Say you're holding a slightly heavy object in your hand.
Now, if you try to hold that object within a container full of water,
you'll notice that it feels lighter.
It doesn't feel heavy at all.
What happens when you put an empty glass into a container
of water?
Of course, the glass fills up with water.
So you're holding the glass of water inside the water
which is in the container.
and now, when you try to take the glass outside the water
what do you notice? Yes, the glass
feels heavier when it's outside the fluid. And the same
glass full of water felt light when
placed inside the fluid and that
brings us to the concept of buoyancy.
What happens when an object is placed inside a fluid?
As we've seen in the previous video, the fluid pressure increases with depth.
Now because the pressure increases as we go deeper,
the force exerted upward due to the pressure will be more than
the pressure exerted downwards due to the pressure.
So the force exerted on the bottom of the object due to pressure will be more than the force exerted
on the top.
So this net upward force exerted on objects
submerged in fluids is called the buoyant
force.
You can test this with a tightly closed empty plastic bottle
and a container of water.
If you drop the bottle on the fluid, you will see that it will
float. That's because the fluid is applying an upward
force on the bottle.
Now, hold the bottle and push it inside the fluid.
You'd notice that the bottle is also exerting an upward
force on your hand. If you leave the bottle,
you'll see that the bottle will come up to the surface on its
own.
I think you can guess why.
Now push the bottle deeper inside the fluid. You will
feel more force against your hand and when you
release the bottle, it'll rush back to the surface.
It'll come up faster than it came up in the previous case.
The upwards force increases as it's pushed deeper
into the fluid and that's because
the pressure also increases as we go deeper into the
fluid
This upward force exerted by the water on the bottle
is known as upthrust or the bouyant force.
Now let's say you simultaneously drop a piece of  balsa wood
and a metal drawing pin of the same mass
into a container filled with water.
What do you think will happen? Will both of them
float? Or will they both sink? Or
one stays afloat and one sinks?
We will understand this example in the next video
