Say you have a beaker filled with water
and you put your index finger inside it.
You’ll observe that the water level rises slightly.
You put in two fingers,
the water level rises a bit more.
And now when you put in your hand up to the wrist level,
you’ll see that the water level increases significantly.
The ancient Greek mathematician Archimedes
observed something similar
when he got into a bath tub
When he got in
he realised that the level of water went up.
In both the cases,
you noticed that the water
is DISPLACED by the object that goes in
Keep this though in mind!
We’ll come back to it soon.
In our previous videos,
we’ve seen that
when an object is placed in or on the fluid,
the fluid exerts a buoyant force on it
which acts upwards.
The Archimedes’ principle goes
just one step further in saying that
the buoyant force applied by the fluid
is equal to the WEIGHT of the displaced Fluid
We can take a simple example to understand this.
Take a 5 kg solid object
and suspend it by a spring balance
The balance will read 5 kilograms.
Let this be the first case.
For the second case, take a beaker of water
which has a small opening at the top
The water in the beaker is filled right up to
this point where the opening is
And we keep a small container under the opening
Now slowly lower the solid object into the water
and you’ll see that the reading will be a
lower number as compared to the previous case
Why is that?
It’s because of the buoyant force acting on the object
You’d have also noticed
that some water was DISPLACED.
Now in this context,
we can say that the weight of this displaced water
is equal to the magnitude of the
buoyant force acting on the object
That’s the Archimedes’ principle!
When an object is immersed fully or partially in a fluid,
the upward force applied by the fluid
on the object is the SAME as the weight of
the fluid displaced by the object
Where is this principle applied?
Can you think of examples?
How do you think large STEEL ships float?
It’s simple
If the weight of water displaced by the vessel
BELOW the keel is equivalent to the vessel’s weight,
then everything ABOVE the keel will remain afloat.
That’s because the water too applies
an upward buoyant force on the ship,
which is SAME
as the weight of the water displaced by it
There’s just
one last important concept left in this section
And that’s relative density,
which we will cover in the next lesson.
