Concepts

Class 9
Chapter 10 Class 9 - Gravitation

## Which occupies more space Cotton of 1 kg or Iron of 1 Kg?

Both Cotton and Iron have mass of 1 Kg

Still cotton occupy more space than iron

This is because density of cotton is more than iron

## What is density of an object?

Density means Mass of an Object per unit volume

Density = Mass/Volume

Example

Iron has a density of 7800 kg/m 3

It means that in a square box of 1 meter which has 1m3 volume , we can store 7800 kg of iron

Example 2

Cotton has density of 450 kg/m 3

It means that in a square box of 1 meter which has 1m3 volume, we can store 450 kg of cotton

## Densities of Different Objects

 Substances Density (kg/m 3 ) Cork 240 Air 1.2 Wood 700 Cooking oil 910 – 930 Ice 916.7 Water 1,000 Glycerine 1,261 Aluminum 2,700 Iron 7,870 Mercury 13,546 Gold 19,320 Glass 2500

## What is Relative Density of an object

It is the ratio of Density of an object to Density of Water

Relative Density = Density of object/Density of Water

## Example

Iron has a density of 7800 kg/m 3

Water has a density of 1000 kg/m 3

Relative Density of Iron = Density of iron/Density of Water

= 7800/1000

= 7.8 kg/m

Example 2

Cork has a density of 240 kg/m 3

Water has a density of 1000 kg/m 3

Relative Density of Cork = Density of Cork/Density of Water

= 240/1000

= 0.24 kg/m 3

## Why do some Objects sink in Water

The object sinks in water because buoyant force is less than force exerted due to weight of object due to its high density

Those objects which have density more than water sink in water

We can also say that those having Relative Density more than 1 sink in water

Example

Iron sinks in water because Iron has more density than water

(Density of iron is 7800 kg/m 3 but water is 1000 kg/m 3

Relative Density of Iron = Density of iron/Density of Water = 7800/1000 = 7.8

We can also say that since relative density of object is more than 1, it sinks in water

## Why do some Objects float in Water

The object floats in water because buoyant force is more than force exerted due to weight of object due to its low density

Those objects which have density less than water float in water

We can also say that those having Relative Density less than 1 sink in water

Example (Would come under reason for floating)

Cork floats in water because cork has less density than water

(Density of Cork is 240 kg/m 3 but water is 1000 kg/m 3

Relative Density of Cork = Density of Cork/Density of Water = 480/1000 = 0.48

We can also say that since relative density of object is less than 1,it floats in water

## Questions

Example 10.7 - Relative density of silver is 10.8. The density of water is 10 3 kg m –3 . What is the density of silver in SI unit?

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