1

That CAN'T be true! / What is "four times less"?

« on: 02/11/2023 20:33:36 »

I've just seen a TV advert for a detergent that requires "four times less scrubbing" to remove grease.

Now one times A equals A  so one times less A = 0. No problem with that.

But what does 4 times less A mean? Does the detergent remove the grease from next week's dinner too?

2

Geology, Palaeontology & Archaeology / Which came first - knives or caucasians?

« on: 16/10/2023 18:45:04 »

Couldn't help noticing a woman with waist-length hair cycling past me today. Then I began to wonder about the practicality of long hair.

It seems that there is no limit to the length of human hair - surely a unique phenomenon among apes if not all animals?

Now cast your mind back  to the dawn of the genus homo. Gradually becoming bipedal and a collaborative hunter, almost naked except for this extraordinary thatch on top. Assuming the thatch has some protective value, to some extent more is good, and plenty of tight curls not only keep the sun off the brain but also act as a crash helmet. 

So far so good, but the genetic mutations that resulted in caucasian and asiatic versions of homo sapiens also produced hair that just hangs down and gets in the way of everything unless you tie it  up or cut it.   

So which came first - hom sap euroasiaticus, or the knife? We can find flint tools up to about 2,000,000 years old, Which kind of suggests that the northward migration of humans was either prompted or assisted by the evolution of hairdressers, without whom white Man would have been unable to walk very far!

3

That CAN'T be true! / Do dogs defy the laws of physics?

« on: 20/09/2023 09:00:05 »

It's an everyday observation but never seems to appear in an exam question.

I've just watched my 80 kg neighbor being pulled along by his 10 kg dog on a lead. The dog was clearly in control of their horizontal velocity, from v = 0 in the vicinity of an interesting gate post, up to about 12 m/s in visual range of another dog. The lead was at 45 deg to the horizontal. They were on a level tarmac sidewalk, so the dog had no frictional advantage (claws work better in soft ground, but dog paw pads and leather shoes probably have the same μ value on tarmac).   

Explain.

4

General Science / Is AI a useful guide to risk assessment?

« on: 24/07/2023 13:05:27 »

A cautionary tale from my first (and probably last) professional encounter with AI.

I've been asked to estimate the risk to a patient from a research program that involves four CT examinations of the skull. Not a problem as we have standard conversion factors from scan parameters → absorbed dose → effective dose → risk.

If you ask Bing for "dose from a skull CT examination" it returns 0.07 millisievert, one thirtieth of the correct value, based on its AI search.

The reason seems to be that it hasn't distinguished between computed tomography and 2D ("film") x-ray imaging, which does indeed result in the lower dose.

So if you aren't an expert, don't rely on AI, and if you are, don't waste your time with it!

5

Just Chat! / A revised law of the sea?

« on: 02/07/2023 12:28:47 »

Cricket commentators have dragged their sport into the 21st century by replacing Batsman, Third Man and Twelfth Man by Batter, Third and Uniformed Reserve. It is therefore proposed that another ancient custom be revised in line with what is hoped to become common parlance:

If you see a someone disappear over the railings,

immediately shout "irrelevantly gendered anthropoid overboard"

nod in their general direction (pointing is discriminatory)

do not throw a lifeperson (it is prejudicial to suppose that they cannot swim)

seek consensus from the passengers and crew as to whether to execute a sharp turn around the point of disappearance (not everyone is popular with everyone else, and most passengers are in a hurry to get somewhere)

remember that a cry for help may be a Cry for Help, so as the vessel circles the victim, shout "How do you Feel?" and respect their answer. 

6

Chemistry / How do "all in one" dishwasher sachets work?

« on: 01/06/2023 11:17:02 »

It seems to me that the three components of a dishwasher sachet are required to "do their thing" at different times and temperatures, but the soluble sachet is dumped into the bowl all at once and there are several fill and drain cycles in a 2-hour wash, so there can't be much of number three by the time you need it.

I misspent part of my youth as a physical chemist in a detergent laboratory but 60 years ago we only worked on one surfactant material at a time - the job was to tailor its molecule, concentration and solvent viscosity to a particular application. This new stuff looks like magic in comparison!

7

Technology / How did they make the first screw?

« on: 17/03/2023 17:56:27 »

So I'm cutting a screw thread on a lathe. The lead screw turns and moves the tool along the workpiece, and the gear ratio between the chuck and the lead screw determines the pitch of my product screw. An everyday occurrence in thousands of workshops all over the world.

But the lead screw needs to be at least as precise as the one I'm making.

So how did they make the first lead screw?

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Just Chat! / Empty words?

« on: 03/03/2023 23:27:48 »

If you delete the word "existential" from a sentence, does it alter the meaning?

Likewise this month's buzzword "performative".

