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[136호] 승인 2015.11.19  
트위터 페이스북 네이버 구글

Have you ever longed to satisfy your curiosity about daily science? Or, are you interested in learning scientific facts? There are two channels on YouTube that may surprise you. SmarterEveryDay and Veritasium are popular Youtube channels dealing with interesting science experiments.
The producers of these two channels once teamed up to tackle the same scientific question: the toilet swirl. Some people insist that the idea that a flushing toilet swirls in opposite directions in the northern and southern hemispheres is a groundless assumption. Is it real, or just a baseless myth, an urban legend? You can these this videos on Youtube or by following this link:

To approach this scientific mystery together, the producers, Derek Muller from Veritasium and Destin Sandlin from SmarterEveryDay, made two separate videos with elements and effects overlapping each other. In order to watch the two videos together, the viewer needs to synchronize them. Synchronizing videos is a bit tricky, so Muller and Sandlin provide another YouTube video explaining how to do it. The link to these instructions on how to synchronize videos is at the bottom of the “Toilet swirl” page. Once you get the two videos synchronized, you’ll have a very interesting YouTube video experience.

Now, let’s learn some things about the “toilet swirl,” and catch some new language, too.

· Blue dialog: Veritasium · Red dialog: SmarterEveryDay(SED)
· Veritasium: The Simpsons did a whole episode based on toilets flushing the opposite direction in Australia. Plus, other shows have supposedly demonstrated this effect.
· SED: So I have seen documentaries that seem to indicate that which hemisphere you’re in determines which way the water is gonna 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 gonna swirl however it wants.
· So is this a real effect or not?
· “The application of this principle to water draining in Earth’s two hemispheres is just bunk.”
· But have you ever just looked for yourself and figured out which way your toilet swirls?
· If you try it yourself, you’ll find inconsistent results.
· Here in Alabama I’ve noticed that some turn counter-clockwise, and some go clockwise.
· This sink sometimes drains one way and sometimes the other way.
· You see most toilets have little jets in it, so the swirl direction is determined by the design of the toilet and not which hemisphere you’re in.
· 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.
· So it’s a myth. Crossing the equator does not mean the toilet’s gonna change directions. But what if we come into the garage and do a more controlled experiment?
· But what if we could eliminate all motion from the water?
· This is a one and a half meter kiddy pool.
· I have here a five foot wide kiddy pool.
· 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.
· I actually filled the pool in the anti-clockwise direction to be sure any clockwise motion we see is not due to how I filled the pool.
· So let’s let the water settle for a complete day so that we know that it’s perfectly still.
· I’ve left this water sitting here for 24 hours. So it seems like I’ve damped all of the motion from the filling.
· I’m not gonna reach in and pull the plug out because that would induce some vorticity. I’m gonna use this valve that I have connected to the bottom of the pool.

· OK water’s been flowing for a couple of minutes and I haven’t seen anything yet, so we’re gonna put some dye in it.
· To help us see where the water is flowing, I’m going to put some food colouring in on the four cardinal directions around the pool.
· Check it out, it’s like a tornado, like right off the bat. We’ve got a counter-clockwise rotation. We filled it up clockwise and now it’s going counter-clockwise.
· 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.
· We have a kiddy pool in my garage and the whole earth is rotating, and the water is going counter-clockwise, because I’m in the northern hemisphere. It’s real! This is real.
· But you can see what a tiny little effect it is and what extraordinary lengths I had to go to see this effect. So really you’re not going to see it in a bathtub or in a toilet or in a sink because there are other sources of angular momentum that totally wash out this effect.
· We just proved it because we just eliminated variables.
· The Coriolis Effect is real! / The Coriolis Effect. There you have it... It works!

To understand how it works, imagine a pool with one edge touching the South Pole. The pool is stationary relative to Earth, but every day it’s actually completing one full revolution. 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. 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. Now imagine we drain the pool. 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. So when the water approaches the drain, it swirls clockwise.

Think about a pool near the North Pole. The earth is spinning on its axis so the pool spins around the pole once a day. The whole 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. Think about these velocities relative to the drain in the middle. When we pull the plug, all the water starts moving towards the middle. 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. So when the water approaches the drain, it swirls counter-clockwise. This is the reason hurricanes swirl counter-clockwise in the northern hemisphere. The center of the hurricane has lower pressure just like a drain, so the hurricane swirls just like our pool. And that’s the truth about toilet swirl.

Sang-yeop Lee
All photos by Veritasium and Smarter Every Day

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