Coriolis, bath water and science

It’s an interesting paradox that, while science is based on observation, any good scientist (or indeed any thinking person) will know that you can’t always believe what you see. This came home to me a while ago when having a light-hearted conversation about which way the bath water swirls as it goes down the plug hole. “This is a real effect”, a colleague said, “I saw it when I was on holiday in Africa”. Now, I’ve never been to Africa but what she was describing, which I have had confirmed by other people, is a way of extracting money from tourists in African countries which lie on the Equator.

When a tour bus arrives in a town bisected by the equator, usually conveniently indicated by a white line down the middle of the main street, a man will appear holding a large tin can with a small hole in the bottom. Holding his finger underneath the can on the hole , he fills it with water and adds a few leaves or twigs on the surface to show movement of the water. Standing on one side of the equator, he releases his finger, the water flows out and everyone can see which way it swirls out of the can. He then repeats the exercise on the other side of the equator and, lo, the water goes the other way. My colleague was convinced this was a real effect. She had seen it with her own eyes.

Now, the physics going on here is the Coriolis effect. I read about it in the “Feynman Lectures on Physics” which some relatives, both librarians, gave me the Christmas before my A-levels (bless them). Since then, nobody has told me that this is wrong (the Coriolis force, I mean, not getting nice Christmas presents). On the surface of the Earth the force varies with latitude – it is zero at the equator and a maximum at the poles. I freely admit I have not done the sums but, given the size of the Earth, I just can’t believe that walking a few yards to the other side of the road is going to change the direction and size of the force enough to affect the movement of a bit of water. If you lookat the way that ocean currents and winds circulate on the Earth, there is not much rotation quite a long way either side of the equator.

So there are two possibilities here. Either this is some effect unknown to physics or it is a trick. Maybe the guy gives the water a bit of a twirl as he pours it into the can. Occams Razor, or even just common sense, tells us that the first explanation is more likely.

So is there a paradox here? A paradox beween relying on obervation and not always believing what you see? Not, I think, if you are doing the science properly. I have said before that an observation on its own is no better than gossip – you need to try to fit it to an hypothesis. The hypothesis here is based on the effect of the Coriolis force, as seen in weather and ocean currents. If, as I suspect, calculation would show that the force is miniscule in a can of water then we need to seek an explanation – we need more data. One possible, and rather sneaky, way of doing this would be to take the man and his can to another location where the ‘equator’ is marked out, unkown to him, a mile or so away from the real equator. I bet he would still demonstrate water going both ways, especially if there were tourists there to give him some money. This isn’t judging such people, by the way. Good luck to them. I would do the same thing if I had to live in the sort of poverty that most people in Africa experience.

Tags: Coriolis force, hypothesis, observation, science