The Coriolis Force: Or Why Toilets Flush the Same Everywhere

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Like many of you readers out there, I seem to overlook the small things in life.  One of those is what direction the water in my toilet rotates in when it flushes.  Why does this matter?  Well, if you are like me, you have likely grown up hearing that toilet water in the Northern Hemisphere flushes in a different direction than toilet water in the Southern Hemisphere.  That is to say that when Australians use the toilet in America, they will aptly notice that the water is flowing in the opposite direction.

Well I learned this past Thursday in Physical Oceanography, that is not the case!

As it turns out, toilet water flushes down in the same direction in both the Northern and Southern hemisphere.  Why is that?  Well, like almost everything else in the world, it is due to physics.  Why though, did this rumor about flushing in different directions start?  As it turns out, it too is based in physics, but it is just inaccurate.

The Coriolis force is a deflection of moving objects when they are viewed on a rotating reference frame.  Well, the Earth is a rotating surface, therefore objects that move in the Northern Hemisphere are deflected towards the right and objects that move in the Southern Hemisphere are deflected to the left.  It is this force that the rumor is based on.  However, whoever started this wives tale didn’t take their Physical Oceanography class very seriously because this only applies to objects that move for a time period of over a day (since that is how long it takes for the Earth to rotate completely) and for a distance to the order of kilometers.

Since toilets do not take over a day to flush and the water in a toilet is moving in a space that is less than even a meter across, it is safe to say that the Coriolis force does not affect what direction the water flushes down a toilet.  Next time you flush, just know that the water moves down that toilet in the same direction as it does when a kangaroo uses a toilet in Australia as well.

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