exchemist

Thanks, um, Kevin. These are nothing to do with Space X, I notice. The NASA one seems to be a kind of artificial photosynthesis, but they don't explain what they aim to react with the CO2. If they want to make methane, they need hydrogen from somewhere. Is this water? Doesn't seem to be specified. All very unclear. The National Geographic report is about capture of atmospheric CO2 (which I have read is being done at pilot plant scale in Iceland) and reacting that with hydrogen from electrolysis. So it requires a lot of electricity - just as producing hydrogen itself does. For rocket fuel I can't see this has much advantage over simply producing hydrogen, which is a well established rocket fuel already. As for Musk's claim, again I struggle to see the point if all it involves is reacting CO2 with a hydrogen source to produce a hydrocarbon fuel. When you burn it, you will get the CO2 back again. So what is the advantage over simply generating hydrogen and burning that? Re plastic waste, there is a pyrolysis method for producing a mixture of light, medium and heavy hydrocarbons, rather analogous to the products of fractional distillation of crude oil. No doubt some of these fractions could potentially be used as rocket fuel, but since methane, propane, gasoline, diesel etc., are not used for rocket fuel today, I suspect the products of this process would not be first choice for rocket engineers.

Um, Kevin, Elon (Musk) is a person. Perhaps you mean Space X is on its way to using CO2 for rocket fuel. However all I can find on this is some throwaway comment on social media from Musk. Do you have information about a concrete proposal from Space X to do this? How do they plan to do it?

Getting an engine to do work is extraneous to the problem in mechanics that the OP was trying to resolve. But never mind, it's solved now anyway.

You can treat an "ideal" flywheel, crank and piston assembly as a closed system, for the purpose of understanding why there are no intrinsic energy losses in the motion. All the other stuff, while obviously true of real engines, is extraneous to the problem.

Yes, that's it exactly. For an isolated mass, you would need to do work on it from outside in some way, to accelerate it. But for the piston in an engine, it just exchanges kinetic energy with the flywheel and crank, twice per revolution of the engine.

If you read my post about this you will see I say the (kinetic) energy of the piston goes into the flywheel (and in some cases other pistons) and then comes back to the piston on its return stroke. Just as in @mistermack's sprung mass example, the kinetic energy goes into potential energy in compressing or stretching the spring and then comes back to kinetic energy again. But the total energy of the system stays constant, if there is no friction or other source of loss. That's what the crank does: transforms linear kinetic energy into rotational energy and back again. Think of a single cylinder engine. Every power stroke, the piston gives a kick to the flywheel, which is then able to move the piston back on the exhaust, induction and compression strokes, slowing down all the while, until it is given another kick by the next power stroke. The flywheel is the kinetic energy store and its speed changes throughout the cycle as it gains energy from, and loses energy to, the piston. When the piston decelerates, it does work on the crankshaft and flywheel, and then the crankshaft and flywheel do work on it to accelerate it again. No net energy loss need occur.

You can judge that for yourself I think, by what they say about such things. If you talk to a Methodist, or an Anglican, or Episcopalian, or a Scottish Presbyterian (I don't know about Presbyterians elsewhere), or a Lutheran or a Catholic, i.e. major recognised denominations with a body of theology and some form of authority structure, you won't find them taking literally all these bloodthirsty stories in the Old Testament.

The caricature of Christian belief and the association of it with US right wing politics. Don't get me wrong: both the caricature and the association seem to have a fair amount of validity in a US context, sad to say. Elsewhere however it would not be seen as a very thoughtful critique of Christian belief, that's all.

This is all very American.

OK, you will know better than I, I'm sure. I suppose what you are saying is that there is something about pure mathematics that corresponds to the physical world and this is not necessarily what one might expect. I shall have to think about that.

There may be a misunderstanding here. There is nothing energy-absorbing about the change of direction of the piston. Its momentum changes, but then so does the momentum of another piston on the crankshaft to compensate. The kinetic energy of the piston is transferred by the crankshaft to the flywheel - and/or to other pistons - and then back to the piston again as it accelerates in the opposite direction. Don't rely on YouTube videos for explanations: many of them are made by idiots or people that can't explain things properly. The main losses in a reciprocating engine, apart from thermodynamic ones, are pumping losses (obviously and inevitably) and friction losses. Both can be considerable. There are many sliding surfaces, e.g. piston rings sliding on the cylinder liner, plain bearings, cam followers against cams, and so forth. On your original question, it makes no difference how the engine is mounted in the vehicle. The momentum transfer takes place internally in a well-designed engine. Any momentum transfer that is not internally compensated will merely make the engine and vehicle vibrate at the frequency of the piston's motion. There is either way no net momentum transfer to the vehicle - unless the engine blows up and throws a piston of course. 😊

True. And Gaussian distributions do crop up in physics. (Euler's Identity shows a connection between e and π which I suspect may be somewhere at the bottom of this, but I don't pretend to be a mathematician) Is all this unreasonable?

