Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Haggai on 23/05/2010 16:30:02
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Haggai asked the Naked Scientists:
Dear Chris,
Why is the vortex of say water, clockwise in the Southern hemisphere and anticlockwise in the northern?
Thanks
What do you think?
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It usually isn't; it's a myth.
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The direction of rotation relative the Earths axis is just the same but when you are in the southern hemisphere your feet are pointing towards the north pole so everything appears to be reversed.
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Not visible in bathtub or toilet bowl, but the phenomenon does occur ...
n 1908, the Austrian physicist Otto Tumlirz described careful and effective experiments which demonstrated the effect of the rotation of the Earth on the outflow of water through a central aperture. The subject was later popularized in a famous article in the journal Nature, which described an experiment in which all other forces to the system were removed by filling a 6 feet (1.8 m) tank with 300 US gallons (1,100 l) of water and allowing it to settle for 24 hours (to remove any internal velocity), in a room where the temperature has stabilized. The drain plug was then very slowly removed, and tiny pieces of floating wood were used to observe rotation. During the first 12 to 15 minutes, no rotation was observed. Then, a vortex appeared and consistently began to rotate in a counter-clockwise direction (the experiment was performed in the Northern hemisphere, in Boston, MA). This was repeated and the results averaged to make sure the effect is real. The report noted that the vortex rotated, "about 30,000 times faster than the effective rotation of the earth in 42° North (The experiments location)" Thus, the Coriolis effect does indeed play a role in vortex rotation for draining liquids that have come to rest for a long time and may be observed under carefully controlled laboratory conditions.
http://en.wikipedia.org/wiki/Coreolis_effect#Draining_in_bathtubs_and_toilets
However these guys (http://www.youtube.com/watch?v=TmVWkoH4W_I&feature=related) are fooling the tourists.
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It usually isn't; it's a myth.
In terms of water going down plug holes, it does seem to be mostly mythology, but in terms of atmospheric conditions, it seems to be real enough.
http://en.wikipedia.org/wiki/Anticyclone
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I'm surprised no-one has mentioned the Coriolis effect. I'm assuming that's the natural force that might influence the direction of flow. It works something like this:
If you were standing on a spinning planet (such as Earth) and you shot a cannon then, from the perspective of someone on that planet, the cannon ball would travel in a straight line. However, because the ball is travelling through a medium which offers resistance (the atmosphere), the cannon ball is actually being pulled round in the same direction the planet is rotating in. So from the perspective of an observer sitting in space the cannon ball will follow a curved trajectory.
The same applies to water falling down a plughole. The water wants to travel in a vortex but it needs something to kick-start it off and decide which direction the vortex will spin. In the absence of any other outside force the Coriolis effect will be the deciding factor - clockwise in the southern hemisphere and anticlockwise in the north. In practise, the planet Earth takes so long to make a revolution, the water is such a small volume and the effect is oblique (unless your bath is actually sitting on the North or South Pole) so it's effect is negligible. Other influences, such as small eddies within the bath water, are what really count.
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It usually isn't; it's a myth.
In terms of water going down plug holes, it does seem to be mostly mythology, but in terms of atmospheric conditions, it seems to be real enough.
http://en.wikipedia.org/wiki/Anticyclone
There are more bath plugs than cyclones; that's why I said "usually". (It affects ocean currents too)
Incidentally, I suspect that the North and South poles and the equator are the only places where the Coriolis effect wouldn't influence the water.
The effect happens when you change latitude.
Also, it doesn't require you to be travelling through a medium that offers resistance. It would apply to a skater on frictionless skates travelling towards the pole from the equator, even if there were no air resistance.
If you start at the equator, facing the pole, you are travelling sideways at roughly 1000 miles an hour because the Earth is spinning.
As you skate in a straight line towards the pole your sideways velocity falls (to zero when you get there).
The Coriolis force is the force that changes your sideways velocity from about 1000 mph to zero.
If you moved fast enough it would be quite a big force. For bathwater the force is tiny.
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Incidentally, I suspect that the North and South poles and the equator are the only places where the Coriolis effect wouldn't influence the water.
Yes, of course you are absolutely right. I remembered that the Coriolis effect applies across changes in latitude but had forgotten the correct mechanism.
So... the reason no-one had mentioned the coriolis effect is because it's irrelevent! [:-'(]
Please excuse me why I find a rock to crawl under.