1

Just Chat! / Re: Not so good YT videos.

« on: Yesterday at 16:20:04 »

And where does the gravitational field come from when a single photon produces a positronium pair?

2

Just Chat! / Re: Would the world be better if we were all vegetarians?

« on: Yesterday at 09:13:31 »

Watch out for a new movie "Eating ourselves to extinction" released this week in cinemas. My original article in Physics World (July 2005) set out the calculations.

Incidentally the annual quiescent CO2 emission of a horse is about the same as a small car travelling 8000 miles. Animals are not green!

3

That CAN'T be true! / Re: Are Flat Earthers really for real?

« on: Yesterday at 09:03:05 »

Not a problem. There is a standard intercept procedure set down by the International Civil Aviation Organisation. If intercepted, I'll happily agree that we are close to the edge, and we'll be able to see the wall that keeps the water in.

4

That CAN'T be true! / Re: Are Flat Earthers really for real?

« on: 15/09/2021 17:35:51 »

Does anyone know a flat earther who would like a flight to the edge? Happy to share costs. Eastward from the UK is preferable for me - saves renting a life raft.

5

Technology / Re: EoN a con?

« on: 08/09/2021 09:43:42 »

Electricity is not essential. Food is. 90% of the UK is farmed for food, and it's not quite enough to sustain the human population.

The choice is obvious: reduce the population to a level that can travel around on solar electricity and eat insects. That's good for the economy.

6

Physics, Astronomy & Cosmology / Re: Lagrangian Mechanics - Why is Nature lazy?

« on: 02/09/2021 23:11:48 »

From any state B a system could evolve into two states A or C where B→A requires the input of energy or results in a decrease of entropy, and B→C is the opposite. A either presumes the existence of an external agent  (so B was not a complete description of the system) or that evolution will lead to a less probable state of the system, which is clearly less likely than C.

We have discussed elsewhere that energy is simply "that which is conserved"  so in a fully described (i.e. closed) system, ΔK = -ΔV by definition, so the most probable path is B→C with ΔS > 0.

7

Just Chat! / Re: Is it wise to call androids slaves/robots!?

« on: 01/09/2021 11:01:51 »

The essence of slavery is ownership. If you own a machine or animal of any kind, it is a slave. You have the absolute right to exchange it for goods or money or to dispose of it whenever you wish, for any reason. The only  limitation in most jurisdictions is that animals must be cared for or humanely killed, but however pointless it may be, starving or beating your own machine is not an offence.

Not a good idea to ponder too much on the origins of English words. "Jungle", for instance, derives from the Hindi for "desert". 

8

Just Chat! / Re: Is it possible to have a short discussion of history on a sciene forum ?

« on: 01/09/2021 00:14:18 »

But how did they cope with the fact that it wasn't actually on/over the Cg?lift?

The CG is generally close to the CL  to minimise elevator drag but it obviously moves around as you burn fuel or expend munitions, so you are frequently adjusting the trim tab en route to or from combat. Cruise trim was less critical in the Mustang which had most of the fuel in the wings ( so you have to switch tanks from time to time in cruise to keep the wings level) , and flying the Concorde involved a lot of pumping fuel between tanks to maintain trim,

The center of lift is actually the sum of all the lift vectors, so as long as the wing configuration is fixed, so is the CL. Variable sweep wings present another problem but the behavior is very similar to deploying flaps on a fixed wing: in landing configuration the CL/CG moment changes so you (or the computer) re-trim to bring the nose down.

9

Just Chat! / Re: Is it possible to have a short discussion of history on a sciene forum ?

« on: 31/08/2021 23:55:37 »

Why did the Soviet Il/yushin-2 Sturmovik have an external/underslung oil cooler/radiator?
Where it could be shot up, from behind, anyway, and U couldn't have a hardpoint.
Was it to take advuntage of the "Meredith effect" or was it a poor man's cabin heater?
And what about the P-51 Mustang and Hurricane, then?

Pretty well the only bulletproof part of a fighter was the secondary windscreen. Spitfires had one or two external radiators under the wings: much the same total area as they used the same engine, but differently configured because the undercarriage folded in the opposite sense.

10

Just Chat! / Re: Is it possible to have a short discussion of history on a sciene forum ?

« on: 31/08/2021 23:34:38 »

What was with the mo/ustache?

Absolutely vital piece of kit, my dear chap. Along with the girlfriend's knickers in one pocket, slightly grubby wings on the chest, and a battered cap with Brylcreem stains. Can't go around looking like a novice!

It's sobering to note that the Old Man in a bomber crew was probably no more than 25 and half the fighter pilots were in their teens.

11

Just Chat! / Re: ufo's/uap's tell us your story

« on: 31/08/2021 05:03:00 »

What intrigues me is that so many photographed  UFOs follow variants of the same design pattern - the top cover of  a  globe light fitting. And just like terrestrial light aircraft, the design has evolved slightly over the last 50 years, from pierced metal (which we earthlings used to ventilate incandescent lights) to solid plastic (as shown in the OP, which is OK for domestic LEDs).

So no worries about contact with vastly superior technology. The little green men have bathrooms just like ours.

