Re: When a sink is full up and the plug is pulled why does the water swirl down?

That's a good question and it has a two-part answer. But first, the short answer: water swirls down the drain because of random motions that it began with and the Earth's rotation does not usually play any significant role. I'll explain why the water swirls in more detail first, and then I'll explain why Earth's rotation isn't very important for this.

Why does water swirl down drain? Because of something called "angular momentum". Angular momentum is quite a bit like normal (or linear) momentum. You can think of angular momentum is how difficult it would be to make an object stop spinning.

So why does this matter? When you fill the sink with water, there's some initial overall spin to the sinkful of water which persists for quite a while. Here's a simple experiment to prove it: fill up the sink and sprinkle pepper across the top of the water. After about a minute or so, notice that the pepper flecks will swirl in one direction or another. The water has an overall spin, although it'll probably be pretty much random which way it goes. (Try the experiment several times and see.)

Whn you pull the plug out, the water moves towards the drain. In doing so, its rotation speeds up just like a figure skater who starts spinning with her arms out-stretched and then draws them in. So the small initial spin to the water is amplified to a larger swirl before it goes down the drain.

So the water has some overall tendency to want to swirl around the drain slowly. Why does it swirl so much faster when you pull the plug out? Because as the water gets nearer the drain, it has to spin faster to keep the same angular momentum. This is like a figure skater pulling her arms in and speeding up. As the water spins faster nearer the drain, it has a hard time actually getting into the drain because it heading off to the side. As it happens, this effect is important to astronomers! The same thing happens when gas tries to fall into a black hole or a forming star or anywhere else. There's always some initial angular momentum to the gas so as it gets close to the star (or whatever) it orbits around the object, unable to fall in. Eventually, both the water in your sink and the gas around the star are able to lose enough angular momentum to fall in and finish the story. However, in astronomy the delay is important: the swirling gas forms a disk around the object. For black holes, the gas rubbing against other parts of the disk heats the disk up to very high temperatures and we can observe the it. This is how we "see" black holes. For forming stars, the disk around the star is where the planets form, something that is important to us on Earth!

So that's why water swirls around the drain before going in. So what's this about Earth's rotation?? Weeeeell, there is actually something going on, there. Since the Earth is spinning, we get some funny behaviors when things move large distance or quickly. The reason is the Coriolis "force" (it's not really a force, but it looks like one if you forget that the Earth is spinning). As things move towards Earth's rotation axis, the tend to deviate from their original path towards the east. So if you're the Northern hemisphere and you head north (towards the pole), you'll find that there's a small force nudging you towards the east. Conversely, heading towards the equator causes a slight force nudging you towards the west.

The Coriolis force has very real effects: large weather patterns (like hurricanes) spin in opposite directions in the two hemispheres. In the ninteenth and twentieth centuries, gunners noticed that shells which were fired north landed east of their intended positions and shells fired south similarly landed to the west. (This was in the Northern hemisphere.) Their shots were being deflected by the Coriolis force. If you want to experiment with the Coriolis force, a small plastic tub of water with a hole in the bottom and a merry-go-round at a local playground should give you a good chance see how spinning in different directions causes different swirling in the draining water.

So why doesn't this affect your sink? Technically, it does. But the Earth's spin is very slow and the size of the sink is small. So the water doesn't move a lot closer to or farther from Earth's spin axis and the Coriolis force is small, much smaller than the effects of the initial random motions in the water. However, if you let the water sit for a long time (ideally, a few days) and were able to remove the plug without making a large disturbance in the water, you should see the Coriolis effect cause the water to spin counter-clockwise in the Northern hemisphere. I have actually seen a film of this being done with a very large tank of water and it does work. Conversely, I once participated in an experiment where we flushed a dozen different toilets in our physics building and recorded which way the water swirled down the drain. Six went one way, six went the other. So it's pretty much random, based on our small experiment.

For more details on the Coriolis force, see Physics World's entry. Also, Phil Plait has a wonderful discussion in his book, Bad Astronomy which I highly recommend. Also, this page has a fantastic discussion of the topic.

As a cool little note, I want to point out that a similar kind of swirling occurs when material tries to fall into a black hole, forming star, or pretty much anywhere else in astronomy. In this case, the material usually forms a disk about the central object because it takes a long time to lose enough angular momentum to fall all the way in. (It's flat for another reason which I won't go into.) The disks are important features in astronomy: the show us where black holes are because the disks heat up from friction and they are where planets form around forming stars, among other things.