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Eureka! Down the Drain

water-783355_640Note:  This is the part of a series of posts about science activities to be done in the bathroom – the lav-oratory if you will.  

You’ve probably heard the old saying that the water in sinks and toilets drain clockwise in the Southern Hemisphere or wondered what would happen if you flushed a toilet at the equator. But have you ever wondered if the old saying is really true?

Try it! (from First Place Science Fair Experiments for Inquisitive Kids)


Fill the sink halfway with water. Turn the water off completely and wait at least 5 minutes so that the water is completely still. Make sure no one walks near the sink or disturbs the water in any way. You want to be sure that the direction in which the water drains is not due to other forces.

Slowly and carefully remove the plug and watch the water drain down the sink. Record the direction in which the water spins. Note whether the direction changes while the water drains.


Find a clean toilet and, just as you did with the sink, make sure the water is very still. Flush the toilet. Record the direction in which the water spins. Also, note whether water pouring back into the toilet affects the direction in which the water drains.

Make sure to test each sink/toilet at least 3 times to make sure your results are consistent. Test as many sinks and toilets as you can to determine if the draining water swirls in the same direction.

Photo by Ken Yee
Photo by Ken Yee

What is happening?
The Coriolis force is caused by the rotation of the Earth. Everything on the planet rotates with the Earth. If you were able to look down on the North Pole of the spinning Earth, you would see that it turns counterclock-wise. If you then look at the South Pole, you would see it spins clock-wise. Check this out on a globe!

But because the Earth rotates relatively slowly—about once every day—this force is extremely small. For very large amounts of slow-moving fluids, such as the atmosphere, it has a larger, lasting effect. Hurricanes in the Northern Hemisphere spin counterclockwise and in the Southern Hemisphere they spin clockwise—just like the Earth. Also like the Earth, hurricanes turn relatively slowly and the direction of the spin is controlled by the direction in which the Earth spins.  Check out this cool demonstration that you can try at home.

For smaller amounts of a fluid, such as the water in your sink, this force is strongly outweighed by even the tiniest disturbances and design flaws in the sink. These factors are more likely to influence the direction in which the water drains than the Coriolis force does.

For more on draining sinks and the Coriolis effect, read Bad Coriolis and this article from Discover.