Derek: A couple of years ago my friend Destin
and I wanted to definitively answer the
question: does water actually swirl the
opposite direction down the drain in the
other hemisphere? At the time, I was
living in Sydney, Australia, and Destin
was in Huntsville, Alabama. Those are
about the same latitude, just opposite
sides of the Equator, so we created two
videos that were meant to be watched
simultaneously, and if you want to
synchronize them yourself, you can click
the link in the description, but we also
recognize that that was a hassle for
some people, particularly watching on
mobile devices, and that's why I'm now
releasing this extra wide video of both
videos synchronized. The Simpsons did a
whole episode based on toilet flushing
the opposite direction in Australia. Plus,
other shows have supposedly demonstrated
this effect.
Destin: So I have seen documentaries
that seemed to indicate that which
hemisphere you're in determines which
way the water is going to swirl, but
there's this other group of people, and
they seem really confident that it
doesn't matter where you're at, the
water's going to swirl however it wants.
Derek: so is this a real effect or not?
Hank: the application of this principle to
draining water in Earth's two hemispheres
is just bunk.
Destin: But if you ever just look
for yourself and figured out which way your
toilets swirls,
Derek: if you try it yourself
you'll find inconsistent results.
Destin: Here in Alabama I've noticed that some turn
counterclockwise and some go clockwise
Derek: this sink sometimes drains one way and
sometimes the other way.
Destin: You see, most will it have little jets in it, so the
swirl direction is determined by the
design of the toilet and not which
hemisphere you're in.
Derek: In any container of
water there's always going to be some
rotation: the water is not perfectly
still, and it is this, rather than the
hemisphere, that determines which way the
water will swirl down the drain.
Derek: so it's a myth: crossing the equator does not
mean the toilet's going to change
directions. But what if we come into the
garage and do a more controlled
experiment?
Derek: But what if we could
eliminate all motion from the water?
Destin: This is a one and a half meter kiddie pool.
Derek: I have here a five foot wide kiddie pool.
Destin: Instead of filling the pool with no vorticity at
all, I'm going against the way it's
supposed to drain.
So I'm trying to fill
it with a flow going clockwise.
Derek: I actually fill the pool in the
anti-clockwise direction to be sure that
any clockwise motion we see is no
due to the way I filled the pool.
Destin: So let's let the water settle for a complete day
so that we know that it's perfectly still.
Derek: I've left this water sitting here for 24
hours. So it seems like I damped all of
the motions from the filling.
Destin: I'm not going to reach in and pull a plug out
because that would induce some vorticity.
I'm going to use this valve that I have
connected to the bottom of the pool.
Derek: I really hope this works. Destin, wish me luck.
Destin: Good luck Derek.
Derek: I'm about to pull
the plug.
Destin: okay opening the valve in
Both: three, two, one
Destin: Water should be flowing
Derek: And the pool is draining. But you can't see any
motion of the water just yet.
been flowing for a couple of minutes and I
haven't seen anything yet, so we're going to
put some dye in it.
Derek: To help us see where the water is blowing I'm going to put
some food coloring in on the four
cardinal directions around the pool.
Destin: Check it out, it's like a tornado like right off
the bat.
We got a counterclockwise rotation.
We filled it up clockwise, and now
it's going counterclockwise.
Derek: You can clearly see that the water is
flowing clockwise, in this direction, and
that makes sense because that's how it
should flow in the southern hemisphere
due to the Earth's rotation.
Destin: We have a kiddie pool in my garage, and the whole
Earth is rotating and the water's going
counterclockwise because I'm in the
northern hemisphere. It's real! This is real!
Derek: But you can see what a tiny little
effect it is and what extraordinary
lengths I had to go to to see this
effect. So really, you're not going to see
it in the bathtub and you're not going
to see it in a sink or in a toilet
because there are other sources of
angular Momentum that totally wash out this effect.
Destin: we just proved it because we
just eliminated variables.
Destin: The Coriolis Effect is real.
Derek: The Coriolis Effect, There you have it. It works!
To understand how it works, imagine a pool
with one edge touching the South Pole.
Destin: Think about a pool near the North Pole.
Derek: the pool is stationary relative to Earth,
but every day it's actually completing one full revolution.
Destin: the Earth is spinning on its axis, so the pool spins around the
pole once a day.
Derek: Now you can see the side
of the pool furthest from the pole
travels much farther every day than the
side right next to the pole.
Destin: the pool is moving but the part that's
closest to the Equator has more momentum,
and the part that's closest to the pole has less.
Derek: So the outer side of the pool is
moving fastest towards the east and as
you get closer to the pole the velocity
decreases down to zero.
Destin: Think about these velocities relative to the drain in the middle.
Derek: Now, imagine we drain the pool
Destin: When we pull the plug, all the water
starts moving towards the middle.
Derek: Water from the far side is moving too fast
relative to the drain, and so it gets out
ahead, whereas water from near the pole
is going too slow, and so it lags behind.
Destin: The side nearest the equator is going
faster, so that water outruns the drain,
but the water nearest the pole is going
slower, so it falls behind.
Both: So when the water approaches the drain it swirls
(counter)clockwise.
Destin: this is the reason
hurricanes swirl counterclockwise in
the northern hemisphere,
Derek: And this is the
reason cyclones swirl clockwise in the
southern hemisphere.
Destin: The center of the
hurricane has lower pressure, just like a
drain, so the hurricane swirls just like our pool.
Derek: The higher pressure air rushes into
the eye of the storm, and just like in
our pool, swirls in the direction
dictated by the hemisphere.
Both: And that's the truth about toilet swirls.
[End of Sync]
Derek: Hey, I hope
you enjoyed that, and thanks for watching.
If you haven't subscribed to Smarter
Every Day, Destin's channel, yet then
click here, or if you want to see his
latest video, click here, and if you're
not subscribed to Veritasium for some
reason, you can do that by clicking here,
or see my latest right here. [disappears]