Simpsons did it!

June 4, 2015 2:52 PM

The definitive internet videos demonstrating the Coriolis Effect, which makes water drains counter-clockwise in the northern hemisphere (by SmarterEveryday) and clockwise in the southern hemisphere (by Veritasium). Necessary to watch them in with youtubedoubler.
posted by numaner (26 comments total)

That was really fun.
posted by theora55 at 3:06 PM on June 4, 2015

Previously on As the World Turns...
posted by Ratio at 3:15 PM on June 4, 2015 [2 favorites]

That was fun, if a little breathless for an old fogey like me who is cynical enough to wonder whether a sample size of two is really adequate for investigating events with only two possible outcomes.
posted by merlynkline at 3:26 PM on June 4, 2015 [6 favorites]

The very wikipedia article forming the first link in the post says:

Contrary to popular misconception, water rotation in home bathrooms under normal circumstances is not related to the Coriolis effect or to the rotation of the earth, and no consistent difference in rotation direction between toilet drainage in the Northern and Southern Hemispheres can be observed. The formation of a vortex over the plug hole may be explained by the conservation of angular momentum: The radius of rotation decreases as water approaches the plug hole, so the rate of rotation increases, for the same reason that an ice skater's rate of spin increases as they pull their arms in. Any rotation around the plug hole that is initially present accelerates as water moves inward.

posted by kenko at 3:30 PM on June 4, 2015 [13 favorites]

On my phone such that I can't load video at the moment, so I am probably missing the point, but doesn't the Coriolis Effect only apply to things like whirlpools & tornadoes? I thought it had been demonstrated that toilets and sinks were more strongly influenced by bowl design than by global inertia.
posted by jsnlxndrlv at 3:32 PM on June 4, 2015 [1 favorite]

I thought it had been demonstrated that toilets and sinks were more strongly influenced by bowl design than by global inertia.

Speaking as someone who has actually tested the phenomenon in both hemispheres, I can say this was my experience as well.
posted by Celsius1414 at 3:37 PM on June 4, 2015

Speaking as someone who watched the videos, I can say that y'all might try that.
posted by pjenks at 3:42 PM on June 4, 2015 [23 favorites]

They make it pretty clear that for the vast majority of drains of water the Coriolis effect is so very weak it is normally drowned out, and even both demonstrate drains in their respective hemispheres that are going against the direction predicted by the effect. They do a decent job of isolating their actual test from interference, and their result is compelling, although I wouldn't say definitive.
posted by ckape at 3:50 PM on June 4, 2015 [2 favorites]

I guess the northern hemisphere one is in Alabama, which is 32°N. His pool is about 2m across, so the south end of his pool goes about 3.3m further each day than the drain (the radii differ by sin(32) * 1m, and their daily travel distance is 2 π r).

That's only a difference of like 40 μ/s. If this were all the way at the north pole, this would be double. It's hard to tell how long the process took, since the video is sped up a lot, but a particle steadily moving straight from the outside of the pool to the drain over half an hour would be going about 10x that speed, which is close enough that I have no idea how legit this is.
posted by aubilenon at 4:11 PM on June 4, 2015 [3 favorites]

I really like Smarter Every Day, but I don't trust this video. The effect, even on a pool that size, is small enough that I expect they just got lucky with all the other random non-isolated factors giving them the results they hoped for.
posted by 256 at 4:40 PM on June 4, 2015 [5 favorites]

I do wish that they'd shown multiple experiments, but the two videos thing is neat.
I had one on each monitor, it worked amazingly well, until the end when they were pointing the wrong way. So, Northern Hemisphere goes on the left monitor!
posted by Just this guy, y'know at 4:44 PM on June 4, 2015

Ack, that doubler link has the left side on the right side and vice-versa. It messed up some of their editing tricks. But still cool.
posted by Hamusutaa at 4:46 PM on June 4, 2015

Yeah... The Coriolis effect, in relation to the planet, presents much bigger than this, and ONLY presents much bigger than this. Hurricane rotation? Yes. Ocean current rotation? Yes. Kiddie pool water rotation? No. Even the former two aren't 100%.
posted by Sys Rq at 4:55 PM on June 4, 2015 [1 favorite]

This meets the Mythbusters standard for scientific rigor, so I'm satisfied.
posted by Mr.Encyclopedia at 5:29 PM on June 4, 2015 [1 favorite]

In many parts of the world, where the equator is marked where it passes through a well-touristed area, there'll be at least one guy hanging around who'll demonstrate the effect for you for whatever amount of money you can be persuaded to part with. They'll typically have a basin with a hole in the bottom, which they'll fill with water, and you can stand with them a few of meters to one side of the equator sign and you can watch the water swirl out. Then they'll walk with you an equal distance to the other side and demonstrate it again.