Where π is concerned, it is not surprising that circles, spheres and periodic phenomena occur in nature. Finding exponential processes in nature is not a surprise either.

Well yes, but that's just because complex numbers are a good way to represent periodic phenomena: being numbers in 2D, a fixed radius rotating is handy for waves etc. via projections along real and imaginary axes and so forth. I don't see that it is surprising that items from the mathematical toolkit are useful to model aspect of the order in nature. It is the order that is the issue, really.

I should have thought that what he should really have marvelled at is the patterns, i.e. the order that we perceive in nature. Human mathematics is just a way of modelling that order, it seems to me. Such constructs as Hilbert space are just our models, after all.

One problem with such a model, surely, is that a non-expanding universe with "tired light" fails to account for either the CMBR or what General Relativity tells us we should expect, and nor does quantum mechanics provide for any process to allow photons to lose energy in the way you describe. So why would science adopt a model that creates three major unresolved problems, in preference to one that accounts well for all three? Choosing a model with weaker explanatory power is not generally the way science progresses. And what does your model predict? How can we test it, in a way that shows its superiority, in some way, to the current one?

Well if this is confirmed we may not need to bother. But let's see. But in fact that reminds me of a piece in the Financial Times the other day regarding BioNTech, saying one great advantage of their mRNA method of production is it obviates the long growing time needed by conventional methods. They were hopeful that for example flu vaccines, which have to be changed each year according to the variant forecast as likely to be dominant each winter, could be made fast enough for the variant to be selected at the end of summer, instead of early spring as now. That should allow each year's vaccine to be much more reliably targeted than at present.

I don't know why the numerous mutations have had this effect but preliminary reports suggest Omicron does not go for the lungs any more, just the upper respiratory tract, like any other "flu" or cold virus.

True. H bonds typically have a bond strength of the order of 10% of covalent bonds, in water about 20kJ/mol. (Though they can range quite widely in strength in particular instances.) But H-bonds are I think believed to be a bit more than purely electrostatic dipole attractions. At least, my understanding is that they have some directionality, associated with the "lone pairs" on the electronegative atom. Whether it is covalent character, or just electrostatic attraction to electron density in the lone pairs, I'm not sure. As far as I know, they remain an object of theoretical study.

Aren't you forgetting the strongest interaction by far, in both cases: hydrogen bonding?

I'm not so sure. This is an ion, HeH⁺, that is isoelectronic with H₂, i.e. with 2 electrons in a σ-bond formed by overlap of the 2 1s atomic orbitals. Though it will be strongly polar, due to the higher charge on the He nucleus (i.e. the 1s on He will be pulled in and won't overlap so well). I'm sure it is highly reactive: as a cation it will tend to pull electrons off whatever it comes into contact with, and it can easily form He by donating the proton to something. What strikes me about it is that as, unlike H₂ it is polar, it will have a vibrational and rotational spectrum, so presumably can be detected in the IR and microwave regions of the spectrum. P.S. I see there is a Wiki article on it: https://en.wikipedia.org/wiki/Helium_hydride_ion#cite_note-Epa-23 according to which the bond strength is 178kJ/mol, about 40% that of H₂ so quite respectable. Also I notice they think it was a constituent of the primordial plasma, 280,000 yrs before the universe became transparent. So presumably it is not expected to fall apart thermally so easily.

Also, it is now common practice for plumbers to freeze the water in a radiator pipe when disconnecting the radiator, thereby avoiding the need to drain the system. (I've had a guy do this to two radiators in my house recently.) So clearly it can be done without cracking the pipe. Copper is ductile so can take a certain amount of stretching before it cracks. From what I have managed to look up quickly, it retains this ductility down to very low temperatures. So my best guess would be it is unlikely to split - provided there are no joints in the vicinity.

Not on the basis of this and other similar poor reviews I've read.

No, it just seems to be a terrible film, that's all: https://www.theguardian.com/film/2021/dec/27/look-away-why-star-studded-comet-satire-dont-look-up-is-a-disaster. According to the review it is worthy, cynical, smug and condescending. And who is it for? According to this review, it seems to be made for people who inhabit the same bubble as the film maker, encouraging them to point fingers at everyone else.

Then you need to put forward a mechanism by which a photon can somehow transfer a part of its quantum of energy to another entity, without being deflected from its trajectory. As I and others have pointed out, the known scattering processes are no good because these deflect the light in all directions, so that it would no longer seem to be coming from the source in question. In other words they would just attenuate the signal rather than reddening it. So you must have some new process in mind, unknown to physics so far. What is it and how does it work?