Water down a plug hole

[kensey]

http://www.straightdope.com/classics/a1_161

I looked up the web to see why water goes down a plug hole clockwise in the southern hemisphere and find it to be a myth. Fair enough.

The questioin then is, why does the water rotate at all. Why does it not just go straight down the plug hole. If the answer is the shape of the bowel or drain, I am not convinced especially if you have a symetrical bowl.

Any answers

[C_K_Dexter_Haven]

Welcome to the Straight Dope Message Board, kensey, glad to have you with us.

A sink may look symmetric to you, but there can be teeny tiny irregularities in the surface that could force the water in one direction or another. Or the sink may not be exactly flat – being off by a teeny tiny bit could make a big diff.

Also, of course, the main factor is the way the water goes INTO the tub. If you plug your sink and turn on the tap, the water is not necessarily going straight down – a slight inclination, and there’s a natural push to one side, so that the water has some residual motion to it. You’d need to wait many hours after filling up the sink before you pull the plug, to watch a rotation direction that didn’t have that residual motion left.

[zut]

That’s actually a darned good question. Dex has got most of it: Even if a sink looks symmetrical to you, there are small irregularities in the sink. Even without those, there are small eddies in your bowl of water that are moving the particles around, so there will be some overall tendency to move in one direction or another.

However, that’s not quite the entire answer, because there’s still the question of: Why do those small irregularities cause the water to swirl? That’s because of conservation of momentum. A small tendancy for a water particle to move in one direction or another will be magnified as the water approaches the drain. This eventually sets up a bulk swirl motion in one direction or another, as the rest of the water is dragged along.

In fact, the situation where water goes “straight down” the drain is unstable – eventually, given time, there will be some force on the water that’s large enough to tip the motion into swirling in one direction or another.

[John_W.Kennedy]

That’s conservation of angular momentum.

[kensey]

@zut

That’s actually a darned good question. Dex has got most of it: Even if a sink looks symmetrical to you, there are small irregularities in the sink. Even without those, there are small eddies in your bowl of water that are moving the particles around, so there will be some overall tendency to move in one direction or another.

However, that’s not quite the entire answer, because there’s still the question of: Why do those small irregularities cause the water to swirl? That’s because of conservation of momentum. A small tendancy for a water particle to move in one direction or another will be magnified as the water approaches the drain. This eventually sets up a bulk swirl motion in one direction or another, as the rest of the water is dragged along.

In fact, the situation where water goes “straight down” the drain is unstable – eventually, given time, there will be some force on the water that’s large enough to tip the motion into swirling in one direction or another.

Thanks for your replys. I like the angular momentum explation.