Concepts

Class 9
Chapter 10 Class 9 - Gravitation

## Who was Archimedes

Archimedes was a Greek scientist who discovered buoyancy

### What is Archimedes Principle

As per Archimedes principle

When an object is immersed in a fluid

fully or partially

it experiences a upward force

This upward force is equal to weight of fluid displaced by the object

Example

Suppose we take glass

We full it to the brim with water

Now if we drop a coin into it

It leads to water falling from the glass

This is called water displaced from glass

As per Archimedes Principle,

the buyout force acting on coin is equal to

quantity of water which flows out of glass

Note

Buoyant Force remains constant After Object is fully immersed in water

Suppose we take a rock

We attach it to stretchable string

We note down the elongation(length of string)

Now we immerse string in beaker contain g water

Elongation(Length of string) decreases

This is because of buoyant force of water

More we immerse string in water,

Elongation(Length of string) decreases more and more

However when we immerse rock completely in water and bring it more down

The elongation does not decrease any further

This shows that

Buoyant Force remaions constant After Object is fully immersed in water

## Different Uses of Archimedes principle

It is used in designing and manufacturing of

Ships and Submarines

Lactometer (a device to test purity or sample of milk)

Hydrometer (A divide to test density of different liquids)

## Questions

NCERT Question 22 - The volume of a 500 g sealed packet is 350 cm 3 . Will the packet float or sink in water if the density of water is 1 g cm –3 ? What will be the mass of the water displaced by this packet?

### Transcript

Question When an aluminum object is immersed in water, it displaces 5 kg of water. How much is the buoyant force acting on aluminum object in newtons? (g = 10 m/s2) Mass of water displaced = m = 5 kg According to Archimedes Principle Buoyant force acting on object = Weight of liquid displaced by that object Weight of object = m × g = 5 × 10 = 50 N Thus, Buoyant Force = Weight of object = 50 N