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Today we're going
to change topics.

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I'm going to talk to you
about fluids,

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hydrostatic pressure
and barometric pressure.

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If, for now, we forget gravity
 

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and I would have
a compartment closed off

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and filled with a fluid--

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could be either a gas
or it could be a liquid--

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this has area A, here--
 

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and I apply a force on it
in this direction,

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then I apply a pressure.

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Pressure is defined
as the force divided by area--

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has units newtons
per square meter
 

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which is also called pascal.

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One newton per square meter
is one pascal.

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Now, in the absence of gravity,

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the pressure is, everywhere
in this vessel, the same.

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And that is what's called
Pascal's principle.

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Pascal's principle says

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that the pressure applied
to an enclosed fluid

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is transmitted undiminished
to every point in the fluid

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and to the walls
of the container.

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Keep in mind, pressure is
a scalar, it has no direction.

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Force has a direction

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and the force exerted
by the fluid on anything--

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therefore also on the wall--

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must be everywhere
perpendicular to the wall,

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because if there were
any tangential component,

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then the fluid
would start to move.

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Action equals minus reaction,
so it starts to move

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and we are talking here
about a static fluid.

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So if I take any element--

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I take one here at the surface,
little element delta A

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and the force must be
perpendicular

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to that surface, delta F,

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and so delta F
divided by delta A--

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in the limiting case
for delta A goes to zero--

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is, then, that pressure P.

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This has some
truly amazing consequences

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which are by no means
so intuitive.

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This is the idea
of an hydraulic jack.

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I have here a vessel
which has a very peculiar shape.

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Ooh, ooh, an opening here.

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And let there be here
a piston on it with area A1

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and here one with area A2.

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It's filled
with liquid everywhere

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and I apply here a force F1
and here a force F2.

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So the pressure that I apply
here is F1 divided by A1.

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So according to Pascal,

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everywhere in the fluid,
that pressure must be the same.

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For now, I just assume
that the effect of gravity,

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which I will discuss shortly,

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doesn't change the situation
very significantly.

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But I will address
the gravity very shortly.

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So the pressure, then,
will be the same everywhere,

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but the pressure due to this
side is F2 divided by A2...

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and so the two must be the same,
if the liquid is not moving.

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So what that means is
that if A2 over A1 were 100,

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it means
that this force could be

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a hundred times less
than that one.

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In other words, I could put
on here a weight,

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a mass of ten kilograms,

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and here I could put
1,000 kilograms

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and it would be
completely in equilibrium.

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That's not so intuitive.

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This is used in all garages.

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What they do is,
they put on top of this--

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if I blow that up here,
so this is this platform,

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there's a rod here
and on top of it is a car.

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And someone pushes here
and then this goes up.

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The car goes up.

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If I push here with a force

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a little bit more
than ten kilograms--

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so that would be 100 newtons--
this level would go up.

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And so your first thought
may be,

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"Gee, isn't that a violation
of the conservation of energy?

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Am I not getting
something for nothing?"

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Well, not really.

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Suppose I push this down
over a distance d1,

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then the amount of fluid
that I displace--

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that is, the volume,
is A1 times d1.

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That fluid ends up here.

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So this one will go up
over a distance d2.

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But the same amount of fluid
that leaves here adds there.

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In other words, Al d1
must be A2 times d2.

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Now, if the force here

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is a hundred times less
than the force there,

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the work that I am doing
on the left side

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is F1 times d1.

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If the force here is
a hundred times less than that,

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the distance that I move is
a hundred times larger than d2,

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because A2 over A1 is 100.

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And so F1 d1 will be
the same as F2 d2--

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100 times lower force but
over 100 times larger distance,

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and so the product is the same.

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So the work that I do
when I push this down

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I get back in terms of
gravitational potential energy

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by lifting the car.

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So if I wanted to move
the car up by one meter

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and if the ratio is 100 to one

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I would have to move
this down by 100 meters.

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That's a little bit impractical

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so these hydraulic presses
are designed in such a way

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that you can just jack it
like that

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and every time
that you bring it up,

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that liquid flows back in again

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into this side
of the hydraulic jack.

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But, indeed, you will have to go
effectively 100 meters, then,

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for that car to go up
by one meter

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if the ratio is 100 to one.

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Now, gravity, of course,

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has an effect
on the pressure in the fluid.

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If you go down into the oceans,

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we know that the pressure
will go up,

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and that is the result
of gravity.

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And I would like to derive
the pressure increase.

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Let this be the direction
of increasing y,

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and I choose a liquid element,
so this is in the liquid itself.

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I can choose it in any shape
that I want to.

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I just take
a nice horizontal slab.

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And this is area A
so the bottom is also A.

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And let this be
at height y plus delta y

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and this is at height y.

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And the pressure here
is P y plus delta y

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and the pressure here is P of y.

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And this object has
a mass, delta m,

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and the liquid has
a density, rho,

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which could be a function of y--

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we will leave that open
for now.

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And so this mass--
the mass that I have here--

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is the volume times the density.

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And the volume is A--
this area-- times delta y,

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and then times the density,
which may be a function of y.

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So if now I put in
all the forces at work here,

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there is gravity, which is delta
m times g in this direction.

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Then I have a force upwards due
to the pressure of the fluid.

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That's what
we want to evaluate.

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It's always perpendicular
to the surfaces.

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We talked about that earlier.

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So in this side,
it comes in like this

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and here it comes
in like this, the force.

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From the bottom,
it comes in like this

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and from the top,
I call this F2.

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I only consider
the vertical direction,

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because all forces
in the horizontal plane

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will cancel,
for obvious reasons.

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So, now, there has
to be equilibrium.

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This fluid element
is not going anywhere--

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it's just sitting still
in the fluid.

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And so I now have that F1--
which is in this direction--

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minus F2 minus delta mg
must be zero.

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Only then is the fluid element
in static equilibrium.

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But F1 is this pressure
times the area--

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so that is P at level y
times the area,

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and F2 is P at level
y plus delta y times the area,

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minus delta m
is A delta y times rho,

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so I get minus A times delta y,
which could be a function...

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rho could be a function of y
times g, and that equals zero.

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Notice I lose my area.

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I'm going to rearrange this
slightly and divide by delta y.

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And so I get that P
at the level y plus delta y

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minus the pressure at level y
divided by delta y equals--

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if I switch that around, so I
bring this to the other side--

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equals minus rho y times g.

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And if I take
the limiting case of this--

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for delta y goes to zero--
then we would call this dP/dy.

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And this tells you that when you
go to increasing values of y,

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that the pressure will go down,
it's a minus sign.

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Very natural.

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If you go with
decreasing values of y,

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then the pressure will go up.

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And we call this
hydrostatic pressure.

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So it's due to the fact
that there is gravity;

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without gravity, there is
no hydrostatic pressure.

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Now, most fluids, most liquids
are practically incompressible.

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In other words, the density of
the liquid cannot really change.

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And so therefore,
you could remove this

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and simply always use
the same density.

