There are videos and articles on the internet which demonstrate that water flows

Question

There are videos and articles on the internet which demonstrate that water flows down a flush clockwise in Northern Hemisphere and anti-clockwise in Southern Hemisphere.

Here are a couple of links which claim to demonstrate this fact : http://www.youtube.com/watch?v=Pb69HENUZs8 http://www.youtube.com/watch?v=z9uN9rcgJ1s

There are others which call this a total myth, and one of them is a very popular website. Link : http://science.howstuffworks.com/science-vs-myth/everyday-myths/rotation-earth-toilet-baseball2.htm

Interestingly, I got introduced to this by a Children's Fact Book. I am not quite convinced that it is a myth.

Can you suggest a way to proceed towards an analytical treatment?

Explanation / Answer

Well, first we know that the Coriolis force acting on the scale of a toilet is going to be a pretty small force. So the question can generally be tackled in the following way:

Estimate the magnitude of the Coriolis force on the toilet water. Use this to estimate the magnitude of effect of the Coriolis force on the toilet water spin.
Enumerate the possible other sources of spin for the toilet water. The two major ones off the top of my head would be any angling of the water jets, and the shape of the toilet bowl. Estimate the magnitude of the effect of these sources of spin.
Compare the two effects. You'll probably find an answer which is something of the form:

"If the angling of the toilet jets is less than ? for a jet velocity of v, then the Coriolis force should dominate"

Essentially what I'm doing is asking "How idealized would this toilet have to be for the Coriolis force to be the dominant source of spin in the toilet water?"

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