12

Chemistry / Re: Did they really use Cordite in Little Boy, AB?

« on: 29/08/2021 22:44:21 »

Cordite was a British invention but widely available as it was manufactured in quantity in Canada. If I was designing a one-off bomb based on wholly new physics, I'd want the least number of engineering unknowns, so a stable, off-the-shelf propellant that has been around for 50 years is a good start. Remember a fair number of Commonwealth citizens were involved at Los Alamos and if you can turn the gun design over to a guy who has done it a hundred times before, let him choose the chemistry he is familiar with, for the best chance of it working first time.   

13

Just Chat! / Re: What exactly is the connection between poverty and illiteracy?

« on: 28/08/2021 23:34:10 »

The point I missed earlier is that literacy gives you access to everything that anyone has ever learned, made, discovered or argued about.

Without it, you are restricted to what the bloke next door tells you. At best, illiteracy produces societal stagnation, at worst, prejudice and superstition.

Not sure about the US "attitude". The Constitution grants free primary and secondary education to all children in the USA, even including those who have no legal right of abode, and with a specific right to be taught English if you are not a native speaker. That doesn't sound like frowning on the poor and illiterate. Given that public schools may not teach religion, it is remarkable how much prejudice and superstition survives in what should be a literate society with a common language.

14

Physics, Astronomy & Cosmology / Re: How does a diffraction grating really work?

« on: 28/08/2021 12:38:36 »

The good news is that we have cheap lasers so you can demonstrate the difference between red and green diffraction and a whole bunch more besides.

15

Physics, Astronomy & Cosmology / Re: How does a diffraction grating really work?

« on: 27/08/2021 23:51:26 »

You can't model diffraction with particulate photons.

At some point, kids have to accept that we need two very different mathematical models for the behavior of electromagnetic radiation. I think it is important not to talk about "wave-particle duality"  as if the term were meaningful. Fact is that em radiation does what it does, and we don't have one model that predicts all of it.

If it were up to me, I'd demonstrate with a real grating and real light. Computer simulations do not inspire awe or inquisitiveness.

16

Chemistry / Re: Did they really use Cordite in Little Boy, AB?

« on: 27/08/2021 22:45:42 »

Practically everyone.

17

Just Chat! / Re: Chickens are over priced

« on: 25/08/2021 16:49:10 »

But dead and therefore surplus to egglaying requirements.  And possibly male, so useless anyway, like the rest of us.

18

Chemistry / Re: Did they really use Cordite in Little Boy, AB?

« on: 24/08/2021 11:06:22 »

The value of cordite as a propellant is that it is a fast "low" explosive, unlike its constituent nitroglycerine which is a "high" explosive. That is to say that the flame rate does not exceed the speed of sound inside the charge so the product gases are released at a smoothly increasing rate, accelerating the missile very predictably without shattering it (or the gun barrel).   

Apparently it is a British invention

A United Kingdom government committee, known as the "Explosives Committee", chaired by Sir Frederick Abel, monitored foreign developments in explosives and obtained samples of Poudre B and Ballistite; neither of these smokeless powders was recommended for adoption by the Explosives Committee.[citation needed]

Abel, Sir James Dewar and W Kellner, who was also on the committee, developed and jointly patented (Nos 5,614 and 11,664 in the names of Abel and Dewar) in 1889 a new ballistite-like propellant consisting of 58% nitroglycerine, by weight, 37% guncotton (nitrocellulose) and 5% petroleum jelly. Using acetone as a solvent, it was extruded as spaghetti-like rods initially called "cord powder" or "the Committee's modification of Ballistite", but this was swiftly abbreviated to "Cordite".

19

That CAN'T be true! / Re: Why are atheists despised and ridiculed?

« on: 19/08/2021 18:06:37 »

The reason prayers are unanswered is because every prayer is asking the omnisicent and omnipotent creator to reverse his irreproachable decisions that led to the status quo, and is thus blasphemy or lese majeste of the most dangerous kind.

I recall a preacher who praised the Lord for answering his fervent prayer that the Blitz might stop so his sick daughter could die in peace. And lo, as she passed away on 14 November 1940, no bombs fell on London. The Nazis heard the Word of the Lord and bombed Coventry instead.

Be careful what you wish for.

20

Physics, Astronomy & Cosmology / Re: Where is the gravitational potential energy? School-level question.

« on: 19/08/2021 17:48:43 »

"Mineralogy Crystallography and Metallurgy" was Tuesday and Thursday afternoon, more than 50 years ago, but IIRC Tommy is right - the process is one of forcing the naturally random moments and domains into alignment, so the work done is principally overcoming the "thermal" randomness of the raw lump of material. Once scrunched, whether they remain aligned or not depends on the impurities present or added to the iron, so you can get "soft" or "hard" magnetic materials by judicious alloying and tempering.

I tried using the analogy to teach 10-year-olds the value of tight binding in a rugby scrum but our masters at Twickenham thought I was mad and wouldn't add it to the curriculum. Don't they learn anything at primary school?

I once managed to shunt a car, just enough to magnetise it without rendering it undriveable. Made navigation easy thereafter as whatever road I took, the compass indicated "south".