Of course they're practiced at getting it to swirl one way or the other at will without you catching on, it's not at all difficult. But hey, y'know, free enterprise.
posted by George_Spiggott at 6:00 PM on June 4, 2015 [2 favorites]

Cecil Adams answered this question in 1983.
posted by fifteen schnitzengruben is my limit at 6:06 PM on June 4, 2015 [2 favorites]

Here's an example of a huckster Coriolis demo from Ecuador (pay attention to what side of the tank she pours the water in).

I recall watching a video demonstration of the Coriolis effect back in undergrad days. Perhaps it was this one, but like Cecil Adams mentions to get it right the water needs to rest a long time after being poured into the tank so that any residual momentum gets dissipated before draining the tank.
posted by plastic_animals at 6:41 PM on June 4, 2015 [1 favorite]

The National Committee on Fluid Mechanics made an educational film on vorticity that discusses this very phenomenon. (skip forward to ~19:40)
posted by indubitable at 7:17 PM on June 4, 2015 [4 favorites]

Quick and dirty swapped youtubedoubler link.
posted by reprise the theme song and roll the credits at 8:43 PM on June 4, 2015 [1 favorite]

Goodness, indubitable, that video was narrated by Ascher Shapiro, who wrote the little Science Study Series paperback on fluid flow I read in jr. high (Shape and Flow [?]) and who I later found out was a very eminent figure in fluid dynamics.

Watching that I realized, among many other things, that I hadn't really understood why airplane wings shed those tip vortices.

Really excellent stuff!
posted by jamjam at 8:58 PM on June 4, 2015

Haha, whoops, my back-of-the-envelope calculations were off by an extra 3‰, because I used the length of the solar day, while clearly it's the length of the nearly 4-minute shorter sidereal day that is relevant.
posted by aubilenon at 12:51 AM on June 5, 2015

Yeah, but if we reverse the spin of the earth, these results become completely invalid!
posted by blue_beetle at 5:44 AM on June 5, 2015

Ack, I had the correct youtubedoubler link with the two videos in the correct places and I pasted the wrong link..., thanks reprise!
posted by numaner at 6:27 AM on June 5, 2015

It's worth noting that they did do multiple experiments, although they only showed one.

It's also not really comparable to the hucksters at the equator, since a) they are much farther from the equator, so the effect may actually be noticeable, b) they intentionally tried to remove any external effects, even going so far as to fill the pools the opposite direction that one would expect them to drain, and c) they aren't really trying to sell you anything.
posted by vernondalhart at 10:13 AM on June 5, 2015 [1 favorite]

Oh they made a both on one page version themselves.

Also, comment from the southern Hemisphere one;

"In case you don't want to read the description: we repeated this experiment at least three times in each hemisphere and saw the same effect every time. We were in fact replicating an experiment conducted at MIT and in the southern hemisphere in the 1960s which used an almost identical setup and got the same results. If you are not convinced by our findings we invite you to prove us wrong. Thanks for watching!"

I clearly did not read the description, which says:

We each repeated the experiment 3 times, and got the same results every time. For those of you who might be skeptical, great! The good news is that right circularly prismatic kiddie pools are only $10 and you can do the experiment for yourself at your latitude. There's really no reason you shouldn't do this on your own, so before you criticize how we've done it, I'd strongly encourage you to do it for yourself.

and also references this site about Coriolis effects on artillery.
posted by Just this guy, y'know at 10:36 AM on June 5, 2015 [2 favorites]

Well, I'm sure if you asked my downstairs neighbors they might have a reason or two for me to not try to replicate this on my own.
posted by ckape at 1:46 PM on June 5, 2015