More rotating toilet bowl insanity

[DaveC426913]

This dumb video is making the rounds of the dumb.

Apparently, it's not enough to promote that water goes down the drain in different directions in different hemispheres, but now apparently, you only need to go 10m to either side of the equator.

I'm thinking about making my own video showing the science of this.

It's pretty easy to show how vanishingly small a counter-force would need to be to eliminate the force - rotating a 15" bowl (48" circumference) at one inch every 30 minutes will completely cancel out the Coriolis force. Doubling that to 2 inches every 30 minutes would completely reverse it. And that's at the poles - the ideal condition where the force would be strongest.

I'm trying to figure out (in vain, I am certain) how much the force is reduced as you approach the equator.

10m is one millionth the distance from pole to equator, so the froce would be reduced by the cos of 999,999/1,000,000ths of 90 degrees or 89.999999 degrees?

Which I calculate to be 1 over 1.74e-8 or 0.0000000174.

 

[kuruman]

Ye olde Nigerian scamme.

 

[russ_watters]

This dumb video is making the rounds of the dumb.

I'm thinking about making my own video showing the science of this.

I bet you could demonstrate it with a small funnel under your kitchen faucet.

 

[anorlunda]

I'm most fond of the following pair of videos. These guys tried hard to eliminate biases and extraneous influences. The fun part is that the two videos, one from the USA and one from Australia, can be viewed side-by-side synchronized. In other words, play both simultaneously clicking start about the same time on each. The setup starts around 2:00 and the demo around 3:00.

 

[russ_watters]

I'm most fond of the following pair of videos. These guys tried hard to eliminate biases and extraneous influences.

Depending on one's formulation of the question, "biases and extraneous influences" may be the entire point of the question -- and even if not, they still might be the answer!

 

[DaveC426913]

Great. They've done two tests, and have a single data point for each.

Essentially, they flipped two pennies, once. One landed heads up while the other landed tails up. Proof!

 

[cjl]

The funny thing about that video is that they got it backwards. For a low pressure system (or a simulation of a low pressure system caused by a drain at the middle of a bowl), you'd expect counterclockwise rotation in the northern hemisphere, and clockwise in the southern.

 

[NTL2009]

Great. They've done two tests, and have a single data point for each.

Essentially, they flipped two pennies, once. One landed heads up while the other landed tails up. Proof!

Agreed. I actually like that guy's videos a lot, but you nailed it. Two data points is not proof. Interesting, but not proof .

 

[DaveC426913]

Agreed. I actually like that guy's videos a lot,

Totally. It was really well done. Which is why such a glaring error was so surprising.

 

[rootone]

I discovered that in the north hemisphere draining bathwater usually develops a clockwise rotation.
However it is very easy to change it into a stable anticlockwise rotation by messing around the water flow with hands.

 

[DaveC426913]

I discovered that in the north hemisphere draining bathwater usually develops a clockwise rotation.

:raises eyebrow suspiciously:
How many factors did you rule out?
Your bathtub may well have pre-existing factors that influence direction.

 

[sophiecentaur]

I discovered that in the north hemisphere draining bathwater usually develops a clockwise rotation.
However it is very easy to change it into a stable anticlockwise rotation by messing around the water flow with hands.

Did you ever repeat the test with the same bath, on the other side of the Equator?

 

[nasu]

I discovered that in the north hemisphere draining bathwater usually develops a clockwise rotation.
However it is very easy to change it into a stable anticlockwise rotation by messing around the water flow with hands.

How did you "discover" this? You mean "discover" as in looking up various sources of information or "discover" by doing experiments yourself?

 

[rootone]

How did you "discover" this? You mean "discover" as in looking up various sources of information or "discover" by doing experiments yourself?

I noticed as a matter of observation that if left to develop naturally the vortex would always be the same direction.
However, I could interfere with it and get it going in the opposite direction.
It would then remain rotating in the abnormal direction, without further assistance from me, it would not try to.revert to normal.

 

[sophiecentaur]

I noticed as a matter of observation that if left to develop naturally the vortex would always be the same direction.
However, I could interfere with it and get it going in the opposite direction.
It would then remain rotating in the abnormal direction, without further assistance from me, it would not try to.revert to normal.

That was less than half of a proper experiment. Did you try it in the southern hemisphere and did you do it with a range of containers?
Your only valid conclusion so far is that your drain has a bias to anticlockwise (or whichever it was).

 

[anorlunda]

Supposedly, the definitive answer to this question was published in Nature. I don't have access to Nature. Perhaps another PF member who does can help us out.

https://www.nature.com/articles/1961080b0

Edit: A more recent 2011 MIT source. National Committee for Fluid Mechanics Films http://web.mit.edu/hml/ncfmf.html also points to this video by Dr. Shapiro. Beginning at 19:45 in the video, he described the experiment.



