# Can somebody finally settle this question: Does water flowing down a drain spin in different directions depending on which hemisphere you're in? And if so, why?

Finally, Thomas Humphrey, a senior scientist at the San Francisco Exploratorium, discusses in more detail the reasons why we do not see the Coriolis effect at work in the bathroom:

"There is an African country near the equator where entrepreneurs have set up two toilets, one just north of the equator, the other just south of it. For a fee, they will allegedly demonstrate that the toilets flush in opposite directions. It is only for show, however; there is no real effect. Yes, there is such a thing as the Coriolis effect, but it is not enough to dominate the flushing of a toilet--and the effect is weakest at the equator.

"The telling comparison is between the magnitude of the Coriolis effect and the initial amount of angular momentum in the water--that is, how much is it spinning anyway, regardless of the earth's rotation. Coriolis acceleration at mid-latitudes is about one ten-millionth the acceleration of gravity. Because it is a very small acceleration, it needs a very long distance for it to produce an appreciable curvature--and hence directionality--to the motion. A toilet or sink is just not large enough. The Coriolis effect influences because wind velocities may be hundreds of times greater than the motions in a sink and because the distances involved are far larger than the tiny draining diameter in a sink or toilet.

"It is impossible to find a cup full of water that does not have some average net motion; it will always be going one way or the other, and that little amount of angular motion is enough to swamp the Coriolis effect. The net motion in the water becomes much more pronounced as the water is forced to move in toward the center of evacuation, causing the normally invisible flows in the water to become visible as the water nears the drain. The ultimate direction of that flow is random--it can go one way one time, the other way the next.

"If you run an experiment in your sink--fill the sink, then pull out the stopper--the water will almost always go down the same way, making you wonder if this is really a random effect. But you will find that the faucet is almost always off center or that there is some other asymmetry in the sink. As a result, filling the sink consistently gives it some net rotation in the same direction, which you see as the normal direction of evacuation. Toilets will always drain and fill the same way, for the same reason.

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1. 1. jjoe348st 04:27 PM 12/18/08

to answer this question of which way it spins call one of your offices in austrailia and ask them to conduct an experiment. i have always seen the water spinning in the same direction, even if you change the direction it will return to the original direction. your guys were way off base.

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2. 2. jjoe348st 04:33 PM 12/18/08

why dosen't anyone just call somebody in austrailia and ask?

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3. 3. zimbo 05:36 AM 1/15/09

why then does my bath and kitchen sink go in one direction and the basin next to my bath go in another ?!?
the force of the gravitational pull from the moon is 10 times stronger than the force of the earth spinning , everyones bath has got a tide but is to small to notice , how then can a smaller force (the earth spinning) have more of an effect than a larger one (gravitational pull fron the moon) on the same body of water , it depends entirerly on the shape of whatever the water is in.

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4. 4. cbsimkins 04:59 PM 6/14/13

To start with, it is a Coriolis effect, not a force per se. The miniscule difference in the radius differences etc. is so tiny that the drain is not really going to be affected. On the larger scale, the atmosphere in particular, there are differences in size of the radius that will show up in the effect. The absolute velocity of an object moving across the surface of the earth is affected because or the angular velocity of the object with respect to the axis of rotation. Everything you do is on a rotating body, the earth, and so is affected by the tangential velocity if the body. When you go up hill or down hill, north or south, the distance from the axis or rotation changes slightly, but we never notice these small changes as they are so minute.

CBS

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5. 5. Realist1948 09:57 PM 9/18/13

If we accept as fact that the Coriolis effect does not cause draining water to rotate, that begs the question: what does cause the rotation? Intuitively, it would seem that the path of least resistance would be along radial lines toward the center of the drain.

Now for some speculation: Imagine an empty conical funnel standing vertically, narrow end down, with no liquid in it. Now suppose a single small steel ball (an air-rifle BB) were released from the rim of the funnel. It would roll straight down the conical surface and out the bottom of the funnel. There would be no rotation (other than the rolling of the BB). Next suppose that we simultaneously released a pair of BBs from points opposite each other (two points where a diameter of the circular rim intersects the rim). Given the elasticity of steel, we'd expect the BBs to bounce off of each other. If they collided exactly head-on, they would bounce in opposite directions. Again, there would not be any spin. Now instead of steel BBs, suppose we had rolling spheres of liquid water. Upon collision, the spherical drops would merge. If the drops did not collide exactly head-on, conservation of momentum says that the combined blob of water would spin (with the spin direction determined by left-to-left or right-to-right nature of the off-center collision).
If, instead of having just one pair of drops of water we had thousands of such pairs, launched from all around the rim, each pair would tend to spin one way or another. The summation of all of the spins (or the integral of all of the angular forces) would cause the mass at the neck of the funnel to spin one way or the other.

Is this a plausible explanation for the rotation, without bringing the Coriolis effect into it?

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Can somebody finally settle this question: Does water flowing down a drain spin in different directions depending on which hemisphere you're in? And if so, why?

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