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It's exceedingly difficult.

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It takes horrendous
forces and pressures

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to change the density of
a liquid, unlike that of a gas.

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A gas is compressible

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and you can very easily change
the density of gas.

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So liquid is incompressible.

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If I have here a piston
and I have here a liquid

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and I put a force on here,

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it would be impossible for me
to make that volume smaller--

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even by a fraction of a percent,
it would be impossible.

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If this, however, were gas,

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then it would be very easy
for me to push that in

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and to change the volume,
make the volume smaller

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and thereby make
the density of the gas go up.

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If I took a sledgehammer

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and I would hit
a plastic pillow, just bang,

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and the pillow was filled with
air, it acts like a cushion

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and I could squeeze it.

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If I hit the sledgehammer
on the marble floor,

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I could not squeeze it

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and the force on the marble
floor and on the hammer

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would be way higher, because I
don't have this cushion action.

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If I take a paint can-- and we
have one here, we have two--

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and this paint can is filled
to the brim with water

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and another one
is filled with air

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and I hit it
with a sledgehammer,

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then this acts like a cushion.

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This one, however, doesn't want
the volume to be decreased,

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so the force, like on the marble
floor, would be way higher.

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But remember that force divided
by area is pressure,

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and according to Pascal,

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that pressure propagates
undiminished in the whole fluid.

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And so if I would shoot
a bullet through here,

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then I get a huge force--

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extremely small area
of the bullet.

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And so the pressure
inside the liquid

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would go up enormously,
and the can might explode,

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provided that it's really filled
to the brim with water,

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because if there is air left,

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then you have
this cushion action.

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Now, I don't remember
whether there's air in here

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or whether there's air in there.

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I'll leave you to decide.

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So we'll fire a bullet
from this side,

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and then we'll see which can
explodes and which does not.

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And the one that doesn't
is the one that has air in it,

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and the one that explodes...
has the water in it,

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provided that we
really filled it to the brim.

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Oh, boy, there's
still something in there.

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(blowing )

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Okay.

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I did something wrong,
but that's okay.

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All right,
there goes the bullet.

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00:15:39 --> 00:15:45
Okay, are we ready for this?

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So you tell me
which can is filled with air

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and which can is filled
with water.

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Three, two, one, zero.

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(gunshot )

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Okay, this one is closed.

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It has a nice hole here
and a nice hole there.

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And this one has a hole here
and a hole there,

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but you saw the top come off.

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So we know
which one had water in it

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and by the way,
it's still there.

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These things are
not so intuitive.

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I will assume from now on

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that liquids are
completely incompressible.

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In other words, I can now use
this law that we have there

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and do a very simple
integration.

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I have now dP,

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which I can integrate
from some value P1 to P2.

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This is y, level P2, level y2,

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level y1,
pressure P1 in the liquid,

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and that equals now minus rho g
dy, integrated from y1 to y2.

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So that's now a trivial integral
because rho is constant--

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rho is not a function of y.

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With the atmosphere of the
Earth, that's more difficult,

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because rho is a function
of altitude with the atmosphere

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but not with liquids.

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And so we get that P2 minus P1

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equals minus rho g
times y2 minus y1,

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and this is called Pascal's law.

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I prefer to write it
slightly differently,

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but it's the same thing.

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I write a plus sign here,
so I switch these around:

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rho g times y2 minus y1.

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So what it means
is I see immediately

292
00:17:41 --> 00:17:47
that if y2 minus y1
is positive--

293
00:17:44 --> 00:17:50
this is higher than this--

294
00:17:46 --> 00:17:52
that the pressure at P1 is
larger than the pressure at P2,

295
00:17:50 --> 00:17:56
but of course they are
completely identical,

296
00:17:53 --> 00:17:59
so this is
the hydrostatic pressure.

297
00:17:57 --> 00:18:03
This has
quite bizarre consequences.

298
00:18:01 --> 00:18:07
Suppose I had a vessel
that I filled with a liquid.

299
00:18:06 --> 00:18:12
It had a rather changed shape,
like so...

300
00:18:09 --> 00:18:15
a ratherstrange shape.

301
00:18:13 --> 00:18:19
So I would fill it
with liquid to this level,

302
00:18:21 --> 00:18:27
and the level here is y2.

303
00:18:23 --> 00:18:29
And let's take the bottom of
this vessel and call this y1.

304
00:18:28 --> 00:18:34
And so inside here
we have pressure P1

305
00:18:32 --> 00:18:38
and right there,
we have pressure P2.

306
00:18:38 --> 00:18:44
Well, what Pascal is saying now

307
00:18:41 --> 00:18:47
is that the pressure here
is everywhere the same

308
00:18:45 --> 00:18:51
because y2 minus y1 is the same
for all these points here.

309
00:18:50 --> 00:18:56
And so you will say, "Well,
that is sort of intuitive."

310
00:18:54 --> 00:19:00
You will say, "Look,
if I take here a column--

311
00:19:00 --> 00:19:06
"nicely cylindrical vertical
column, which has area A,

312
00:19:07 --> 00:19:13
"and I call this separation h,
for simplicity--

313
00:19:13 --> 00:19:19
"then the weight
of that column--

314
00:19:16 --> 00:19:22
"that's the weight
of the liquid--

315
00:19:20 --> 00:19:26
"would be the area times h--
that's the volume--

316
00:19:26 --> 00:19:32
times the density
of the liquid, rho, times g."

317
00:19:30 --> 00:19:36
And so you would say,
"That's a force."

318
00:19:33 --> 00:19:39
The weight... the bottom here
has to carry that weight

319
00:19:38 --> 00:19:44
and so the pressure
at the bottom

320
00:19:40 --> 00:19:46
is that weight
divided by the area,

321
00:19:43 --> 00:19:49
so that is rho hg.

322
00:19:44 --> 00:19:50
So you would say,
"That's very clear."

323
00:19:48 --> 00:19:54
Yeah, maybe, but how about here?

324
00:19:51 --> 00:19:57

325
00:19:53 --> 00:19:59
The pressure is the same,
it couldn't be any different.

326
00:19:56 --> 00:20:02
If the pressure
were different here,

327
00:19:58 --> 00:20:04
then the liquid
would start to flow.

328
00:20:00 --> 00:20:06
But here you don't have
that column h over you.

329
00:20:03 --> 00:20:09
You only have it here.

330
00:20:05 --> 00:20:11
And how about here?

331
00:20:08 --> 00:20:14
The consequence of Pascal's law

332
00:20:11 --> 00:20:17
is that if you had
a vessel like this

333
00:20:19 --> 00:20:25
and you filled it all the way
with liquid,

334
00:20:23 --> 00:20:29
that the pressure
here at the bottom

335
00:20:27 --> 00:20:33
would be exactly the same
as this vessel,

336
00:20:31 --> 00:20:37
which is filled with liquid
all the way to the bottom.

337
00:20:34 --> 00:20:40
And yet the weight of this

338
00:20:36 --> 00:20:42
is way more
than the weight of this.

