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Q & A: Flushing toilets south of the equator

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Most recent answer: 06/07/2013
Q:
When you flush the toilet in Australia, does the water move counter-clockwise? If so, why?
- Lisa (age 34)
Toronto
A:

Hi Lisa,

This is an age-old myth.   Although hurricane motion in the northern hemisphere is counter-clockwise and cyclones in the southern hemisphere rotate in a clockwise direction the Coriolis effect on water in a flushed toilet is minuscule compared to the effects of even a slight toilet bowl asymmetry.   Take a look at http://www.abc.net.au/science/articles/2010/03/24/2853824.htm#.UbXdQspdMhY for some more information.

LeeH

Hi Lisa,

We've answered this before. The Coriolis force is a real effect, but is proportional to velocity, so it usually is only noticeable if you are launching serious projectiles. It is strongest on the North and South poles, where it acts in opposite directions, and is zero on the equator.

But I wanted to add, I know that tourists on the equator are often shown demonstrations to "prove" that water  does flow in opposite directions on either side of the equator (for example, https://www.youtube.com/watch?v=m3jZ3Dx9m5Y). A big line is usually drawn which says "equator". Water is first emptied from a tub placed above this line, and it doesn't swirl much at all. Then, the water is drained on either side of the equator, and sure enough, the flow rotates in opposite directions on either side!

Now, the thought of all these tourists learning bad physics makes me furious. At the same time, however, the video is almost amusing; first of all, if you wanted to measure such a small effect as the Coriolis force, you wouldn't do it at the equator where the force is almost zero! And second of all, their manipulation of the "experiment" is extremely transparent.

The plug is first pulled while the tub is over the equator. Previously, the water had been sitting in the tub for a long enough time that the residual random motions of the water had died out. So, by conservation of angular momentum, almost no vortex was formed. Then, when the tub is moved on one side of the equator, the water is blatantly poured into the tub on one side of the drain-hole, giving the water an initial rotation in the desired direction. On the other side of the equator, the water is poured on the opposite side of the drain-hole, giving the opposite initial rotations. So, sure enough, the water rotates in opposite directions on either side of the equator!

But that's not the Coriolis force, that's simple conservation of angular momentum. You can test all these things for yourself in the kitchen.

If you ever see a demonstration like this, I suggest you do a magic trick, and embarass the presenter by making their water magically reverse direction!

(By the way, you can find more information about water going down tubs here: http://van.physics.illinois.edu/qa/listing.php?id=22302)

Cheers,

David Schmid


(published on 06/07/2013)

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