water down the plughole... background information
The science of sinks...
Any one who's ever pulled a plug out of a sink full of water will know that water spins as it goes down the plughole.
But why? Scientists have investigated this and discovered lots of possible reasons, but one which has always been controversial is called the Coriolis Force. This force influences weather across the globe, and causes systems like cyclones and hurricanes to rotate in different directions on opposite sides of the globe.
For many years people have also debated whether it can also influence rotation on a smaller scale - for example, which way water spins when it goes down the plughole!
more about the coriolis force
It's the way the Earth rotates on its axis that creates this Coriolis Force. It is named after a scientist called Gustave-Gaspard Coriolis, who first described the force in 1835.
It’s also known as the ‘deflective force’- this name gives us a clue to what it’s all about. Here's how it works.
If you were to fire a missile from the ground, aiming in a straight line at a place a long distance away, you would soon get frustrated because mysteriously, you would miss your target. From where you were standing it would look as though the missile had not travelled in a straight line at all, but had somehow veered off course in a bent line. This would seem weird to you - until you remembered that the earth rotates. From the time you fire the missile until the time it lands, the earth has rotated. Meaning the place you are firing at is now no longer where it once was!
From our perspective on the ground, it looks as though the line of travel has bent. This effect is called the Coriolis Force.
It is quite difficult to see from the earth's surface, as we move along with the ground too. But you can see it happening on a roundabout, if you 'fire' balls at another person sitting opposite you.
From above the ball is seen to move in a straight line. When seen from the roundabout itself the ball appears to be deflected to the right.
so why does all this happen?
It's all to do with the shape of the Earth: a sphere.
The circumference of the Earth at the equator is larger than it is at the poles. But the whole Earth rotates once every 24 hours, so the surface of the earth at the equator actually moves faster than it does at the poles. This difference in speeds causes an object like a missile, moving over the Earth’s surface, to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Currents and winds are also deflected in this way. Cyclones and hurricanes spiral as air rotates around a point of low pressure. In the Northern Hemisphere the air around a hurricane or cyclone spins anti-clockwise, and clockwise in the Southern hemisphere.
So we know the Coriolis Force exists - but because the Earth’s rotation (only once per day) is small, the effect is also small. For things on the surface of the earth, the effect is even smaller. And on the scale of your sink, it's likely to be very, very, very small indeed. Can the Coriolis force influence which way water goes down the plughole in your sink? It all depends on who you ask!
the debate
Scientists argue over whether or not the Coriolis effect can be seen in your sink or bath tub. Trials in 1962 by AH Shapiro, a physicist at Mississipi's Technology Institute, showed an anti clockwise spiral after he let the water rest for several days. Shapiro's experiments were confirmed by five scientists at Sydney University in Australia in 1965, who found water drained in the clockwise direction. However, another scientist, Merwin Sibulkin was not successful in his experiment, when he used a small bathtub.
For more information on the debate go to our web links page.
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