339
00:20:38 --> 00:20:44
But yet, according
to Pascal's law,

340
00:20:41 --> 00:20:47
the pressure differential
is the same.

341
00:20:44 --> 00:20:50
It is not intuitive.

342
00:20:46 --> 00:20:52

343
00:20:49 --> 00:20:55
We live at the bottom
of an ocean of air.

344
00:20:56 --> 00:21:02
So here's the Earth
and here's air.

345
00:21:00 --> 00:21:06
And when we go up in y,

346
00:21:02 --> 00:21:08
then we also expect
that the pressure will go down.

347
00:21:07 --> 00:21:13
It doesn't go down linearly,
like liquids do.

348
00:21:11 --> 00:21:17
Liquids are linear
because rho doesn't change.

349
00:21:15 --> 00:21:21
In the case of air, the density
does change with altitude.

350
00:21:19 --> 00:21:25

351
00:21:21 --> 00:21:27
But if I can take
one square centimeter cylinder

352
00:21:30 --> 00:21:36
all the way to the top
of the atmosphere

353
00:21:33 --> 00:21:39
just like I did here,
in the liquids...

354
00:21:35 --> 00:21:41
I take a one square centimeter--
I could have taken area A--

355
00:21:40 --> 00:21:46
and I weigh all this air,

356
00:21:42 --> 00:21:48
then I would get the right
answer for the pressure here,

357
00:21:46 --> 00:21:52
because I do that there
and that works,

358
00:21:49 --> 00:21:55
so that should work here.

359
00:21:50 --> 00:21:56
And what I find, then...

360
00:21:54 --> 00:22:00
then I find that
at the bottom... at sea level,

361
00:21:59 --> 00:22:05
I find roughly one kilogram
force, which is ten newtons,

362
00:22:04 --> 00:22:10
per square centimeter.

363
00:22:06 --> 00:22:12

364
00:22:08 --> 00:22:14
It means, then,
that this whole column...

365
00:22:10 --> 00:22:16
If I take a one square
centimeter tubing

366
00:22:13 --> 00:22:19
all the way to the top
of the atmosphere--

367
00:22:16 --> 00:22:22
a few hundred kilometers--

368
00:22:17 --> 00:22:23
that that would weigh one
kilogram only, all that air.

369
00:22:22 --> 00:22:28

370
00:22:25 --> 00:22:31
One kilogram
per square centimeter.

371
00:22:27 --> 00:22:33
If you convert that
to newtons per square meter,

372
00:22:31 --> 00:22:37
then you get roughly
ten to the fifth pascal.

373
00:22:34 --> 00:22:40
And that is called, generally,
one atmosphere.

374
00:22:38 --> 00:22:44
It's called
the atmospheric pressure.

375
00:22:40 --> 00:22:46

376
00:22:45 --> 00:22:51
So the air pushes down on us--

377
00:22:47 --> 00:22:53
that gives us
the atmospheric pressure--

378
00:22:50 --> 00:22:56
not unlike the way
that liquid pushes down

379
00:22:53 --> 00:22:59
because of its weight

380
00:22:55 --> 00:23:01
and increased pressure
as you go down.

381
00:22:57 --> 00:23:03

382
00:22:59 --> 00:23:05
This atmospheric pressure

383
00:23:02 --> 00:23:08
is also often called
barometric pressure.

384
00:23:05 --> 00:23:11

385
00:23:11 --> 00:23:17
The idea of a barometer.

386
00:23:12 --> 00:23:18

387
00:23:16 --> 00:23:22
Here's my hand,
and my hand has an area

388
00:23:18 --> 00:23:24
of roughly
150 square centimeters.

389
00:23:22 --> 00:23:28
Force is always perpendicular
to the surface.

390
00:23:25 --> 00:23:31
I discussed that several times.

391
00:23:27 --> 00:23:33
For each square centimeter,

392
00:23:28 --> 00:23:34
there is an equivalent weight
of one kilogram

393
00:23:31 --> 00:23:37
due to the air above me.

394
00:23:33 --> 00:23:39
That means 150 kilograms
is pushing down on my hand.

395
00:23:37 --> 00:23:43
Why is my hand not going down?

396
00:23:39 --> 00:23:45
Well, because there's
also 150 kilogram pushing up.

397
00:23:42 --> 00:23:48
And so, I feel very comfortable.

398
00:23:45 --> 00:23:51
I don't even notice it

399
00:23:46 --> 00:23:52
that there is this huge force
in this direction

400
00:23:49 --> 00:23:55
and this huge force
in that direction.

401
00:23:52 --> 00:23:58
So how can I measure
this atmospheric pressure

402
00:23:56 --> 00:24:02
if I can't feel it?

403
00:23:57 --> 00:24:03

404
00:24:01 --> 00:24:07
The way that you can measure it
is by the following experiment.

405
00:24:09 --> 00:24:15
You take liquid, and you put
a hose in the liquid,

406
00:24:14 --> 00:24:20
as I will do very shortly.

407
00:24:17 --> 00:24:23
This is cranberry juice,
and this is the hose.

408
00:24:20 --> 00:24:26
And we're going to immerse the
hose completely into that liquid

409
00:24:25 --> 00:24:31
so that it's completely filled
with the liquid.

410
00:24:29 --> 00:24:35
And then we lift it out,
and as we lift it out,

411
00:24:34 --> 00:24:40
we will see that the liquid
will stay there.

412
00:24:38 --> 00:24:44

413
00:24:40 --> 00:24:46
It's the barometric pressure
that's pushing it in.

414
00:24:43 --> 00:24:49
And I pull it out
and pull it out

415
00:24:45 --> 00:24:51
and pull it out and pull it out

416
00:24:47 --> 00:24:53
and there comes a time that it
will not stay in there anymore.

417
00:24:54 --> 00:25:00
So it's way down there now,
the vessel, and then it lets go.

418
00:24:59 --> 00:25:05
And this is now empty,
and here is the liquid.

419
00:25:02 --> 00:25:08
And this is a way that we can
measure the barometric pressure

420
00:25:06 --> 00:25:12
and I will show you shortly
how that works.

421
00:25:08 --> 00:25:14
But let me first convince you

422
00:25:11 --> 00:25:17
that if I let all
this cranberry juice

423
00:25:15 --> 00:25:21
inside the tubing that I have...

424
00:25:18 --> 00:25:24
I put my finger on top
of the tubing and I lift it out.

425
00:25:22 --> 00:25:28
Notice that the cranberry juice
stays there.

426
00:25:25 --> 00:25:31
It's not running down.

427
00:25:27 --> 00:25:33
It's only when I take my finger
off the top--

428
00:25:31 --> 00:25:37
(whooshes )
then it goes down.

429
00:25:33 --> 00:25:39
But as long as I hold my finger
on the top, it isn't going down.

