An Urban Myth

There is an urban myth that water will swirl down a sink in an anti-clockise direction in the Northern hemisphere, and in a clockwise direction in the Southern hemisphere. This is due to a force called the Coriolis effect, discovered by a French scientist by the name of Gaspard-Gustave Coriolis, in 1835. Here is a definition of the Coriolis effect:

“An effect whereby a mass moving in a rotating system experiences a force (theCoriolis force) acting perpendicular to the direction of motion and to the axis of rotation. On the earth, the effect tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern and is important in the formation of cyclonic weather systems.”

But the myth is not true. At least not for small bodies of water like sinks, toilets, or baths. It only works on large bodies of water, or mass, such as whirlpools in the ocean, or hurricanes.

You can test this in your bathroom, next time you take a shower. Watch which direction the water swirls down the plughole, then take the shower head and direct the water in the other direction. The water will follow whichever direction has the greatest “push” (like the jet of water from the shower head). Now try the sink – in a sink full of water, let the water drain, then turn the direction around using your finger. See? It’s not the Coriolis effect at work, it’s just the shape of your sink or bath that causes it – and that would be the case whether you were in the Northern or Southern hemisphere.

As they say on one of my favourite TV programmes, “Myth Busted” :0)

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