Archimedes Principle
We are all aware of the fact that some objects
float in water while others sink. Let's look
at a balloon. Do you know what makes a balloon
float on water? Well, it's the upward force
exerted by water that makes the balloon float.
To experience this, try pushing the balloon
under water. You will feel an upward force
that makes it difficult to push the balloon
down.
Now release the balloon, it bounces back to
the surface.
What happens here is that the applied force
acts in the downward direction, pushing the
balloon down, but the water exerts an upward
force on the balloon, pushing it up.
This upward force exerted by water on the
balloon is known as upthrust or buoyancy,
and the upward force is called buoyant force.
Have you ever wondered why an iron nail sinks,
while a huge iron ship floats on water? Let's
see.
The iron nail sinks, as it is solid and compact
with less volume in water, and its density
is
greater than water. Thus the upthrust exerted
by water on the nail is less than the weight
of the nail. This causes it to sink.
An iron ship floats as it is hollow, filled
with air and occupies more volume in water,
and its density is lesser than water. Thus,
the upthrust exerted by water on the ship
is greater than the weight of the ship, letting
it float.
These examples indicate that the magnitude
of the buoyant force depends on: the volume
'V' of the immersed part of the body, the
density of the fluid and the acceleration
due to gravity 'g'. Thus, upthrust or buoyant
force
In general, an object experiences a loss of
weight in water due to the upthrust and it
is equal to the weight of water displaced
by it. This is better answered using the law
of buoyancy, also known as Archimedes' principle,
named after the Greek scientist who discovered
it.
Archimede's principle states that, When an
object is immersed in a liquid the apparent
loss of weight of an object is equal to the
upthrust and this is also equal to the weight
of the liquid displaced.
We see the uses of this principle in our daily
lives. Ever caught a fish with a fishing line?
When pulling the line, you would have felt
the weight of the fish in water is lighter
than when the fish has surfaced.
Say the weight of the fish in water is 4 kilo,
its approximate weight outside would be 5
kilo. Well, the water level too has come down!
This difference in the volume of the water
level will equal the loss in weight of the
fish when in water.
That is, the weight of the fish when outside
minus its weight when immersed in water, equals
the loss in weight of the fish. So the loss
in weight of the fish will equal the weight
of the decreased water.
This is because the up thrust or buoyant force
is equal to the weight of the decreased amount
of water. Thus Archimedes principle is verified.
Certain technologies today work based on Archimedes'
Principle. To name a few - the designing of
ships and submarines, hydrometer- is used
to find the specific gravity of liquid and
lactometers used to determine purity of milk.
Things to Remember
The upward force exerted by a fluid is called
up thrust or buoyant force.
Up thrust or buoyant force equals V?g
Archimedes principle states that, When a body
is partially or fully immersed in a fluid
it experiences an upward force that is equal
to the weight of the fluid displaced by it.
Archimedes Principle is also known as the
Law of Buoyancy.
