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Interesting stuff

...on serious topics that don't fit anywhere else at present.
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animist
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Re: Interesting stuff

#81 Post by animist » February 16th, 2011, 2:11 pm

Latest post of the previous page:

does "Interesting stuff" include mathematical impossibilities which actually are possible? If anyone knows about bowlers' averages in cricket (ie runs conceded/wickets taken) they will assume that bowler A, who has a better average than bowler B in both innings, will have a better average overall. In fact, it is possible for the reverse to be true - if anyone want to know how, or challenges this, I will dig out the old book which explains it!

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Alan C.
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Re: Interesting stuff

#82 Post by Alan C. » February 16th, 2011, 4:20 pm

I'd be interested in seeing that, numbers fascinate me.
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animist
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Re: Interesting stuff

#83 Post by animist » February 16th, 2011, 4:39 pm

Dave B wrote:Problem and solution:

I have this futuristic steel and glass triangular corner computer desk - problem, is with its 10mm thick glass top it weighs lots and only sits on four round feet about 1.5 inches across. There is no way I can lift this and move it over the carpet on my own.

:puzzled:

:idea2:

Up into loft for a coupla of bits of hardboard, a piece of sheet metal, a short bit if 1"x1" wood and a very large screwdriver.

Get under desk and use the s/driver and the 1x1 wood to lever up the back feet - slide bit of metal under them. Ditto front feet with bit of hardboard (shiny side up) under these. Carefully slide desk over hardboard - making sure the bit of metal moves with the desk. When edge of hardboard neared rotate the sheets for new direction and slide again. Leave on hardboard so no new dents in carpet.

So, job done with no lifting or straining - just how I like it!

(One thing leads to another, I did the above to get the steps in place to fit new blind. Hmm, whilst I am here I'll just fill in those old screw holes, clean the windows (inside and out), touch up the paint work . . . . . . . . . )
wish you lived near me, Dave - we would have interesting conversations while you fixed various things!
:laughter:

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Dave B
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Re: Interesting stuff

#84 Post by Dave B » February 16th, 2011, 4:47 pm

animist wrote:does "Interesting stuff" include mathematical impossibilities which actually are possible? If anyone knows about bowlers' averages in cricket (ie runs conceded/wickets taken) they will assume that bowler A, who has a better average than bowler B in both innings, will have a better average overall. In fact, it is possible for the reverse to be true - if anyone want to know how, or challenges this, I will dig out the old book which explains it!
I consider cricket scoring on the same level as quantum mechanics!

Mysterious manipulations manufacture marvellous maths.
"Look forward; yesterday was a lesson, if you did not learn from it you wasted it."
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Re: Interesting stuff

#85 Post by animist » February 16th, 2011, 5:14 pm

Alan C. wrote:I'd be interested in seeing that, numbers fascinate me.
ok, the two bowlers take all the wickets between them. A takes 5 first-innings wickets for 30 runs (so average is 6) and B takes 5 for 31 (6.2). In the second innings, A gets 3 for 12 (average 4) and B gets 7 for 29 (4 + 1/7) so A has the better average for both innings. But aggregating them, A's 8 for 42 (average 5.25) is less good than B's 12 for 60 (5). Weird! But did once happen in Australia long ago...

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Re: Interesting stuff

#86 Post by Nick » February 19th, 2011, 10:01 pm

The King James Bible and the Book of Common Prayer are not actually the Word of God, but are covered by Crown Copyright. :laughter:

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animist
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Re: Interesting stuff

#87 Post by animist » March 3rd, 2011, 7:35 pm

apparently it is possible for hot water to freeze faster than cold water, even if the two are cooled at the same way and in identical containers. This is called the Mpemba effect, and there are several reasons why it can happen, eg the hot water loses volume through evaporation so that there is less to freeze.

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Re: Interesting stuff

#88 Post by Dave B » March 3rd, 2011, 7:58 pm

Simple evaporation has a cooling effect, which is why we sweat in hot weather.

Wonder if enclosed containers of water with no air space in them, one warm one cold, would behave in the same way?
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Fia
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Re: Interesting stuff

#89 Post by Fia » March 3rd, 2011, 11:09 pm

Well I've learnt this winter, after my hall radiator freezing, that leaving the pump on will stop that as running water doesn't freeze as easily.

