I expect answers by 22nd

[Ohdrat]

OK whilst I am at large destroying all and sundry anywhere in the vicinity of Bute I am setting you merry guys and girls a wee problem I have been mulling on everytime I watch the bath water drain..

In the Northern hemisphere the eddy around the plug hole is anti clockwise... except and when almost all of the water has gone and then it reverses and is clockwise!.. what causes it to change? infact why is it anti clockwise in the first place.. ?

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[Twister_Ken]

Coriolis effect* and the shape of your bath/basin

*but don't ask me to explain.

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[AlanPound]

... I can explain, but (hic), wait til I'm sober (burp)....

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[AlanPound]

Ok, you asked for it.... (hic)

The Coriolis effect with respect to water, is the same as with respect to air masses....

... boaty relevance, the WEATHER... (so lets talk air, rather than water....)

A parcel of air, stationary at the surface, has both angular and linear velocity around the axis of rotation of the Earth, against which it is constrained to lie by gravity.

That same parcel of air has mass (weight) - a surprising amount of mass, for a reasonable chunk of air (and water even more so).

If it is pursuaded, for whatever reason (usually pressure differential) to move, say, due North, then (in the northern hemisphere, for a decent distance from the equator) its distance from the axis of rotation will decrease, and potentially its linear velocity would decrease, except that its momentum wants it to carry on moving at the same speed, causing its angular momentum to increase (much like an ice skater in a spin, pulling his/her arms in)....

... so with respect to an observer at its starting point and facing North, although that parcel of air is persuaded to go due North, it will also tend to move/bend to the right...

... using a similar argument, you can also convince yourself how a similar parcel of air, in the Northern hemisphere, pursuaded to move south, will also move right (for an observer at its point of origin, and facing South), as its angular momentum wants to decrease.

I also have a much less obvious/convincing mental model for air masses trying to move East, or indeed West, but I'll hurry past those, and simply say that...

... the net effect of this is that, given a low pressure (in the Northern Hemisphere), and air masses (water masses) moving from areas of higher pressure towards it, the effect of the Earth's rotation is to cause them to tend to rotate around it, in an anti-clockwise direction... (and viky verka in the Southern...)

.... I tinks thats done now... (dunno what it'll look like in the morning tho')

... In fact, I am beginning to recall that our entire weather system, the polar front, polar front depressions, hurricanes, typhoons, probably earthquakes, monsters from the beginning of time, God and the Devil locked in mortal combat, all knds of bad stuff, can be derived from the thermal machine fed by the sun, plus the Earth's rotation....

... ok, time for bed now... G'night!

PS now the 11th (will that do?)

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[jimi]

Weel if ye hae a wee problem everytime yer in ra bath ah'd gang tae ra loo furst.

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[BrendanS]

Re: Urban Myth I\'m afraid

Your sink shape is correct, Coriolis is def not.

Coriolis force works on things like weather systems - huge , over hundreds of miles in diameter. Coriolis force won't work on a 'system' such as a bath or sink which is inches, the effect is just too small compared to the direction in which you pulled the plug out, or minor imperfections in the surface of the bath or sink which will cause the water to move in a certain direction.



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[tcm]

Coriolis for dummies

Coriolis was a frenchie, born about 250 years ago, and not a roman. His explanations of angular kinamatics are blamed for all sorts of things, notably water down the sink, a myth propagated ever since, and in the Simpsons cartoons. But the Coriolis "force" remains a mystery. Until now?

Get a cricket ball and lob in northwards from the equator a long way. When it started, on the equator, the ball was whizzing eastwards at 1000mph ish, like everthing else on the equator. As it travels further north, its easterly speed does not decrease. So relative to the ground below (which is moving slower, innit) it seems to deflect sideways, to the east. In fact, relatively, it actualy IS moving to the east. But nobody pushed it that way at all. Weird, eh?

OK, now get another cricket ball, and start from the north pole. Lob this due south. It had almost zero easterly component where it started and you threw the ball, but soon, as it travels south, the ground below is whizzing eastwards, faster and faster as the ball moves south. So the southbound cricket ball actually moves (relatively) to travel a bit to the west, again as if some weird force is making it go that way.

the throwing of cricket balls east and west requires that you think in 3d, which is a bit hard. Take it from me that although gravity pulls the ball into the centre of the earth, it does not affect the north/south/east or west components. This makes no difference at the equator - it is going due west or east. But if a ball is thrown to the east in northern hemisphere, the easterly spin sort of makes it seem to move south a bit, relatively, once the earth underneath has turned around a bit. Think about it.

Likewise a westerly flung cricket ball seems, once the ball has been let go, and as the earth underneath turns a bit, to moves a bit north, again as seen from the ground underneath. It's not a massive effect, but it will cros lines of latitude, you have to admit.

This is about the best explantion i can give regarding how (in the northen hemisphere) things moving north turn to the east, things moving south tuns a bit west, thngs moving east turn a bit south, and things moving west turn a bit north.

