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Bathtub vortex

by Himal kharel
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Himal kharel
#1
Oct21-11, 08:29 PM
P: 79
Is it true that bathtubs in northern hemisphere the water drains out in clockwise rotation and those in southern hemisphere in anticlockwise direction?
If so then why?
And in equator?
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DaveC426913
#2
Oct21-11, 08:30 PM
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False. A big honkin' stinky myth that just will not die.

Snopes has a good description of this silliness.
http://www.snopes.com/science/coriolis.asp

Frankly, I think even they are too generous. They talk about it as a "miniscule" force that is "easily overwhelmed" by larger forces.
Unfortunately, this getting-a-foot-in-the-door caution leads people to "...OK but a perfectly symmetrical bowl filled with reeeeeally still water would - in theory - still exhibit blah blah blah..."

Do this thought experiment.

Pick up a bucket of water. Hold it still.
Over the course of ten minutes, rotate it 2.5 degrees - a turn of about two centimeters.

That's less than 2cm over ten minutes. That's how much torque is applied to the water by the Earth turning. Now, the 10 seconds it takes for a toilet to drain is 1/60 of that - 300 micrometers - on the order of the width of a human hair.
DaleSwanson
#3
Oct22-11, 03:13 AM
P: 351
Wikipedia details the lengths necessary to observe the Coriolis effect in draining water:
In 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.[28] 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-foot (1.8 m) tank with 300 US gallons (1,100 l) of water and allowing it to settle for 24 hours (to allow any movement due to filling the tank to die away), in a room where the temperature had 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 Boston, Massachusetts, in the Northern hemisphere). This was repeated and the results averaged to make sure the effect was real. The report noted that the vortex rotated, "about 30,000 times faster than the effective rotation of the earth in 42 North (the experiment's location)". This shows that the small initial rotation due to the earth is amplified by gravitational draining and conservation of angular momentum to become a rapid vortex and may be observed under carefully controlled laboratory conditions.
http://en.wikipedia.org/wiki/Corioli...bs_and_toilets

Danger
#4
Oct22-11, 05:07 AM
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Bathtub vortex

Quote Quote by DaleSwanson View Post
Wikipedia details the lengths necessary to observe the Coriolis effect in draining water:

http://en.wikipedia.org/wiki/Corioli...bs_and_toilets
If I ever had a toilet that took more than 15 minutes to flush, I would have to install at least 2 more bathrooms in my house. In any practical situation, the hemispheric influence is unmeasurable.
You could also counteract any such Coriolis effect by training your turds to curl in the opposite direction and thus redirect the fluid flow.

Quote Quote by Himal kharel View Post
And in equator?
At the equator, the water just hovers over the drain because it can't make up its mind.
DaveC426913
#5
Oct22-11, 10:03 AM
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Quote Quote by Danger View Post
...training your turds to curl in the opposite direction...
And I now have my weekend project!
Danger
#6
Oct22-11, 10:46 AM
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Quote Quote by DaveC426913 View Post
And I now have my weekend project!
Keep it fair too, Dude. You're not allowed to swallow a Bratwurst whole.
Lsos
#7
Oct22-11, 11:35 AM
P: 774
My earth science teacher in high school specifically taught us that the toilet water spins due to the coriolis effect. When they teach it in school, I'm not surprised the myth keeps going.
Danger
#8
Oct22-11, 11:55 AM
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Quote Quote by Lsos View Post
My earth science teacher in high school specifically taught us that the toilet water spins due to the coriolis effect.
Yeah... I couldn't figure out why my Grade 12 chemistry teacher was such an idiot until about 10 years later when I found out that once you have your degree in Education they assign you to what you will teach, whether or not you know anything about it.
HallsofIvy
#9
Oct22-11, 12:15 PM
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Most states (in the United States, specific requirements for teaching are set by the state and so vary from state to state), a secondary school teacher must have an undergraduate degree in the subject, with a minor in education. Of course, once they are teaching, they may be assigned by the principal to teacher a course in different area. Too much of that can cause trouble with the school's accreditation.

In the elementary grades, teacher's typically have degrees in education.
Grizzled
#10
Oct22-11, 01:13 PM
P: 33
Never mind that vortex. The best myth I heard in that area was that this same force determines... the direction your cat wraps its tail around when sitting! I always knew cats are neat and picky but THAT picky? Hmm.
Danger
#11
Oct22-11, 01:43 PM
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Quote Quote by Grizzled View Post
this same force determines... the direction your cat wraps its tail around when sitting!
That is true, and very irritating. Normal cats are ambidextrous, so they don't care. Mine is dyslexic, so she just stands there whipping it around until she get too tired and falls over.


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