 Hello and welcome to the session. In this session we will discuss about density. Suppose we take same volume of different substances. Now let us take one cube of wood and one cube of iron. Each cube has volume that is one cubic centimeter. So we have taken same volume but substances are different. When we measure the two cubes we see that their weights are not same. That is iron will be heavier than wood. So we can say that density of iron is greater than density of wood. So density tells us which substance is heavier. So different materials have different densities. Thus density is a comparison of how much matter there is in a certain amount of space. So we define density is the amount of matter that an object has in a given unit of volume or we can also say density is mass per unit volume. Density of an object is calculated by dividing its mass by its volume. The SI unit of density is kilogram per cubic meter but gram per cubic centimeter is also commonly used. Now let us calculate density when mass of an object is 35 grams and it takes up 7 cubic centimeters of space. That is we need to find the density of an object when mass of the object is given as 35 grams and its volume is given as 7 cubic centimeters. As we know that density is mass per unit volume. So here density will be equal to the mass that is 35 grams over volume which is equal to 7 cubic centimeters and this is equal to 5 grams per cubic centimeter. Now we are going to discuss population density. Suppose we are given area of the city in square miles and we are also given population of the city. And if we want to find the number of people living in per square mile we shall use population density. Population density gives us the number of people living in per square area. The formula for finding population density is given by number of people divided by area of the land. Now let us consider the following example. Suppose population of the city is 621,760 and its area is 9,600 square miles and we need to find the population density of the city. That is we need to find the number of people living in per square mile and we are given the population of the city that is the number of people living in the city and also the area of the city. Now to find population density we use the formula that is number of people divided by area of the land which is equal to number of people that is the population of the city which is given as 621,760 divided by area of the land which is given as 9,600 square miles and this is approximately equal to 64.8 people per square mile. So we can say that there are about 65 people per square mile in the city. So in this session we have learnt about density based on area and volume. This completes our session. Hope you enjoyed this session.