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I'm aware of the previous answer to this question. But my question is concretely about this video. Is it fake? How?

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    The first time the water is completely still, while in the other two cases the recipient is emptied while the water is still moving. Moreover, the second time the recipient is loaded from the left side while in the third experiment the water is poured from the right. – Dr. belisarius Mar 7 '15 at 15:30
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    As the answer to the previous question noted, it is very easy to influence the outcome of a small-scale Coriolis demonstration. – Mark Mar 7 '15 at 22:03
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    Note how he puts the leaves which help you see the water movement only after opening the drain. Otherwise you'd clearly see it's moving even before. – vartec Mar 12 '15 at 18:54
  • Water may not be a very good example since there are many influences besides the correole-effect. It is verified though that low pressure zones (low pressure forms as the air contains more water) move in different directions on the two hemispheres. – user25379 Mar 13 '15 at 14:09
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The video is likely fake.

Watch closely how he pours the bucket. We don't see him pour it the first time. However, the second time, he pours it into the left side of the sink (from his perspective). This causes the water to have a clockwise motion (since it flows upwards on the left side). The second time, he pours it into the right side of the sink. This causes the water to have a counterclockwise motion (since it flows upwards on the right side). It is likely that the first time, he poured it into the middle. The nature of the hoax is covered in more detail by astronomer and skeptic Dr. Phil Plait at The Bad Astronomy Blog

There is a similar trick I saw in Kenya, where they turn one way before putting a bucket down on side of the equator, turn the other way on the other side, and put it down without turning at all in the middle. Dr. Phil Plait also covers this hoax in his book, Bad Astronomy (google books link to the chapter)

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    Please provide some references to support your claims. – Oddthinking Mar 11 '15 at 23:40
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    I provided a reliable source that said it was definitely fake. – TheBlackCat Mar 12 '15 at 9:43
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    So we can't call something a trick unless the trickster admits to it? – TheBlackCat Mar 12 '15 at 12:03
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    I have provided two reliable sources from an expert in a relevant field that provides multiple studies showing that the given explanation could not possibly be correct and explains in detail why this could not be anything other than an intentional hoax. You have added what, by wikipedia standards, would be "weasel words", but have provided no sources that cast any doubt on my source. This would seem to me to be a clear case of original research on your part, which, as Oddthinking pointed out, is not allowed. – TheBlackCat Mar 13 '15 at 8:22
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    This page (dvandom.com/coriolis/sink.html) provides a calculation that indicates that coriolis cannot explain the rotation in a sink. This paper (nature.com/nature/journal/v207/n5001/abs/2071084a0.html) also suggests that the effect is too small (I think it is the study that Plait refers to). Note corolis effect will be weaker at the equator than anywhere else as that is where the earth is best locally approximated by a cylinder, so the effect will be smaller there than in the experiments in the paper. – Dikran Marsupial Mar 13 '15 at 17:08
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While TheBlackCat's answer (+1) is fully sufficient, this letter that appeared in Nature describes the authors experiments which show that (in Boston) if you set up the experiment very carefully, you can show that the Coriolis force reliably dictates the direction in which water drains from a basin specifically designed to negate other influences (see also southern hemisphere replication in this Nature letter). However, the rotation was only counterclockwise if you let the water settle for 24 hours and only started 12-15 minutes after opening the drain. The interesting fact is that the letter concludes:

"Incidentally, those who claim to have seen the direction of swirl change as a ship crosses the equator are surely pressing the case too far. At the equator the Coriolis forces vanish, and it would be virtually impossible to perform a valid experiment a short distance from the equator" [emphasis mine]

The Coriolis acceleration is proportional to the sine of the latitude (see e.g. wikipedia) so the last place on Earth the direction of draining would be dictated by the Coriolis force would be a couple of meters from the Equator.

So yes, we can be certain that the demonstration in the video is a trick, in the sense that it is not a legitimate demonstration that the direction of the vortex is governed by the rotation of the earth. The Coriolis effect is way too small to have created a vortex of that speed that quickly, and also the effect vanishes as you approach the equator anyway! The effect in this case is almost certainly due to the fact that the water is poured into the basin from the side that produces the desired vortex direction. This is something that was dealt with in the properly performed experiment by pouring the water into the vessel to create an initial rotation in the wrong direction, that would have to be overcome by the Coriolis force.

  • A very good answer. – Sklivvz Mar 14 '15 at 15:32

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