Does the word "celebrity" mean anything more than "someone who has no function in your life"?

Is there any kind of experience that is not a "lived" experience?

9

Geology, Palaeontology & Archaeology / Why do insects fly?

« on: 20/02/2023 11:07:22 »

Not a completely stupid question. Last night's BBC2 program about Jurassic dinosaurs threw up an unexpected aside: there were no flowering plants in the Jurassic period.

Flying demands a high energy density fuel, which many insects get from nectar or pollen. Nectar-bearing plants certainly benefit from flying insects but that poses a chicken-and-egg question. In the absence of flowering plants, there seems to be little if any advantage in flying - flightless insects are very successful without leaving the ground and even beetles, fully capable of flight, rarely take to the skies.

So: chicken (bees, butterflies, etc) or egg (flowering plants)? which evolved first, and why? 

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That CAN'T be true! / Can I get infinite "green" heat from my barn?

« on: 11/02/2023 19:58:43 »

I have converted an old barn into a music room. Some of the instruments being almost as old and fragile as myself, I have installed an air-source heat pump to maintain a reasonably constant indoor temperature of 18°C.

The heat pump uses electricity to run a refrigerator system that extracts heat from the outside air and uses it to circulate warm water under the floor. So most of the time there is a temperature difference between indoors and outdoors - unless the outside temperature happens to be exactly 18°C, in which case it is unnecessary.

Suppose I have a huge thermocouple, with one junction inside the barn and the other outside. Let us suppose that Tin>Tout, which it will be as soon as I walk into the room (Tout is just above freezing tonight). So a current will flow through the thermocouple pair. OK, the voltage will be small but I could make a lot of junctions in series (a thermopile) to get more volts, and/or use a very thick wire and fat junctions to get a lot of current. Enough, in principle, to start the heat pump. Which will increase the temperature differential, and thus the power available to drive the heat pump.....

Never mind the engineering practicalities: I'm in physicist mode and assume that my alter ego can build the necessary inverters etc. so I can heat my barn to any temperature I like without expending any additional energy - it all comes from the fact that it starts off slightly hotter than  the air outside.

And why not?

11

The Environment / Can anyone please enlighten me on pressure broadening?

« on: 03/02/2023 13:03:27 »

There has been an ongoing debate in this forum (and a few other places, too) about the influence of carbon dioxide on global climate.

As I see it, the principal  CO2 absorption lines in the atmospheric infrared spectrum have been saturated for as long as we have any valid data - at least 70 years. But every so often someone claims "pressure broadening" means that the line width has increased so significantly in that time that the world is in peril.

But as I understand it, pressure broadening in a very dilute mixture is not a function of the partial pressure of the subject gas, which has admittedly increased from .0002 to .0003 bar in the last 10,000 years, but a function of the total pressure of the mixture (1 bar), which has if anything decreased as the temperature has risen.

Comments and elucidation welcome.

12

Just Chat! / Another yuletide maths puzzle

« on: 27/12/2022 00:13:54 »

The Boss always buys a 1000 piece jigsaw puzzle which we start on Christmas Eve as an alternative to the "seasonal" (i.e. recorded in August)  garbage and re-re-recycled movies on television. Interspersed with plenty of food and wine, it can last until the dreary Celebrity (who?) Best of 2022 and mawkish New Year Countdown fades from the screen. 

At some point towards the end of the business we move from logic (edges and corners, then features with straight lines, then textures....) to picking up a piece at random and looking for a hole it can fit.

Now imagine a machine that can scan the boundaries of all the pieces and translate and rotate their images to fit them together. Let it work at "human speed" - say 1 second to see if B fits to A. Assume the pieces are all white - no picture for reference - and scattered at random.

1.How long would it take to solve the puzzle, based on geometry alone and considering one piece at a time?

2. suppose n pieces are upside down.Let the machine have the capability of flipping any piece over. How long now?

3. what is the effect of introducing one item of prior knowledge: there is no point in fitting another piece to a straight edge?

4. what is the effect of now adding a strategy - look for a corner piece before you start? 

And a joke, in memory of Terry Wogan:

Four rednecks start a-whoopin' and a-hollerin' in a bar. Bar tender says
"Why the noise, boys?"
"We just finished this here jigsaw puzzle"
"Darn took y'all long enough. You guys been workin' it for the last three months."
"Hell yeah, but the label on the box says 'five to eight years' "

Seriously, folks, returning to the random fit machine, given just four shapes, two pieces of string and some glue, how long would it take to evolve a strand of DNA?   

13

General Science / Advent calendar maths question!

« on: 14/12/2022 23:21:07 »

This might amuse those of a mathematical persuasion.

We have a novelty advent calendar. For anyone brought up in a different faith, it counts down the days from 1 to 25 December. Ours consists of two numbered wooden cubes which you can rearrange to display every integer from 25  to zero.