Edit: I crossed posts with @baldersnatch. See the following post.

 

[Bandersnatch]

I noticed as a matter of observation that if left to develop naturally the vortex would always be the same direction.
However, I could interfere with it and get it going in the opposite direction.
It would then remain rotating in the abnormal direction, without further assistance from me, it would not try to.revert to normal.

The point the questions were trying to convey, is that looking at a poorly-controlled sample of 1, in one particular place is an extremely poor basis for drawing conclusions on how placement affects the direction of water.
So, did you take the same bathtub to various northern and southern latitudes, eliminated all possible interferences (level placement, water pressure in the tap, contaminations on the bathtub surface and in the water, temperature etc.), making several observations in each instance?
Or, did you observe a large number of different bathtubs at different latitudes (the higher the number, the more reliable the conclusions)?

Because looking at one bowl can't tell you anything whatsoever about the Coriolis effect on water direction, given how complex is the water flow in the tub. It's not a simple machine, like the Foucault pendulum. The direction may just as well be determined by the design of the bathtub. Then, your interference does not interfere with the Coriolis force, but with those design peculiarities.
With low sample size, or without controlling for all possible interferences*, you are not justified in making statements about the effect the fictitious force has. That's purely anecdotal evidence, and bad science.

*Ascher Shapiro did that in 1962; you can read about it here:
http://classic.scopeweb.mit.edu/articles/shapiros-bathtub-experiment/

 

[rootone]

I am not at all claiming this was a valid proof of anything, I was just messing around to see if the vortex necessarily had to be in the same direction.
It turned out that that the vortex could be established in either direction, and once established it stays that way.

 

[anorlunda]

I am not at all claiming this was a valid proof of anything, I was just messing around to see if the vortex necessarily had to be in the same direction.
It turned out that that the vortex could be established in either direction, and once established it stays that way.

We don't want to discourage amateur science. Certainly, one experiment is worth a hundred vehement opinions. But this particular case is infamous, with contested results dating back long before any of us were born and will no doubt continue long after we're long dead. It seems that Dr. Alfred Shapiro is the only one to have performed the experiment carefully enough to get published in a respected peer reviewed journal. But even Dr. Shapiro said, "You can't do it in an ordinary bathtub."

 

[brainpushups]

Two winters ago I filled up our spare tub to use the water in case we were without power for days during a big storm. We didn't end up losing power and I forgot about the tub for a few days. When I drained the water I noticed something I had never seen - two counterrotating whirlpools formed on opposite sides of the drain. I remember thinking Wow! The water really must not have any angular momentum about the drain before I opened it! Of course this is a different situation then the pools shown above.

I wish I had taken a video and we don't live in the same house now. To my recollection they moved relatively slowly toward each other and disappeared after they met. I don't recall additional whirlpools forming after these first two.

 

[DaveC426913]

The funny thing about that video is that they got it backwards. For a low pressure system (or a simulation of a low pressure system caused by a drain at the middle of a bowl), you'd expect counterclockwise rotation in the northern hemisphere, and clockwise in the southern.

You're right!

In anorlunda's post #16, at around 20:30, they explain that rotation in the northern hemisphere should be CCW.

 

[DaveC426913]

When I drained the water I noticed something I had never seen - two counterrotating whirlpools formed on opposite sides of the drain.

I've seen videos of this.

 

[Mister T]

I discovered that in the north hemisphere draining bathwater usually develops a clockwise rotation.

Did you test this idea with mare than just the one bathtub?

However it is very easy to change it into a stable anticlockwise rotation by messing around the water flow with hands.

In the 1960's a set of hemispherical bowls were carefully prepared and filled with water. After waiting several days for the water in the bowls to come to rest, a stopper was removed from the bottom of the bowls allowing the water to drain. Statistically, there was a greater likelihood that the water would rotate counterclockwise when the experiment was done at MIT. The entire apparatus was transported to Australia where there was a greater likelihood that the water would rotate clockwise.

From this we get the claim that it rotates in opposite directions on opposite sides of the equator. In fact, all we can say is that a tendency exists, and to realize that tendency takes more than simply watching a tub drain after you've taken a bath.

 

[DaveC426913]

Statistically, there was a greater likelihood that the water would rotate counterclockwise when the experiment was done at MIT. The entire apparatus was transported to Australia where there was a greater likelihood that the water would rotate clockwise.

Any news on those stats? How significant were they?

 

[happyhacker]

To get away from this bog standard way of discussing the effects, is this the way ocean swirls are created?

