View Full Version : Coriolis Force

29th Oct 2002, 15:17
I'm having some troubles convincing a friend of mine that the Coriolis Force is not responsible for the water in the sink/toilet going down clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.
Does anybody out there know of a website that goes through the numbers to show just how small the effect is on those timescales?
I have had no luck thus far, as most websites seem to only enter into a qualiatative discussion of the problem.
Any suggestions would be greatly appreciated.

Cheers, Captianowie

29th Oct 2002, 15:19
I suggest you watch the excellent Simpsons episode on this subject. It was shown last weekend on Sky, but, knowing Sky, I'm sure it will be shown again very shortly....




29th Oct 2002, 15:40
Bad Coriolis (http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html)

Bad Astronomy (http://www.badastronomy.com/bad/misc/index.html)

tony draper
29th Oct 2002, 15:43
Its not that small, large tankers have to take corilois force into account when navigating.you have to feed in opposit rudder or you find yourself sailing at 45degrees from where you intended. ;)

ps, this only effects big ships, so the Royal Navy don't need to bother with it now. ;)

29th Oct 2002, 15:44
Don't know about a website but here goes as an engineering solo!

Coriolis acceleration equals 2 times the product of velocity and the rotational speed of the platform on which the object (i.e. the particle of bathwater) is travelling.

The effect is maximum at the poles, where the earth's rotational vector (vertical) and the water velocity (horizontal) are 90 degrees apart. But the earth's rotation, in radians per second, is 2 times pi, divided by the number of seconds in a day which is 24x60x60.

If you assume that bathwater travels towards the plughole at one sixth of a foot per second (pretty speedy...), multiplying 2 by the earth's rotation by one sixth gives an acceleration of 0.0000242 feet per second. Thus, to travel down the plughole in a straight line, some force would have to be acting on the water particle to give it that acceleration. Because nothing exists to produce that force, the water accelerates sideways and misses the plughole! That's why, theoretically, it goes one way in the northern hemisphere and the other in the south.

But the numbers are so small it is generally accepted that the natural turbulence in the water exerts far greater forces than the Coriolis reaction ever could. So in reality it makes no difference to bathwater. To an aircraft, however, travelling at 600 feet per sec over the rotating earth, the effect is big enough that you have to take account of it in inertial navigation systems.

It's actually a vector cross-product, which is why I set my example at the poles. Head north or south in an aircraft at the Equator and the effect is nil.

Phew! Hope that helps!

29th Oct 2002, 15:57

I've also noticed that after drinking several pints of beer extreme opposite rudder is required & even then it's difficult to maintain an accurate course :D :D

Send Clowns
29th Oct 2002, 21:52
I had heard this some time ago, and working in the kitchen of an Hotel, happened to notice that I was draining the two sinks simultaneously, and each was spiralling in a different direction. If your friend will believe me then I can assure him you are correct (trust me, I'm a pilot ... :eek: )

tony draper
29th Oct 2002, 22:09
Indeed Mariner, also known as departure from controlled walking.

Anthony Carn
29th Oct 2002, 22:12
If I recall, one can reverse the existing direction of rotation by swirling ones hand in the opposite direction, in the water spiral directly above the plughole.

I'll have a go and report back, unless someone beats me to it.

It feels good to have a purpose in life.

29th Oct 2002, 22:27
Personally I think it's nonsense.

From what I understand, while there is a Coriolis effect on water in the sink, the shape of the sink and other factors are more influential in determining which way the water goes down.

Kalium Chloride
29th Oct 2002, 22:34
I think Scudhunter's correct. Having just spent a few priceless moments repeatedly filling up the bathroom basin, dropping small bits of sponge on the surface, and pulling the plug out, I've discovered that the water in the basin spirals both clockwise and anticlockwise in more-or-less equal measure.

Sounds like this Coriolis thing is a crock unless you're firing shells at Paris from 80km away.

30th Oct 2002, 00:03
No one has spotted yet that I should have said 0.0000242 feet per second squared. Divide by 32.2 to express it as "g". Not much is it? The swirlers have it, I think.


30th Oct 2002, 00:44
See my link above:

Is it possible to detect the Earth’s rotation in a draining sink?

Yes, but it is very difficult. Because the Coriolis force is so small, one must go to extraordinary lengths to detect it. But, it has been done.

You cannot use an ordinary sink for it lacks the requisite circular symmetry: its oval shape and off-center drain render any results suspect. Those who have succeeded used a smooth pan of about one meter in diameter with a very small hole in the center. A stopper (which could be removed from below so as to not introduce any spurious motion) blocked the hole while the pan was being filled with water. The water was then allowed to sit undisturbed for perhaps a week to let all of the motion die out which was introduced during filling. Then, the stopper was removed (from below). Because the hole was very small, the pan drained slowly indeed. This was necessary, because it takes hours before the tiny Coriolis force could develop sufficient deviation in the draining water for it to produce a circular flow. With these procedures, it was found that the rotation was always cyclonic.

30th Oct 2002, 22:51
Thanks guys, I think he's finally coming around.
He still wants to try it for himself, but experimental physics is not really his thing, so I'm not holding my breath!!:D

Cheers, Captainowie