430
00:25:37 --> 00:25:43
If my hose were long enough--

431
00:25:39 --> 00:25:45
and we will know shortly
how long--

432
00:25:41 --> 00:25:47
it turns out to be more
than ten meters--

433
00:25:43 --> 00:25:49
then we would see the cranberry
break loose from the top.

434
00:25:46 --> 00:25:52

435
00:25:48 --> 00:25:54
And this is a way that you can
measure the atmospheric pressure

436
00:25:53 --> 00:25:59
and I will now be
more quantitative about that.

437
00:25:57 --> 00:26:03

438
00:26:00 --> 00:26:06
I will leave this equation
because I like that equation.

439
00:26:03 --> 00:26:09

440
00:26:06 --> 00:26:12
So imagine that
we have this experiment--

441
00:26:09 --> 00:26:15
which in the old days
wasn't done with plastic hoses,

442
00:26:12 --> 00:26:18
which was done
with glass tubes--

443
00:26:15 --> 00:26:21
and suppose I end up
here with liquid

444
00:26:20 --> 00:26:26
and here with such a tube

445
00:26:24 --> 00:26:30
and that the liquid had broken,
so it's empty here...

446
00:26:30 --> 00:26:36
and here is the liquid.

447
00:26:36 --> 00:26:42
This is y1, this is y2,
this is the pressure P1

448
00:26:42 --> 00:26:48
and right inside here is the
pressure P2 which is zero,

449
00:26:47 --> 00:26:53
because it's empty,
there is nothing.

450
00:26:50 --> 00:26:56
A little bit of vapor pressure,
but that's very small.

451
00:26:56 --> 00:27:02
And so this distance here...
let that be h.

452
00:27:00 --> 00:27:06
And so now what is
the pressure here at P1

453
00:27:03 --> 00:27:09
which, of course,
is the barometric pressure?

454
00:27:07 --> 00:27:13
It's just exposed
to the atmosphere.

455
00:27:12 --> 00:27:18
Well, P1 minus P2 equals
rho of the liquid times g,

456
00:27:19 --> 00:27:25
times y2 minus y1, which is h.

457
00:27:24 --> 00:27:30

458
00:27:25 --> 00:27:31
But P2 is zero,
so P1 equals rho gh

459
00:27:30 --> 00:27:36
and that is
the barometric pressure.

460
00:27:34 --> 00:27:40
So all I have to do
is take a liquid,

461
00:27:36 --> 00:27:42
know the density of the liquid,

462
00:27:39 --> 00:27:45
measure how far I have
to pull that hose up

463
00:27:42 --> 00:27:48
before the liquid breaks loose,

464
00:27:44 --> 00:27:50
and then I know
what the barometric pressure is.

465
00:27:48 --> 00:27:54
Now, this was done
in the early days,

466
00:27:52 --> 00:27:58
in the 17th century
by Torricelli.

467
00:27:55 --> 00:28:01
He used mercury
and he found that...

468
00:27:58 --> 00:28:04
Mercury, by the way,
has a density

469
00:28:01 --> 00:28:07
of 13.6 times ten to the third
kilograms per cubic meter.

470
00:28:07 --> 00:28:13
So this is mercury.

471
00:28:09 --> 00:28:15
He found that h is about 76
centimeters-- 0.76 meters.

472
00:28:16 --> 00:28:22

473
00:28:18 --> 00:28:24
This is a fact.

474
00:28:19 --> 00:28:25
It changes a little bit
from day to day.

475
00:28:21 --> 00:28:27
It could change
by a few centimeters--

476
00:28:22 --> 00:28:28
a little down, a little up.

477
00:28:24 --> 00:28:30
If it's up, the barometric
pressure is higher

478
00:28:26 --> 00:28:32
than when it's down.

479
00:28:29 --> 00:28:35
And so the barometric
pressure, P1,

480
00:28:32 --> 00:28:38
is then 13.6 times ten
to the third times g--

481
00:28:36 --> 00:28:42
for which I will take ten--
and the height is 0.76,

482
00:28:41 --> 00:28:47
and that is 1.03
times ten to the fifth pascal...

483
00:28:48 --> 00:28:54
which comes very close

484
00:28:49 --> 00:28:55
to the one-kilogram force
per square centimeter

485
00:28:52 --> 00:28:58
that I mentioned to you earlier.

486
00:28:54 --> 00:29:00

487
00:28:55 --> 00:29:01
One atmosphere's pressure is
defined in a very special way

488
00:29:00 --> 00:29:06
in a very precise way--

489
00:29:01 --> 00:29:07
namely, that it is
exactly the pressure

490
00:29:05 --> 00:29:11
when the column here
is 760 millimeters of mercury.

491
00:29:11 --> 00:29:17
Then we call the pressure here--
that's the definition--

492
00:29:15 --> 00:29:21
one atmosphere.

493
00:29:16 --> 00:29:22
Now you can do the same
experiment with water,

494
00:29:19 --> 00:29:25
whereas we tried to do it
with cranberry juice.

495
00:29:22 --> 00:29:28

496
00:29:24 --> 00:29:30
The density of water is 13.6
times lower

497
00:29:27 --> 00:29:33
than that of mercury,

498
00:29:29 --> 00:29:35
so the column has to
be 13.6 times higher

499
00:29:32 --> 00:29:38
than 76 centimeters,
which is about ten meters.

500
00:29:36 --> 00:29:42
So you would have
to raise this thing

501
00:29:38 --> 00:29:44
up to ten meters
before you would see the break.

502
00:29:40 --> 00:29:46
But you would have...

503
00:29:41 --> 00:29:47
then you've built yourself
a water barometer.

504
00:29:44 --> 00:29:50
If you do it with mercury,
you have a mercury barometer.

505
00:29:46 --> 00:29:52
You would see
this level go down.

506
00:29:48 --> 00:29:54
And if the pressure is high,
the weather is good;

507
00:29:50 --> 00:29:56
and if the pressure is low,
the weather is not so good.

508
00:29:53 --> 00:29:59
So you could build yourself
a water barometer--

509
00:29:56 --> 00:30:02
has to be ten meters long.

510
00:29:59 --> 00:30:05
The story has it that Pascal,
who was French,

511
00:30:02 --> 00:30:08
did the whole thing
with red wine.

512
00:30:05 --> 00:30:11
So he had a red wine barometer.

513
00:30:07 --> 00:30:13
It's very good to remember

514
00:30:10 --> 00:30:16
that ten meters of water
produces a hydrostatic pressure

515
00:30:15 --> 00:30:21
of one atmosphere.

516
00:30:16 --> 00:30:22
So if you go down
into the oceans by 100 meters,

517
00:30:20 --> 00:30:26
then the hydrostatic pressure
increases by ten atmosphere.

518
00:30:24 --> 00:30:30
So every ten meters
is one atmosphere.

519
00:30:27 --> 00:30:33

520
00:30:35 --> 00:30:41
Cornelis Van Drebbel--

521
00:30:36 --> 00:30:42
and I know how to pronounce
that name because I'm Dutch;

522
00:30:40 --> 00:30:46
he was a Dutch inventor--

523
00:30:42 --> 00:30:48
is usually credited with
building the first submarine

524
00:30:45 --> 00:30:51
in the very early 17th century,
around 1622.