 

[rsk]

Ah, I spent a couple of years south of the equator and this is what everyone wanted to know - does the water go the wrong way round? In a past life I studied the Coriolos effect as a meteorologist but after more years than I care to remember working as a high school physics teacher, I wouldn't remember much about it now. Too much time in the company of surly teens kills brain cells ;)

I must confess that the movement of the stars and the sun was something that confused me immensely at first in the southern hemisphere - and then again in the north on return, but plug holes and toilet bowls I never lost much sleep over.

Sun clocks are definitely the wrong way round though

 

[George Jones]

From an Exercise 14.23 in the beautiful grad-level tome "Modern Classical Physics: Optics, Fluids, Plasmas, Elasticity, Relativity, abd Statistical Physics" by (Nobel laureate) Thorne and Blandford:

"One often hears the claim that water in a bathtub or basin swirls down a drain clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. In fact, on YouTube you are likely to find video demonstration of this ... Show that ... it is necessary that the water in the basin initially be moving with speed less than ..."

$$v \approx r \omega \sin \theta$$
Here, $r$ is the radius of the basin, $\theta$ is latitude, and $\omega$ is the angular velocity of the Earth's rotation. Using a basin that has a diameter of 1 metre, and $\omega = 2 \pi /\left( 24 \mathrm{ hours} \right)$ gives ...

 

[Dr Whom]

A fascinating phenomenon. Have you ever actually tried rotating a toilet bowl at even 1" every half an hour?
And can I hold you liable for the cost of the repairs to my bathroom?
Keep up the good work:)

 

[DaveC426913]

$$v \approx r \omega \sin \theta$$
Here, $r$ is the radius of the basin, $\theta$ is latitude, and $\omega$ is the angular velocity of the Earth's rotation. Using a basin that has a diameter of 1 metre, and $\omega = 2 \pi /\left( 24 \mathrm{ hours} \right)$ gives ...

Yeah, I back-of-napkin'd this in the OP. It's on the order of 1" per 30 minutes at the pole, and dropping to zero as you approach the equator.

 

[256bits]

Sun clocks are definitely the wrong way round though

CW or CCW?
It all depends if the clock is facing up, or facing down.
Most people look at the rotation as if a top view is the most natural perspective, but a bottoms up view is just as valid.
Even the videos from the science guys ( Get Smarter and V... ), at least I did not notice, do not state their reference perspective.

 

[rsk]

CW or CCW?
It all depends if the clock is facing up, or facing down.
Most people look at the rotation as if a top view is the most natural perspective, but a bottoms up view is just as valid.
Even the videos from the science guys ( Get Smarter and V... ), at least I did not notice, do not state their reference perspective.

Yes. I'd have liked to see it from underneath, but couldn't work out how.

 

[sophiecentaur]

but a bottoms up view is just as valid.

Valid but not intuitive. I always need to look twice at my Planisphere, just to make sure of where to find things in the night sky. That's the only example that I come across of the upside down world. Looking upwards at the vortex in my bath drain would upset the flow and require me to hold my breath a long time.

 

[bland]

It's not true to say that you cannot test this yourself. I remember as child over 50 years ago watching the water spiral down the bathtub drain and forcing it to spiral in the opposite direction by swirling it by hand, then gradually it closed up and resumed it's normal direction. I did not need a giant wading pool to see this.

 

[256bits]

Valid but not intuitive. I always need to look twice at my Planisphere, just to make sure of where to find things in the night sky. That's the only example that I come across of the upside down world. Looking upwards at the vortex in my bath drain would upset the flow and require me to hold my breath a long time.

Like @rsk stated also, it is difficult to visualize.
We look at hurricanes from a satellite view and for the NH they are spinning CCW - no argument there from anyone. ( anyone ? )
But tornadoes we look at from the side and up, and yet most of us, if not all, would still say the spin is CCW for the NH ( a few do spin the other way ).
I base my conclusions on at least 3 data points - you, me and rsk.

 

[sophiecentaur]

It's not true to say that you cannot test this yourself. I remember as child over 50 years ago watching the water spiral down the bathtub drain and forcing it to spiral in the opposite direction by swirling it by hand, then gradually it closed up and resumed it's normal direction. I did not need a giant wading pool to see this.

But what were you testing? You had just one bath tub in one location. That's not the basis for a proper verification of any rule - just the opportunity for a nice relaxing bath. But kids don't know about those.
I could build you a bath that would behave in exactly the opposite way in the same position and I doubt you would spot the difference (certainly not the 'you' of 50 years ago). If you can accept the calculation for the Coriolis Force for the sort of water movements in a bath then that tells you the force is way too small to be relevant. If y ou find it hard to accept that sort of calculation then why do have faith that the computer you are using will work and get your message back to me? The calculations used in making that system are way more complicated.