I'll look up the Mpemba effect...

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Re: Interesting stuff

#90 Post by Alan H » March 4th, 2011, 1:25 am

Fia wrote:Well I've learnt this winter, after my hall radiator freezing, that leaving the pump on will stop that as running water doesn't freeze as easily.
I suspect the reason why your radiators may be less likely to freeze if you circulate the water is that you are equalising the temperature throughout the system. So, if one room is slightly warmer (maybe because it is south facing, even if you don't have the boiler on), then the circulating water will extract heat from the room making it slightly warmer, which then circulates through the whole system.
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Re: Interesting stuff

#91 Post by Dave B » March 4th, 2011, 11:32 am

Reckon so, Alan. As a kid I always thought that running water did not freeze as easily, then I saw pictures of Niagra Frozen, the Thames frozen . . .

I did wonder, though, whether the thermal energy from the I2R heating in the pump (warming the pump body), the friction and the energy from the mechanical action of the vanes might add significant energy into the system?

One of our less physics orientated blokes at work, using our large test rig, nearly blew the pump up because he shut the top isolating valve, thus thrashing the water in the pump chamber somewhat. But that was a 52kW pump unit running at full speed. The water had almost hit boiling point! I felt the heat from it as I walked by - lucky I did.
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Re: Interesting stuff

#92 Post by animist » March 4th, 2011, 1:47 pm

Dave B wrote: Wonder if enclosed containers of water with no air space in them, one warm one cold, would behave in the same way?
sorry, Dave, not sure what you mean - why would lack of space between the containers be relevant?

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Re: Interesting stuff

#93 Post by Dave B » March 4th, 2011, 3:28 pm

animist wrote:
Dave B wrote: Wonder if enclosed containers of water with no air space in them, one warm one cold, would behave in the same way?
sorry, Dave, not sure what you mean - why would lack of space between the containers be relevant?
Not lack of space between the containers, animist, lack of air space (room for evaporation and air circulation) above the surface of the water.

As in any experiment (well, the ones I have done anyway) you reduce the number of variations and variables to start with, then introduce them one at a time and compare the results. If two samples of water, one warm and one cold, the variables must be the same (if that is not an oxymoron!) for each; same size & material of container, same volume of water from same source (probably de-mineralised since contamination can cause nucleation points for gas bubbles and ice crystals), same cooling bath for both containers, with coolant (car anti-freeze fluid) stirred to prevent temperature gradients . . . .

I have done that sort of thing a few times at work, not to check the freezing point of water but the performance of fluid flow measurement devices at low temperature (and high as well.)
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Re: Interesting stuff

#94 Post by Nick » March 4th, 2011, 3:37 pm

I'm a bit puzzled by this talk of moving water freezing...

Without any real justification, I would say that moving water is less likely to freeze than still water. Maybe it's just that the movement would raise the temperature of the water slightly, but I don't know... Any answers?


ETA: A quick google yields the surprising disclosure that agitation can in fact lead to the freezing of water which, because it is so still, has remained liquid below 0 degrees C. Also, that water is at its most dense a 4 degrees, not at 0 degrees. Ooooh!

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Re: Interesting stuff

#95 Post by animist » March 4th, 2011, 3:44 pm

Dave B wrote:
animist wrote:
Dave B wrote: Wonder if enclosed containers of water with no air space in them, one warm one cold, would behave in the same way?
sorry, Dave, not sure what you mean - why would lack of space between the containers be relevant?
Not lack of space between the containers, animist, lack of air space (room for evaporation and air circulation) above the surface of the water.
sorry, misread what you said. Yes, I think the lack of air space might make a difference, don't know. This - evaporation - was just one relevant factor; others included differences in amounts of dissolved gases. A third factor was if both containers sat on frosty ground: the hot water container would melt this, making a more effective heat sink, I think; you'd be better off looking at Google than talking to me!

