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Does Coriolis really cause water to drain one way?

  1. Aug 29, 2007 #1
    We've all heard that water spirals down the drain in a clockwise direction in the Southern hemisphere and counter- (or anti-) clockwise in the Northern hemisphere, due to the Coriolis effect from the rotation of the Earth. I've also heard that this is a myth, and that if you calculate the actual forces involved, those due to Coriolis are negligibly small compared to the random perturbations that typically occur in a basin of water that was filled relatively recently (or otherwise disturbed). The latter sounds likely to me, yet the "myth" persists, and moreover, you can see demonstrations of it on YouTube: .

    In this video, it appears that the water behaves as advertised, even though the basins are displaced from each other by no more than 10 meters or so. Yet, the one on the North side of the equator drains clockwise, the one on the South the opposite, and most amazingly, the one right on the equator doesn't swirl at all, but just drains straight down the hole!

    How can this be? I thought the Coriolis force depended on a change in the radial component of position, but in these cases it's hard to see how that's not completely negligible.
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Aug 29, 2007 #2

    Danger

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    The water effect is a myth; Coriolis forces are nowhere near that strong. If you do a forums search, you'll find quite a comprehensive thread about it elsewhere in PF.
     
  4. Aug 30, 2007 #3

    cristo

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    If you look at the third "experiment" there is a bit of dirt in the bowl before he lets the plug out. This is already moving in a circular fashion, so it suggests that the water is poured into a bowl in a clever way so that it drains however they want it to.

    This is the most common misconception, in my opinion. In the past, no-one has believed me when I tried to argue against it, so I've given up trying now! As danger says, a search of the forums will return a lot of threads on the topic.
     
  5. Aug 30, 2007 #4
    Okay, thanks for confirming my suspicions. We can drop this thread, since it's been covered before.
     
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