[MUSIC PLAYING]
Hi.
I'm Aaron Johnson, a PhD
student in the Department
of Aeronautics and
Astronautics here at MIT.
And it's about 2
o'clock in the morning.
We're in the library, but
we're not working on homework
or not working on a project.
We're actually proving that
the Earth is rotating around
on its axis.
How are we doing that?
It's light task, I know.
We're using a
160-year-old device
called a Foucault's
pendulum that we built here
this evening.
So we'll show you how we
built it, how it works,
and like I said, prove
that the Earth really
is spinning around.
And so in the words
of Foucault himself,
"You are invited to come
see the Earth turn."
[INAUDIBLE] is off getting
ready for our demonstration.
But how exactly is
this going to work?
I mean, how can a
Foucault's pendulum
show that the Earth is
rotating around on its axis?
Let's take a look.
The pendulum is
composed of two parts.
There's a long string,
or wire in our case,
with a bob at the
end, a heavy weight.
When you start it
swinging, a pendulum
swings back and forth until
acted upon by an outside force.
This is called inertia.
A Foucault's pendulum,
if left for a long time,
will appear to
precess, or rotate.
So if we look at the
pendulum from the top,
you would start
swinging like this.
And then in the
Northern Hemisphere,
after time, it will appear like
this, and after more time still
like this.
It's appearing to
precess clockwise
in the Northern Hemisphere.
It's opposite in the
Southern Hemisphere.
So what if we have a pendulum
at the North or South Pole?
On this spot, the
Earth is twisting,
and that includes the
pendulum building.
But the pendulum
doesn't actually move.
So while the pendulum
appears to rotate
to an observer in
the building, it's
actually the building turning
while the pendulum stays
in the same position.
The pendulum precession
period is 24 hours.
What if the pendulum
is at the Equator?
This spot on Earth
isn't twisting,
but it's traveling eastward
on Earth's surface.
Here, the pendulum
won't precess at all.
So these are the
two limiting cases.
But what about in between?
At this spot on Earth, there
is twisting and traveling
eastward.
The pendulum will show this
twisting motion, but not
the traveling component
of the motion.
As a result, the
precession period
will be greater than 24 hours.
There's a simple formula
that lets us find the angular
velocity of our pendulum,
represented by omega,
in degrees per day.
Omega is equal to 360 times
the sine of the latitude.
Here in Boston, our latitude
is 42.36 degrees North.
So this gives us
an angular velocity
of 242.56 degrees per day.
This tells us that our
pendulum will precess
10.1 degrees every hour.
This is called the
Coriolis effect.
Looking from the top down,
pendulums in the Northern
Hemisphere precess clockwise,
while pendulums in the Southern
Hemisphere precess
counterclockwise.
This has effects on Earth
that we can see every day.
Take hurricanes and
typhoons, for example.
If we have little bits of wind
in the Northern Hemisphere
rotating clockwise, we're going
to get an overall rotation
that's counterclockwise.
So hurricanes and typhoons
in the Northern Hemisphere
rotate counterclockwise.
In the Southern Hemisphere,
the opposite is true.
Hurricanes and typhoons
rotate clockwise.
Many people think that the
Coriolis effect also causes
the water in your toilet bowl
to rotate counterclockwise
in the Northern Hemisphere
and clockwise in the Southern
Hemisphere.
It's not really true.
The Coriolis effects are
there, but they're so small
that they're really
overwhelmed by other factors,
such as which way the jets
in the toilet are pointing
and how the bowl was filled.
So it's, unfortunately,
not true.
[MUSIC PLAYING]
When we start our
Foucault's pendulum,
we need to make sure not to
put any sideways motion on it.
Otherwise, we're going to
see that motion, and not
the rotation of the Earth.
So we're going to start our
pendulum the traditional way.
[MUSIC PLAYING]
We got the pendulum
swinging, but it's
going to take a lot for there
to be any visible precession.
So let's skip ahead one hour.
In one hour, the pendulum has
precessed about 10 degrees.
But we can also see
that its amplitude
is smaller than before.
This is due to damping.
We used a heavy
bob and a long wire
to reduce the
effects of damping,
but you can never
completely eliminate them.
Foucault's pendulums that you
see in science museums have
an electromagnet that gives
the wire a kick each swing
and keeps the pendulum swinging.
[MUSIC PLAYING]
There's also a bit of sideways
oscillation in the pendulum.
You can see it traces
out a narrow ellipse.
This is likely because the
wire is made of many smaller
wires twisted together, and
then as [INAUDIBLE] the tension,
it tends to untwist a bit.
This introduces a
sideways torque,
leading [? through ?]
the elliptical motion.
However, we can
still clearly see
the precession of the
pendulum, and thus
the rotation of the Earth.
Ah.
Well, I'd say with
that experiment
we proved that the
Earth really is
rotating around its axis, which
is what we hoped to discover.
So I guess that's good.
Not bad for a night's work.
But I really think I'm
going to head out and get
some sleep now.
[MUSIC PLAYING]