What numbers are painted on each cube? 

14

General Science / Why is gold gold?

« on: 17/11/2022 22:48:11 »

Metals - arrays of atoms with a shared delocalised electron cloud that accounts for high electrical and thermal conductivity. 

The electrical conductivity of a metal at a given temperature is independent of the voltage gradient - no band gap or any evidence of quantum behavior: the conduction electrons are free and indistinguishable.

So why do different metal surfaces have distinct optical absorption spectra? Gold is gold, sodium is white, silver is silver, and titanium is grey. Surely they should all be equally and totally reflective (white) across the entire spectrum,or totally absorbent (black)?

15

General Science / How does shoe polish work?

« on: 17/11/2022 22:39:18 »

I polish my shoes with Kiwi Parade Gloss (other premium finishes are available).

Standard procedure: remove loose dirt, wipe with damp cloth, leave to dry, apply polish and brush to an even shine.

Now comes the bit I don't understand. Sprinkle some water on the now water-repellent waxy surface, and brush again. This produces an almost mirror-like gloss, as every soldier knows.

What's going on in the last "spit and polish" phase? 

16

Just Chat! / Unanswered questions in classical physics!

« on: 01/11/2022 13:37:14 »

Isaac Newton analysed white light with a glass prism that he bought in Cambridge market, and thus established spectroscopy and the wave model of light.

If anyone else had ever bought a prism, wouldn't they have noticed how it analyses white light?

If nobody else had ever noticed it, why was anyone selling prisms in a market?

17

Technology / Are trains greener than green?

« on: 02/10/2022 18:03:40 »

A couple of days ago I spent an enjoyable few hours in the first-class waiting room at Newcastle railway station because the wonderful wind that makes the electricity to run the trains, had exceeded expectations and blown down the wires.

Whilst there, I contemplated the attached notice, and rejoiced. Apparently LNER's trains, on the days that they actually run, produce 513% less carbon emissions than a plane.

On my planet, "100% less" = 0, so it seems that these remarkable machines actually absorb five times as much carbon dioxide as a plane emits!

Is that why they sometimes proceed at the speed of a growing tree?

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Technology / Is sustainable energy really possible?

« on: 30/08/2022 22:02:48 »

I've copied this from my own contribution in a  "New Theories" thread because I think it is too important to languish among fantasies and perpetual motion machines.

All renewable energy, apart from tidal power (which has been around for over a century with very little useful output), ultimately comes form solar radiation. 

Let's look at some numbers.

Mean solar input to the earth is about 3 kWh/square meter per day. About 6 x 10¹³ square meters of the planet is habitable or within reasonable reach of humans, i.e. where we could plant some means of turning solar input into useable power. Let's allow 1% of useable surface for this purpose, since we need as much as possible to grow food. So we have the potential to harvest 3 x 10¹¹ kWh per day of renewable energy, whether from wind, direct heat, or photovoltaics.

To sustain a reasonable western lifestyle requires about 120 kWh per day per capita. So for 8 x 10⁹ people we need 9.6 x 10¹¹ kWh per day.

So even if we devote 1% of the surface to generating renewable energy at 100% efficiency, we are short by a factor of 3.2 of the ability to sustain a lifestyle to which most people can reasonably aspire. Given that nobody has consistently achieved 10% conversion efficiency to date, this means that we need to devote at least one third of the habitable surface to power generation (and build a suitable energy storage and distribution system) if we are to eliminate fossil fuels and make everyone happy.

There seems to be little point in pursuing a goal that is so obviously unachievable.

19

Physics, Astronomy & Cosmology / shape change with no energy transfer?

« on: 11/05/2022 23:02:58 »

As an aside from the discussion on energy transfer between ice and water, consider a cube of ice immersed in water, all at 0 deg C.

All the molecules are jiggling about at random. Those at the edges and corners of the cube have the fewest bonds to other members of the cube so are most likely to acquire sufficient kinetic energy to leave the solid and enter the liquid.

It takes 334 joule/gram (latent heat of fusion)  x 3 x 10^-23 gram (mass of a water molecule) to release one molecule from ice. So by leaving, our exceptionally energetic molecule reduces the temperature (the average kinetic energy of all the remaining molecules) of the ice by a tiny amount.

So a water molecule could now attach itself to the ice to restore thermal equilibrium. But the most probable place for the impact to occur is on a flat surface, not an edge.

Thus if we maintain absolute equality of temperature between the ice and the water, the cube will gradually turn into a sphere!

20

General Science / Why do waves move towards the beach?

« on: 10/05/2022 20:02:03 »

Waves are caused by the interaction of wind and tide.

Wind direction varies, tides flow in and out.

But the waves always travel towards the beach.

Why?