1.Density

Density: It is defined as the ratio of mass per unit volume of the substance. It is denoted by the symbol ρ can be pronounced as rho. Mathematically it can be written as 

ρ = (m/v)

Where,

ρ - Density of the substance.

m - Mass of the substance.

V - Volume of the substance. 

Units:
We will derive the units in S.I. system. 
 In SI system mass is in Kg.
In SI system Volume(v) is in m³.

Therefore,

Density(ρ) = mass(kg)/Volume(m³)

Units of ρ are kg/m³.

Note:

1. Density is also called as mass density or volume mass density.
2. Density of pure water is 1000kg/m³.
3. Density of air is 1.28 kg/m³.
4. Density of mercury is 13600kg/m³.  

Video:

Example Problem:

Find the density of the substance whose mass is measured to be 5kg and a volume of 3.91 m³.

Solution: We know that density is the ratio of mass and volume. Therefore,density(ρ) =mass(5kg)/volume(3.91m³).
 = 5/3.91(kg/m³)
 = 1.28(kg/m³) = Density of air.

The density of the fluid is found to be 1.28(kg/m³)

Python Program:

#!/usr/bin/python
# -*- coding: utf-8 -*-
def density(mass,volume):
 density = float(mass)/float(volume)
 print u"Density(ρ) of the substance is found to be {}".format(density)

mass = raw_input("Enter the value of mass > ")
volume = raw_input("Enter the value of volume > ")

density(mass,volume)

 Output:

Enter the value of mass > 46546
Enter the value of volume > 6546
Density(ρ) of the substance is found to be 7.11060189429