525
00:30:48 --> 00:30:54
And he successfully operated
this submarine

526
00:30:52 --> 00:30:58
at a depth of about five meters.

527
00:30:54 --> 00:31:00
Imagine, five meters.

528
00:30:57 --> 00:31:03
The hydrostatic pressure
there is half an atmosphere.

529
00:31:00 --> 00:31:06
Ten meters, one atmosphere--
five meters, half an atmosphere.

530
00:31:04 --> 00:31:10
Nowadays, submarines go...

531
00:31:07 --> 00:31:13
It's a little secret
how far they go,

532
00:31:10 --> 00:31:16
but they have gone up to 3
feet, which is 900 meters,

533
00:31:14 --> 00:31:20
where the hydrostatic pressure
is 90 atmospheres.

534
00:31:18 --> 00:31:24
On every square meter
of that submarine,

535
00:31:22 --> 00:31:28
if it is at 900 meters,

536
00:31:24 --> 00:31:30
there is a force of 900 tons--
900,000 kilograms.

537
00:31:31 --> 00:31:37
Now, Van Drebbel's submarine

538
00:31:32 --> 00:31:38
was an enormous accomplishment
for the 17th century,

539
00:31:36 --> 00:31:42
because how are you going
to seal a vessel

540
00:31:39 --> 00:31:45
whereby the inside of the vessel
is one atmosphere--

541
00:31:43 --> 00:31:49
that's the air
that he was breathing--

542
00:31:46 --> 00:31:52
is five meters below the level,

543
00:31:49 --> 00:31:55
and so the outside pressure
is one and a half atmosphere?

544
00:31:53 --> 00:31:59
Namely, one atmosphere
barometric pressure

545
00:31:57 --> 00:32:03
and half an atmosphere
from the hydrostatic pressure.

546
00:32:00 --> 00:32:06
So there's an overpressure
on the vessel

547
00:32:02 --> 00:32:08
of half an atmosphere.

548
00:32:04 --> 00:32:10
So that means
on every square centimeter,

549
00:32:07 --> 00:32:13
there is a force pointing
inwards of half a kilogram--

550
00:32:11 --> 00:32:17
the equivalent
of half a kilogram weight.

551
00:32:13 --> 00:32:19

552
00:32:17 --> 00:32:23
Force is always perpendicular
to the surface,

553
00:32:20 --> 00:32:26
and if you would take two square
meters of his submarine,

554
00:32:23 --> 00:32:29
that would be a force
of 10,000 kilograms.

555
00:32:25 --> 00:32:31
It's amazing
that he managed to do that

556
00:32:28 --> 00:32:34
and that he could
actually operate

557
00:32:30 --> 00:32:36
his submarine successfully.

558
00:32:31 --> 00:32:37

559
00:32:33 --> 00:32:39
I can show you here in 26.100

560
00:32:36 --> 00:32:42
what kinds of forces Van Drebbel
was dealing with.

561
00:32:40 --> 00:32:46
You see there in front of you
a paint can.

562
00:32:46 --> 00:32:52
And I'm going
to evaporate the...

563
00:32:49 --> 00:32:55
not evaporate, I'm going
to evacuate the paint can.

564
00:32:54 --> 00:33:00
I'm going to pump the air out.

565
00:32:56 --> 00:33:02

566
00:32:59 --> 00:33:05
And so here is the paint can,
about 25 centimeters by 15.

567
00:33:06 --> 00:33:12

568
00:33:09 --> 00:33:15
And so it has equilibrium--

569
00:33:10 --> 00:33:16
there's one atmosphere outside,
one atmosphere inside.

570
00:33:13 --> 00:33:19
Paint can is happy.

571
00:33:14 --> 00:33:20
I'm going to suck the air out,
so I get an underpressure here.

572
00:33:18 --> 00:33:24
In other words, the pressure
outside is higher than inside--

573
00:33:22 --> 00:33:28
exactly the problem
that Van Drebbel had.

574
00:33:25 --> 00:33:31
The pressure outside is higher
than inside.

575
00:33:27 --> 00:33:33
You get an implosion.

576
00:33:28 --> 00:33:34
He managed to counter that,
to build it strong enough.

577
00:33:32 --> 00:33:38
When we take out the air here,
you can argue,

578
00:33:35 --> 00:33:41
well, then, the overpressure
is really one atmosphere

579
00:33:39 --> 00:33:45
and he only dealt
with half an atmosphere.

580
00:33:41 --> 00:33:47
Well, before we reach
this to be a vacuum,

581
00:33:45 --> 00:33:51
believe me, it already implodes.

582
00:33:47 --> 00:33:53
So the forces that we're dealing
with are very comparable

583
00:33:51 --> 00:33:57
to what Van Drebbel
was dealing with

584
00:33:53 --> 00:33:59
when he built his submarine.

585
00:33:55 --> 00:34:01
And so this can
will start to crumble

586
00:33:58 --> 00:34:04
when we take the air out,

587
00:34:00 --> 00:34:06
and that's another way of really
seeing the atmospheric pressure.

588
00:34:05 --> 00:34:11
So I take the pressure out
of the inside

589
00:34:08 --> 00:34:14
and the can
will literally be squeezed

590
00:34:11 --> 00:34:17
because of the ocean of air
that is hanging on us

591
00:34:16 --> 00:34:22
and is pushing down on us.

592
00:34:19 --> 00:34:25

593
00:34:21 --> 00:34:27
Okay.

594
00:34:22 --> 00:34:28

595
00:34:25 --> 00:34:31
It has to be
properly sealed,

596
00:34:28 --> 00:34:34
which is always a bit
of a problem.

597
00:34:31 --> 00:34:37
And so I have here a vacuum
pump, and let's pump on it.

598
00:34:35 --> 00:34:41
(can buckling )

599
00:34:39 --> 00:34:45
You can already hear
the crushing.

600
00:34:43 --> 00:34:49
The force on the front cover
alone...

601
00:34:45 --> 00:34:51
this is 375 square centimeters.

602
00:34:48 --> 00:34:54

603
00:34:52 --> 00:34:58
If the pressure inside
were zero,

604
00:34:54 --> 00:35:00
that would be a force
of 375 kilograms.

605
00:34:57 --> 00:35:03
But look--
it's not very happy, that can.

606
00:35:00 --> 00:35:06

607
00:35:04 --> 00:35:10
And these are the kind
of forces very comparable

608
00:35:07 --> 00:35:13
to what Van Drebbel was dealing
with in the 17th century,

609
00:35:10 --> 00:35:16
and he was able
to even operate his submarine

610
00:35:13 --> 00:35:19
under these forces,
without collapse.

611
00:35:15 --> 00:35:21

612
00:35:22 --> 00:35:28
Okay, I think we...

613
00:35:26 --> 00:35:32
you want to take this
as a souvenir?

