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Coriolis effect & why toilets flush clockwise

 
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mistux
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Joined: 25 Jun 2004
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Location: South Bend, Indiana USA

PostPosted: Fri Aug 19, 2005 1:28 pm    Post subject: Coriolis effect & why toilets flush clockwise Reply with quote

Claim: Thanks to the Coriolis effect, toilets flush clockwise in the northern hemisphere and counterclockwise in the southern.
Status: False.


Because the Earth is a globe spinning on an invisible axis, a point at its equator will make a circuit of 25,000 miles in the space of a day, but any point not on the equator will make a shorter round trip; the closer that point is to either of the poles, the shorter its trip will be. Put another way, a pencil at the equator travels 1,030 miles an hour, whereas another at Sarasota, Florida, moves at 930 miles per hour, and one at the North pole doesn't move at all.

This apparent difference in speed results in the Coriolis force, an effect that imparts a twist to largish events happening away from the equator. Best known is its effect on air masses: as they move away from the equator, their speed (which matches that of where they started from) appears to be faster than that of the surface over which they are traveling. This sets them turning in a clockwise twist in the northern hemisphere and a counterclockwise twist in the southern. (Before you complain we have that backwards, please click here.) Likewise, a cannonball fired due north will veer a teeny bit to the east, and one fired to the south will deflect ever so slightly to the west, something a skilled gunner would make an adjustment for.

The twisting effect of the Coriolis force is real and does influence certain large things like the movement of air masses, but the effect is so small that it plays no role in determining the direction in which water rotates as it exits from a draining sink or toilet. The Coriolis effect produces a measurable effect over huge distances and long periods of time, neither of which applies to your bathroom. Toilets and sinks drain in the directions they do because of the way water is directed into them or pulled from them. If water enters in a swirling motion (as it does when a toilet is flushed, for example), the water will exit in that same swirling pattern; as well, most basins have irregular surfaces and are not perfectly level, factors which influence the direction in which water spirals down their drains. The configuration of taps and drains is responsible for the direction of spin given to water draining from sinks and bathtubs to a degree that overwhelms the slight influence of the Coriolis force.

The belief that the Coriolis force influences the direction in which water drains from plumbing fixtures is widespread and has been repeated as fact in a number of venues, including popular television shows (such as world traveler Michael Palin's Pole to Pole) and even in textbooks. We can only speculate on why people are so enamored of this snippet of misinformation, guessing that it has something to do with the desire to find some of the mysteries of science in the realm of the everyday.

Source: Here
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CNCJimmy7
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Joined: 05 Jul 2004
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Location: Minneapolis, MN

PostPosted: Fri Aug 19, 2005 1:38 pm    Post subject: Reply with quote

Do bathtubs drain counterclockwise in the Northern Hemisphere? 15-Apr-1983
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Dear Cecil:

My sources tell me that water, when it spirals out of a drain, flows in one direction only in the Northern Hemisphere and in the opposite direction in the Southern Hemisphere. Is this so? Why? Is the spiral especially pronounced at the poles? Subdued at the equator? If I carried a drain across the equator would the spiral reverse directions? How do drains work in outer space? I know this probably seems like a lot of questions, but I have an unquenchable thirst for learning. --Victor C., Chicago


Dear Victor:

Lucky you found me then. Given the limitations of a once-a-week column, it is maybe an exaggeration to describe myself as an everlasting fountain of knowledge. But I am definitely an unstoppable leak.

The erroneous bit of folk wisdom you refer to says that water always drains in a clockwise direction in the Southern Hemisphere, and in a counterclockwise direction in the Northern Hemisphere. The supposed reason for this "fact" is the Coriolis effect, which has to do with the effect of the earth's rotation on moving objects.

Well, there is such a thing as the Coriolis effect. It explains why macroevents such as hurricanes rotate in a clockwise direction in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. However, when you get down to itty-bitty phenomena such as the water draining out of your bathtub, the Coriolis effect is insignificant, amounting to roughly three ten-millionths of the force of gravity (in Boston, at least, which is where they happened to do the measuring).

The boring truth is that water drains every which way no matter what hemisphere you're in, for reasons which have to do mostly with the shape of the drain, the way you poured in the water in the first place, and so on.

All this was demonstrated way back in 1962 by one Ascher Shapiro, a researcher at the Massachusetts Institute of Technology. Shapiro filled a circular tank six feet in diameter and six inches high in such a way that the water swirled in a clockwise direction. (Remember, now, that Coriolis forces in the Northern Hemisphere act in a counterclockwise direction.)

Shapiro then covered the tank with a plastic sheet, kept the temperature constant, and sat down to read comic books or whatever scientists do while they wait for their experiments to percolate. When he pulled the plug after an hour or two, the water went down the drain clockwise, presumably because it still retained some clockwise motion from filling.

On the other hand, if Shapiro pulled the plug after waiting a full 24 hours, the draining water spiraled counterclockwise, indicating that the motion from filling had subsided enough for the Coriolis effect to take over. When the plug was pulled after four to five hours, the water started draining clockwise, then gradually slowed down and finally started swirling in the opposite direction.

Needless to say, unless you are a consummate slob, you do not wait 24 hours (or even 4-5 hours) to drain your bathtub. Hence the influence of the Coriolis effect may be safely described as slight.

But I'm sure the myth of the bathtub spirals will endure. Shapiro did his work in 1962 and I proclaimed it to the world in 1983. Yet next to the mystery of where all the baby pigeons are, this remains the commonest question I get.


Source: CECIL ADAMS http://www.straightdope.com/classics/a1_161
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