In other words, in the northern hemisphere, everything turns a bit to the right - PROVIDED THAT IT IS MOVING. There's none of tis aparent deflection if you don't chuck the cricket ball. So no Coriolis.

Note also that Coriolis is a deflection of physical thing with a mass. So it has the units of a "force"- yet it is a force with no real source - a bit weird really. Nothing has bashed it sideways, and it's not a gravity thing either. So often referred to as "Coriolis" or "Coriolis effect" rather than "Coriolis Force".

Anyway, as explained, everthing traveling in any direction sort-of moves to the right , in the N hemisphere. And everything travelling moves to the left in the southern hemisphere. Especially big lumps of air! Which promotes nasty anticlockwise-spinning low pressure, shite weather and hurricanes up here in t'North. Unlike in the southern hemisphere where the weather is nicer, cept in the roaring forties but that's different. Anyway, in south america and Oz the weather is lovely, ish. Well, not many hurricanes eh? Alright, not as many as in the northern hemisphere.

None of this explains the bath really, cos the Coriolis effect is quite small, and sinks aren't really big enough. That's more to do with the shape of the bath or sink, and hardly at all Coriolisy. The reversing at the last bit shows it is more to do with local currents than coriolis. Otherwise it wouldn't reverse would it? No. I mean, gravity (which is another force, you know) doesn't -oops! - reverse at the last minute, now does it? No. So, that's that.

If anyone says aha Coriolis about the bathwater, you can now ridicule them. If they elimnate all the anticlockywise spinning of the water, cos even when it looks still it isn't really, and also the strange bath curves, it would go straight down, pretty much, but tends to spin one way or t'other. Completely different.

 

[BrendanS]

Re: Coriolis for dummies

Ooh! You're getting all technical and scientific, in a non way. Like it.

We'll have to get you to do the Christmas lectures next (wonderful series that's on Beeb 2 every year over xmas week explaining serious science to kids and grown up kids)

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[MainlySteam]

Dear Prof TCM

Dear Professor TCM

No doubt, as a fellow physicist, you are familiar with Professor Richard Feynman, previously of the California Institute of Technology and who conducted the well known Feynman Lectures on Physics. Unfortunately Prof. Feynman passed away in 1988 and his fine works, although still in print, have suffered the ravages of time as their mass now exceeds the speed of light squared.

Just by chance I have recently been asked to find a suitably physical personality to update, present and republish the lectures and it appears to me that you might be just the chappy. I have to mention that the pay is not very good, sort of emeritus in magnitude but sufficient I am sure for the scholarly nature of the assignment.

I understand that your good friend Professor Claymore is also into this game and I am sure he would be a mine of information. I would be quite happy for you to seek his assistance if you so wished, and I am sure it would give him the opportunity to contribute to the physical world in the same manner as his fellow countryman Lord Thomson, who again is unfortunately no longer with us, did.

I have to say that your description of your balls was just fascinating and I am sure during the representation of the lectures a similar demonstration using them to show the curly path nature of electrons, quarks, sparks, etc would be quite entertaining. That would be especially so if you could demonstrate, with the same, the physics behind Newton's Cradle, and indeed would be mesmorising if you could incorporate into that the effects of imbalance due to misaligned suspension over time.

I do notice, and I trust that you understand my need to know, that to qualify for such an emeritus position all of your discharges have to be honourable, and I trust that you will be able to assure us of that.

Sincerely yours in much anticipation, etc

Dr MainlySteam

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[roger]

Re: Urban Myth I\'m afraid

It is, as far as I remember, possible to set up a tank to demonstate Coriolis BUT its not an easy job. On the small scale coriolis forces are very weak. You need a large circular tank, a drain in the centre, no irregularies - in fact perfect circular symmettry. You have to fill the tank so as not to introduce swirl and you have to let the water settle for ages.
Residual motion of the water due to the parting bum is quite sufficient to explain any vortex effects.

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[peterb]

Re: Urban Myth I\'m afraid

One of the Oxbridge universities (can't remember which) is alleged to have set up a 'Coriolis Room'. No windows, just an access door, but rotatable about an axis through the centre. Floor was curved to counter centrifugal force. In the centre was a snooker table. Can you imagine playing snooker on a table on which every shot was deflected to the right?

Coriolis doesn't just affect the weather, though. It also affects tidal streams, which is why the tides on the French side of the Channel are higher than on the English side.

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[AlanPound]

Urban Myths as urban myths...

... was the pithy subject line I was hoping to post, however...

... I just filled the five washbasins in the accomodation I laughably refer to as my home (sad, or what?), I skipped the baths (perhaps would show too much commitment), and, guess what (too many commas, too many parentheses, probably been drinking....) on emptying, two went anti clockwise, and three (I said three!!!) went clockwise (although one changed its mind at the last moment, and went anti as the last dregs disappeared.... as per the original post, but in reverse)...

... well, *clearly* (a hand-wave), washbasins have no education... have never heard of Coriolis...


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