614
00:35:28 --> 00:35:34
Oh, no, I can't give
that to you.

615
00:35:30 --> 00:35:36
We have to first take this off,
but you can pick it up later.

616
00:35:33 --> 00:35:39
That... that mouthpiece
is quite precious for us,

617
00:35:37 --> 00:35:43
because we have to use that
again, of course.

618
00:35:40 --> 00:35:46
So you see what tremendous
forces are at stake

619
00:35:43 --> 00:35:49
when you deal
with barometric pressure.

620
00:35:45 --> 00:35:51
If you go scuba diving, you go
to a depth of ten meters--

621
00:35:49 --> 00:35:55
could you stick a tube
in your mouth,

622
00:35:54 --> 00:36:00
which could go all the way
to the surface,

623
00:35:57 --> 00:36:03
and could you breathe?

624
00:35:58 --> 00:36:04
Well, there's no way.

625
00:36:00 --> 00:36:06
If you were here
and you have a tube...

626
00:36:06 --> 00:36:12
here's the water level...

627
00:36:08 --> 00:36:14
and if this is ten meters,
then the overpressure here

628
00:36:13 --> 00:36:19
between your lungs and the water
is one atmosphere, overpressure.

629
00:36:19 --> 00:36:25
So here is one atmosphere
barometric pressure.

630
00:36:21 --> 00:36:27
Here is one atmosphere
hydrostatic pressure

631
00:36:23 --> 00:36:29
plus the barometric pressure,

632
00:36:25 --> 00:36:31
so here you have
two atmospheres.

633
00:36:27 --> 00:36:33
So there is ahuge force
on your chest.

634
00:36:30 --> 00:36:36
Inside your lungs
is one atmosphere,

635
00:36:32 --> 00:36:38
outside is two atmospheres

636
00:36:34 --> 00:36:40
and there is no way
that you could breathe.

637
00:36:36 --> 00:36:42
If the area of my chest is
some 30 by 30 centimeters--

638
00:36:39 --> 00:36:45
which is a thousand
square centimeters--

639
00:36:41 --> 00:36:47
it would be like having
a hundred-kilogram weight

640
00:36:44 --> 00:36:50
on my chest, and also
on my back, of course,

641
00:36:46 --> 00:36:52
because it's
in both directions--

642
00:36:48 --> 00:36:54
it pushes like this
and it pushes like this

643
00:36:50 --> 00:36:56
and like this and like this.

644
00:36:52 --> 00:36:58
So you're really being
squeezed to death.

645
00:36:55 --> 00:37:01
So what do you do
when you go scuba diving?

646
00:36:57 --> 00:37:03
You need pressured air
with you in the tank

647
00:37:00 --> 00:37:06
and that you breathe,

648
00:37:02 --> 00:37:08
and so now,
with the pressured air,

649
00:37:04 --> 00:37:10
you can obviously counter

650
00:37:06 --> 00:37:12
the hydrostatic pressure
from the water.

651
00:37:09 --> 00:37:15

652
00:37:11 --> 00:37:17
Now suppose we go snorkeling.

653
00:37:13 --> 00:37:19
That's different.

654
00:37:16 --> 00:37:22
Then we do have a little tube
in our mouth, and we snorkel.

655
00:37:23 --> 00:37:29
How deep do you think
we could snorkel--

656
00:37:26 --> 00:37:32
that our lungs
could easily accommodate

657
00:37:29 --> 00:37:35
the hydrostatic pressure?

658
00:37:30 --> 00:37:36
Any idea?

659
00:37:32 --> 00:37:38
Do you think we could snorkel
maybe three meters?

660
00:37:36 --> 00:37:42
Who thinks we could
easily do three meters?

661
00:37:38 --> 00:37:44
Okay, who thinks
maybe only one meter?

662
00:37:42 --> 00:37:48
Who thinks
way less than one meter?
 

663
00:37:45 --> 00:37:51
Well, we know it's not way less,

664
00:37:47 --> 00:37:53
because snorkels
are this long, you know,

665
00:37:49 --> 00:37:55
so you know you can
at least do 30 centimeters,

666
00:37:52 --> 00:37:58
so it can't be
all that much less.

667
00:37:55 --> 00:38:01
Well, we can measure
how deep we could snorkel

668
00:38:00 --> 00:38:06
and we can measure
what the capacity...

669
00:38:03 --> 00:38:09
the capabilities
of our lungs are

670
00:38:06 --> 00:38:12
in order to counter
the hydrostatic pressure.

671
00:38:09 --> 00:38:15

672
00:38:11 --> 00:38:17
If I'm underwater, there
is pressure on my chest,

673
00:38:15 --> 00:38:21
and so to let the air out
is easy.

674
00:38:18 --> 00:38:24
You just...
I'm squeezed in, right?

675
00:38:20 --> 00:38:26
That goes out.

676
00:38:21 --> 00:38:27
But in order to inhale,
to suck in the air...

677
00:38:24 --> 00:38:30
(inhales )

678
00:38:25 --> 00:38:31
I would have
to push out my chest

679
00:38:29 --> 00:38:35
with a force that counters
the force due to the water.

680
00:38:35 --> 00:38:41
And so the question is,

681
00:38:40 --> 00:38:46
what kind of pressure
could I generate with my lungs

682
00:38:46 --> 00:38:52
to overcome
the hydrostatic pressure?

683
00:38:50 --> 00:38:56
And we are going
to measure that today

684
00:38:52 --> 00:38:58
with an instrument
that we call a manometer.

685
00:38:55 --> 00:39:01

686
00:38:58 --> 00:39:04
A manometer is a simple tube--

687
00:39:01 --> 00:39:07
it could be made of anything,
but a plastic tube will do fine.

688
00:39:07 --> 00:39:13
And we have liquid in here.

689
00:39:09 --> 00:39:15

690
00:39:11 --> 00:39:17
It's open here
and it's open here.

691
00:39:15 --> 00:39:21

692
00:39:18 --> 00:39:24
I put my mouth here,
and I'm going to see

693
00:39:21 --> 00:39:27
how much overpressure
I can produce in my lungs

694
00:39:25 --> 00:39:31
by pushing, by blowing.

695
00:39:26 --> 00:39:32
And so I'm going to blow in here

696
00:39:29 --> 00:39:35
and then this level will go down
and this level will go up.

697
00:39:33 --> 00:39:39

698
00:39:35 --> 00:39:41
And this height difference--
let's call that h--

699
00:39:40 --> 00:39:46
and the density
of this fluid is rho.

700
00:39:42 --> 00:39:48
We will use water for that,
colored water.

701
00:39:45 --> 00:39:51

702
00:39:47 --> 00:39:53
So the pressure here at y1
equals P1-- that's here--

703
00:39:55 --> 00:40:01
and the pressure here
at y2 equals P2.

704
00:40:00 --> 00:40:06
And so P1 minus P2
equals rho times h.