On Nick's query, I intuitively feel the same about running water, but icicles would seem to disprove this (albeit the water is flowing slowly) - can someone settle this point? (later)Nick's ETA thing maybe does settle it. I knew about water being more dense at 4 degrees above freezing point, presumably beause ice is less dense than water and so the water would be starting to freeze at 0 deg C

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Re: Interesting stuff

#96 Post by Alan H » March 4th, 2011, 3:58 pm

Dave B wrote:Reckon so, Alan. As a kid I always thought that running water did not freeze as easily, then I saw pictures of Niagra Frozen, the Thames frozen . . .

I did wonder, though, whether the thermal energy from the I2R heating in the pump (warming the pump body), the friction and the energy from the mechanical action of the vanes might add significant energy into the system?
Of course - I had forgotten about the not insignificant heat from the pump.
One of our less physics orientated blokes at work, using our large test rig, nearly blew the pump up because he shut the top isolating valve, thus thrashing the water in the pump chamber somewhat. But that was a 52kW pump unit running at full speed. The water had almost hit boiling point! I felt the heat from it as I walked by - lucky I did.
52 kW? Bloody hell.
Alan Henness

There are three fundamental questions for anyone advocating Brexit:

1. What, precisely, are the significant and tangible benefits of leaving the EU?
2. What damage to the UK and its citizens is an acceptable price to pay for those benefits?
3. Which ruling of the ECJ is most persuasive of the need to leave its jurisdiction?

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Re: Interesting stuff

#97 Post by Dave B » March 4th, 2011, 4:33 pm

Aw, 52kW was just a baby pump in the D&D lab compared to the two big jobbies in the main calibration lab, they were 250kW each and had their own substation!

We made flow measuring devices with bore diameters from 3mm to 2.2 metres. We used one of the big ones as an entrance arch at exhibitions and parked a mini in one as an advertising stunt, a la "The Italian Job".
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Re: Interesting stuff

#98 Post by Dave B » March 4th, 2011, 8:22 pm

Actually a thought strikes me (ouch!).

With regards to the freezing of moving water: where a stream, river or even the sea is concerned it is not the moving bit that actually freezes. The ice builds up from the sides, water splashes onto rocks etc. and builds up layers as a stalagmite/tite does - ditto icicles. Gradually those layers get thicker and wider, obstructing the flow more, causing more turbulence and thus providing more splashed water that freezes and obstructs . . .

In the end it can even stop Niagara!

IIRC totally de-gassed super pure water, with no contaminating material whatsoever (expensive stuff) does not actually freeze at all - unless you knock the container or drop something as small as a virus particle in it to create a nucleation point for the first crystal to grow.
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Re: Interesting stuff

#99 Post by Fia » March 4th, 2011, 8:39 pm

Whoa all you engineers with your big pumps and all :) There are a couple of variables I see here:

Whether the water is enclosed (as a heating system) or exposed (as in waterfalls).

The speed of the water movement must surely make a difference? A gentle drip drip of meltwater on an icicle will cause it to grow but if you hosed water on it from the top it would melt. But then why do waterfalls freeze?

Interesting stuff indeed.

edited to change 'bug' to 'big'.. damn spellcheckers..

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Re: Interesting stuff

#100 Post by Alan C. » March 4th, 2011, 8:48 pm

IIRC totally de-gassed super pure water, with no contaminating material whatsoever (expensive stuff)
I thought I was in the homoeopathy thread for a minute :laughter:
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Re: Interesting stuff

#101 Post by animist » March 4th, 2011, 8:59 pm

Dave B wrote:Actually a thought strikes me (ouch!).

With regards to the freezing of moving water: where a stream, river or even the sea is concerned it is not the moving bit that actually freezes. The ice builds up from the sides, water splashes onto rocks etc. and builds up layers as a stalagmite/tite does - ditto icicles. Gradually those layers get thicker and wider, obstructing the flow more, causing more turbulence and thus providing more splashed water that freezes and obstructs . . .

In the end it can even stop Niagara!

IIRC totally de-gassed super pure water, with no contaminating material whatsoever (expensive stuff) does not actually freeze at all - unless you knock the container or drop something as small as a virus particle in it to create a nucleation point for the first crystal to grow.
so you are now saying that flow DOES inhibit freezing? Can you explain about pure water not freezing - surely it must at some temperature? And it's about time I learnt what IIRC meant!

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