above table.
Density of a substance is the mass of the substance per unit volume. Mathematically, we can write it as-
\[\rho =\dfrac{M}{V}\]
Where, \[\rho \] is the density, M is the mass and V is the volume. Now, according to the SI measurement units, mass is given by Kilogram i.e. Kg as we can see from the above table. We know that volume is the capacity. In SI units, it is given by cubic metre i.e. ${{m}^{3}}$ . Therefore, we can write the SI unit of density as-
\[\rho =\dfrac{Kg}{{{m}^{3}}}\]
We can understand from the above discussion that the SI unit of density is $Kg/{{m}^{3}}$or we can also write it as $Kg\text{ }{{\text{m}}^{-3}}$.

Therefore, the correct answer is option [B] $Kg\text{ }{{\text{m}}^{-3}}$ .

NOTE: Like the International System of Units, we also have another unit of measurement known as the CGS unit. CGS uses centimetre gram and second for length, mass and time respectively. However, SI unit is internationally accepted and the most used unit worldwide. The density in the CGS unit would be $g/c{{m}^{3}}$.