705
00:40:10 --> 00:40:16
I apply the law that
we still have here--

706
00:40:14 --> 00:40:20
P1 minus P2,
rho times h times g.

707
00:40:17 --> 00:40:23

708
00:40:19 --> 00:40:25
I know that the P2 level is
one atmosphere, that's correct.

709
00:40:26 --> 00:40:32
This is one atmosphere--
that's open to the world.

710
00:40:31 --> 00:40:37
So P1 equals one atmosphere
plus rho hg.

711
00:40:37 --> 00:40:43
And so what my manometer
indicates

712
00:40:39 --> 00:40:45
is how much pressure
I can generate

713
00:40:42 --> 00:40:48
over and above
the one atmosphere,

714
00:40:44 --> 00:40:50
and we call that overpressure.

715
00:40:46 --> 00:40:52
We often work
with overpressure gauges.

716
00:40:49 --> 00:40:55
When you go to the gas station

717
00:40:51 --> 00:40:57
and you have
your tire pressures measured--

718
00:40:54 --> 00:41:00
the pressure in your tires--

719
00:40:55 --> 00:41:01
there's also a gauge which
measures the overpressure.

720
00:40:58 --> 00:41:04
And so right there...
we have such a manometer.

721
00:41:04 --> 00:41:10
I'm going to blow in here,

722
00:41:06 --> 00:41:12
and that's going to tell
us immediately

723
00:41:09 --> 00:41:15
how deep I can snorkel.

724
00:41:10 --> 00:41:16
If I was able to make this
height difference ten meters,

725
00:41:17 --> 00:41:23
then I could snorkel at a depth
of ten meters in the water,

726
00:41:24 --> 00:41:30
because it means
that I could generate

727
00:41:27 --> 00:41:33
an overpressure
of one atmosphere.

728
00:41:29 --> 00:41:35
If this would only be
five meters,

729
00:41:31 --> 00:41:37
then I could only snorkel
down to five meters.

730
00:41:34 --> 00:41:40
If this is only a sad one meter,

731
00:41:36 --> 00:41:42
then I could really
only comfortably breathe

732
00:41:39 --> 00:41:45
one meter below the surface.

733
00:41:41 --> 00:41:47
And that's what you see here
and I want you to...

734
00:41:48 --> 00:41:54
Already you know it's not going
to be so fantastically high.

735
00:41:52 --> 00:41:58
Otherwise, these hoses
would be longer.

736
00:41:54 --> 00:42:00
This is the level
that we have now.

737
00:41:57 --> 00:42:03
On this side is atmospheric
pressure, that's open there,

738
00:42:00 --> 00:42:06
and on this side is also
atmospheric pressure, it's open.

739
00:42:03 --> 00:42:09
That's why the levels
are the same.

740
00:42:05 --> 00:42:11
This mark is 50 centimeters
above here and this is 50 below.

741
00:42:10 --> 00:42:16
So if I can generate
an overpressure in my lungs

742
00:42:13 --> 00:42:19
of one-tenth of an atmosphere,

743
00:42:15 --> 00:42:21
then this one would come up
50 centimeters

744
00:42:18 --> 00:42:24
and this one would go down
50 centimeters.

745
00:42:21 --> 00:42:27
That's one meter of water.

746
00:42:22 --> 00:42:28
One meter of water
is equivalent

747
00:42:24 --> 00:42:30
to a tenth of an atmosphere,
remember?

748
00:42:26 --> 00:42:32
Ten meters of water is one
atmosphere hydrostatic pressure.

749
00:42:30 --> 00:42:36
So if I can manage that,

750
00:42:32 --> 00:42:38
then I can generate an
overpressure in my lungs

751
00:42:35 --> 00:42:41
of a tenth of an atmosphere

752
00:42:38 --> 00:42:44
and I could snorkel, then,
at a depth of one meter.

753
00:42:42 --> 00:42:48
Let me try it.

754
00:42:44 --> 00:42:50
(inhaling deeply )

755
00:42:45 --> 00:42:51
(exhaling )

756
00:42:47 --> 00:42:53
(inhaling )

757
00:42:49 --> 00:42:55

758
00:42:56 --> 00:43:02
(exhaling noisily )

759
00:42:58 --> 00:43:04
A meter is impossible.

760
00:43:00 --> 00:43:06
You can have it maybe
for a few seconds,

761
00:43:02 --> 00:43:08
but not for very long.

762
00:43:04 --> 00:43:10
This is very disappointing.

763
00:43:05 --> 00:43:11
So you cannot even snorkel
at one meter.

764
00:43:08 --> 00:43:14
If someone else wants to try,

765
00:43:09 --> 00:43:15
I will cut this off,
so it's very hygienic.

766
00:43:12 --> 00:43:18
Maybe some of you can do better.

767
00:43:14 --> 00:43:20
You are the strong guy,
remember?

768
00:43:16 --> 00:43:22

769
00:43:17 --> 00:43:23
Now, when you blow,
don't make the liquid oscillate,

770
00:43:20 --> 00:43:26
because then
you can squirt it out.

771
00:43:23 --> 00:43:29
You should really try
to take a deep breath

772
00:43:25 --> 00:43:31
and then push
as hard as you can.

773
00:43:28 --> 00:43:34
STUDENT:
Okay.

774
00:43:29 --> 00:43:35
LEWIN:
Go ahead.

775
00:43:30 --> 00:43:36

776
00:43:37 --> 00:43:43
Strong man!

777
00:43:40 --> 00:43:46
You were about one meter
and 20 centimeters.

778
00:43:45 --> 00:43:51
This is great, terrific.

779
00:43:47 --> 00:43:53
(class applauds )

780
00:43:51 --> 00:43:57
How about sucking air?

781
00:43:55 --> 00:44:01
How much underpressure
could I generate in my lungs?

782
00:43:58 --> 00:44:04
Well, we can measure it
with this instrument.

783
00:44:01 --> 00:44:07
(inhales )

784
00:44:02 --> 00:44:08
I can go like this
and, of course,

785
00:44:04 --> 00:44:10
the liquid will go
the other way around.

786
00:44:07 --> 00:44:13
Maybe I can domuch better

787
00:44:08 --> 00:44:14
in terms of underpressure
than in overpressure.

788
00:44:10 --> 00:44:16
Let's try.

789
00:44:11 --> 00:44:17

790
00:44:20 --> 00:44:26
(class laughs )

791
00:44:22 --> 00:44:28
About the same,
a lousy one meter.

792
00:44:25 --> 00:44:31
When you're underwater, it's
never a problem to let air out,

793
00:44:30 --> 00:44:36
because due
to the hydrostatic pressure,

794
00:44:32 --> 00:44:38
there is a force on your chest.

795
00:44:35 --> 00:44:41
So letting the air out is easy.

796
00:44:37 --> 00:44:43
The problem is to expand
your lungs again

797
00:44:41 --> 00:44:47
to raise your chest.

798
00:44:43 --> 00:44:49
That means the problem
is that you can't...

