Today we're going to do an experiment
to demonstrate Pascal's law of hydrostatic pressure.
So here we've got a 50 litre glass jug of water.
I'm going to try to break this
using just this amount of water.
Who is the little bit skeptical?
Oh, there, I know there are some skeptics
in the crowd, OK!
How many people here believe that
this is going, like really believe that
this is going to work?
We have faith in physics!
Let's see who wins, Katerina or the barrel!
But wait...
How did such a small amount of water
break such a large barrel?
Let's go back to the beginning to understand this.
So, what is hydrostatic pressure?
You know how when you dive into a
swimming pool or in the ocean and you
go deep and you feel pressure in your
ears pushing your eardrums?
As you go deeper you feel more
and more pressure in your ear.
And that's pretty intuitive, right?
The deeper you go the more pressure.
And when I teach pressure in my class
my students have no trouble understanding
the correlation between depth,
or another way to think about it is
the height of the water above you
when you're diving.
More height more pressure.
But what they do have trouble really
understanding and grasping and believing
is that the height is the only thing
that affects the pressure.
Think about the hoover dam.
They built a huge thick wall there
to block the water from falling.
That's not because Lake Mead behind it
has so much water in it which is pushing
with a huge mass of water. In fact if you
took away Lake Mead completely and kept
only a thin film of water right where
the dam is, going as deep as the damn,
you would feel the same pressure as with
the whole Lake.
And another example is if you
had a long thin straw extending a
hundred and fifty feet, the pressure at
the bottom of the straw would be the
same as if you dove in the ocean a
hundred and fifty feet deep.
I started looking online for videos to
try to find other people who have done
demonstrations of this, and I actually
couldn't find any. And then I tried to look up
how Pascal himself, who's the guy who
thought about this experiment.
Blaise Pascal in 1646 he came up with these
laws. I tried to find out how he did it.
And there was a lot of uncertainty as to
how exactly he did it, and even if he
did it at all.
So here we've got a 50 litre glass jug of water,
and it's already full as you can see.
And on top of that we have a
hose that extends the 155 feet
up into the top of Fine Hall.
And here we've got
the water that I'm going to use.
One more thing you should know to understand this
experiment is that when the pressure of
the water goes down it pushes down on
top of the liquid right? But that
pressure is then evenly distributed
throughout the whole volume of the water
so the entire jug will feel that same
pressure. And in particular every point
on the surface of the glass will also
feel that pressure. So basically
we're channeling the pressure from this
hose throughout the entire
glass and if we reach a high enough
pressure in theory it should shatter.
Even though it's a simple idea, there
were a lot of technical challenges to
overcome. For instance, as you pour water
down a narrow tube, air bubbles can form
the tube, and then the surface tension of
the water prevents them from escaping.
Adding soap to the water breaks the
surface tension, and this allows the air
bubbles to travel up the tube more
easily. Since this is the first time that
we've attempted this experiment there
were some unforeseen surprises...
the water would sometimes get
pushed back out of the tube!
This was caused by the air bubbles,
which at first got compressed by
the incoming water, and when they
decompressed, like a metal spring, they
push the water back up.
And things don't always go as planned.
*Oh no!!*
Scientific results are almost always
presented in their final polished form,
and the struggles to achieve them are
swept under the rug.
It's important for young students to
realize this, so they don't get discouraged
when they inevitably hit a roadblock.
Perseverance!
Perseverance will win the end!
Take two!
*Applause*