799
00:44:46 --> 00:44:52
(inhales deeply )

800
00:44:47 --> 00:44:53
suck in the air,
and so it's really

801
00:44:50 --> 00:44:56
the second experiment that I did

802
00:44:52 --> 00:44:58
that determines how deep
you can snorkel underwater,

803
00:44:56 --> 00:45:02
and we found that
it's about one meter.

804
00:44:59 --> 00:45:05
So it's not the blowing out,
but it is the...

805
00:45:01 --> 00:45:07
(inhales deeply )

806
00:45:02 --> 00:45:08
sucking in.

807
00:45:03 --> 00:45:09

808
00:45:05 --> 00:45:11
And so, this weekend--
and this is a true story--

809
00:45:09 --> 00:45:15
I said to myself,

810
00:45:11 --> 00:45:17
"Gee, suppose I see someone...

811
00:45:14 --> 00:45:20
"From the second floor,

812
00:45:16 --> 00:45:22
"I look down on someone
on the first floor

813
00:45:19 --> 00:45:25
"having a great glass of juice
or wine or whatever, beer...

814
00:45:24 --> 00:45:30
and I would like to steal that
by sucking it up with a straw."

815
00:45:29 --> 00:45:35
(class laughs )

816
00:45:30 --> 00:45:36
Could I do that?

817
00:45:33 --> 00:45:39
And the idea would then being...

818
00:45:37 --> 00:45:43
I would be standing here.

819
00:45:40 --> 00:45:46
The person being unaware
of this glass down here

820
00:45:43 --> 00:45:49
with some great stuff in it...

821
00:45:45 --> 00:45:51
This would be my straw,
and I would just suck it up.

822
00:45:49 --> 00:45:55
And I decided over the weekend

823
00:45:51 --> 00:45:57
that the straw could not
be much longer, then,

824
00:45:54 --> 00:46:00
than about that one meter
that you just saw--

825
00:45:56 --> 00:46:02
that's the underpressure.

826
00:45:57 --> 00:46:03
In fact, when you go
to supermarkets

827
00:46:00 --> 00:46:06
and you buy yourself
this stuff for kids,

828
00:46:03 --> 00:46:09
you know that this can be done.

829
00:46:05 --> 00:46:11
You can suck up at least...

830
00:46:07 --> 00:46:13
this is maybe
40, 50 centimeters.

831
00:46:08 --> 00:46:14
That you should be able to do; 

832
00:46:10 --> 00:46:16
otherwise
they wouldn't sell them.

833
00:46:11 --> 00:46:17
But I didn't think that I could
do much more than a meter.

834
00:46:16 --> 00:46:22
And so I went to the supermarket
and I bought myself a hose

835
00:46:22 --> 00:46:28
and I bought the hose
to be two meters

836
00:46:25 --> 00:46:31
and I stood on the...
in the kitchen, like this.

837
00:46:29 --> 00:46:35
And I had a glass there
and I managed to do it.

838
00:46:34 --> 00:46:40
And it surprised me.

839
00:46:36 --> 00:46:42
So I went back to the super...
to the hardware store,

840
00:46:41 --> 00:46:47
got myself a three-meter
hose... tube.

841
00:46:44 --> 00:46:50
I knewfor sure that there
was no way I was going to do it,

842
00:46:47 --> 00:46:53
so I went to the second floor
of my home--

843
00:46:49 --> 00:46:55
I can look down
on the first floor; 

844
00:46:51 --> 00:46:57
that's the way
the house is built.

845
00:46:53 --> 00:46:59
And I can't believe it,
I can't believe it--

846
00:46:58 --> 00:47:04
why I can suck so well.

847
00:47:00 --> 00:47:06
It looks like it's almost a
violation of what I showed you,

848
00:47:05 --> 00:47:11
and so I need some help
from someone.

849
00:47:08 --> 00:47:14
And I'm going to demonstrate

850
00:47:10 --> 00:47:16
how good I am
in stealing someone's drink.

851
00:47:13 --> 00:47:19

852
00:47:16 --> 00:47:22
Could you assist me with that?

853
00:47:18 --> 00:47:24
Because you have to hold my
straw in that juice, you see,

854
00:47:22 --> 00:47:28
because I will go very high.

855
00:47:25 --> 00:47:31
And so you...
well, just stand here,

856
00:47:27 --> 00:47:33
and I will throw you
the hose in a minute.

857
00:47:30 --> 00:47:36
Stand a little bit on the side,
so that the class can see you.

858
00:47:33 --> 00:47:39
I'll see you shortly.

859
00:47:35 --> 00:47:41

860
00:47:42 --> 00:47:48
Hello.

861
00:47:44 --> 00:47:50
(class laughs )

862
00:47:47 --> 00:47:53
Okay, here's my straw.

863
00:47:50 --> 00:47:56
Can you put it in there?

864
00:47:53 --> 00:47:59
Now, as I'm going to try
to get this liquid up to me,

865
00:47:57 --> 00:48:03
I want you to think
about why I can do that

866
00:48:01 --> 00:48:07
whereas there I could
only do one meter.

867
00:48:03 --> 00:48:09
There is something very special.

868
00:48:05 --> 00:48:11
There's no way
that in my lungs--

869
00:48:07 --> 00:48:13
this is five meter almost--

870
00:48:09 --> 00:48:15
there's no way that I could have

871
00:48:11 --> 00:48:17
half an atmosphere
underpressure in my lungs;

872
00:48:13 --> 00:48:19
that is not possible.

873
00:48:14 --> 00:48:20
So somehow I don't do it
with my lungs,

874
00:48:19 --> 00:48:25
and maybe I won't even make it
in the first place.

875
00:48:21 --> 00:48:27
I can't talk when I do it.

876
00:48:24 --> 00:48:30
I cannot talk.

877
00:48:25 --> 00:48:31
There we go.

878
00:48:27 --> 00:48:33

879
00:48:54 --> 00:49:00
(class laughs )

880
00:48:56 --> 00:49:02
LEWIN:
Mmm.

881
00:48:57 --> 00:49:03
Mmm.

882
00:48:58 --> 00:49:04

883
00:49:01 --> 00:49:07
(class laughs )

884
00:49:04 --> 00:49:10

885
00:49:07 --> 00:49:13
Okay, I drank cranberry juice,
believe it or not.

886
00:49:11 --> 00:49:17
Think about all this
and try it at home, it's fun.

887
00:49:15 --> 00:49:21
Buy yourself a hose
that is even longer.

888
00:49:18 --> 00:49:24
Now watch it...
watch it, hold it.

889
00:49:20 --> 00:49:26
If I take my finger off,

890
00:49:21 --> 00:49:27
what do you think will happen
with the cranberry juice?

891
00:49:25 --> 00:49:31
It will run down, there it goes.

892
00:49:27 --> 00:49:33

893
00:49:29 --> 00:49:35
Okay, thank you.

894
00:49:31 --> 00:49:37
See you Wednesday.

895
00:49:32 --> 00:49:38

896
00:49:33 --> 00:49:39.000