1

General Science / Re: Do rockets violate conservation of energy?

« on: 22/04/2022 02:12:47 »

Hi.

Nice diagram  @wolfekeeper .    Out of interest, what was the blue line?

Blue line is the instantaneous energy efficiency (force times vehicle speed/half the exhaust velocity squared) of the rocket engine expressed as a percentage of the internal engine efficiency at turning available chemical heat energy into fast moving exhaust.

2

The Environment / Re: Is hydrogen a better fuel source for the environment?

« on: 23/01/2022 23:02:14 »

The Mirai is not really very competitive in fact, not being able to refuel at home is a complete show stopper for most people shopping for a green car:



The Mirai is selling very badly.

3

Plant Sciences, Zoology & Evolution / Re: Why is chocolate bad for dogs (& cats &...)?

« on: 01/10/2021 22:33:14 »

As evan has already said, theobromine is just a stimulant in the same family as caffeine. Dogs livers don't break it down quickly and they get overstimulated and overheat and can have convulsions. They can eat a little bit, but best not to give them a taste for it.

Fun fact: When I had swine flu I had an awful cough. I ate half of a dark chocolate bar, and my cough STOPPED for about 2 hours or so. Theobromine suppresses the vagus nerve which causes coughing. Doesn't cure the cough in any way, but you stop coughing for a bit.

4

General Science / Re: Can someone healthily raise a cat or dog with meatless food?

« on: 01/10/2021 22:19:28 »

No, cats can eat an entire non meat diet if they have to. But there are issues with things that are normally missing from plant diets (e.g. taurine, vitamin D, etc. etc.) and things that are normally in plant diets in too high quantities (carbohydrates).

But these are not show-stoppers, taurine is routinely synthesised (it's in Red Bull) and is already added to cat food. Things like carbohydrates aren't in all plant foods. But they are in game meat, because muscles and livers contain glycogen.

Basically, cats are mammals, and their dietary requirements aren't THAT dissimilar to human ones, but the proportions they've evolved to need are different.

The issue is not so much that it can't be done, it CAN be done, at a cost. The other issue is that we don't necessarily know the exact nutritional requirements cats need to stay healthy.

5

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

« on: 27/09/2021 01:57:09 »

Yes, most of them are for real. They just don't know any science. It's all magic to them. It's like dyscalculia- the inability to do maths, but physical. They don't get anything, so for them the idea that the Earth is flat make more sense.

They're often but by no means always, homeschooled by very religious parents, reading textbooks written by people who believe that the Old Testament is literally true. The Old Testament was written by people who hadn't heard the research the Greeks had done to measure the diameter of the Earth. So they believe in Noah's ark, the whole Enchilada.

There are plenty of other routes to flat Earthism but that's a royal road there.

One of the key attributes of religions is that they're based on the supernatural. That means that 'minor' things like completely violating all physical laws, gravity, momentum etc. etc. is a non issue.

6

General Science / Re: Is science a religion.........well if not why is it defended as though it were

« on: 24/09/2021 01:05:47 »

Newtonian and Lagrangian mechanics give pretty good prediction on the movements of those moons for quite long period. Given the same information, will Schrodinger equation give similar prediction?

The Schrodinger equation has actually been used to numerically predict the position of the moon and other objects.

How many solutions are provided by Schrodinger equation, if only sun, earth, and its moons are considered, and nothing else? Are those solutions packed close to each other, like Gaussian, or scattered spikes like combs?

I'm not sure, it probably depends on the boundary conditions and how you visualize it. In principle the solution could contain all possible orbits over space time. For practical purposes you have to artificially increase the size of the wavelengths by enormous amounts; the particles of the real moon has wavelengths in the subatomic size, whereas numerically you would probably have to use many miles.

7

General Science / Re: How can some planes can fly higher than others?

« on: 16/08/2021 04:20:01 »

I believe that one of the limits is the engines. You have to compress the thin air so you can reach a combustible mixture. But when you compress the air, the air gets hotter. Eventually you have problems just because it gets too hot at the inlet/compressor and the air's nitrogen starts reacting with the oxygen. I believe nitrogen burns endothermically, so you're losing lots of energy at the compressor. That's really bad, by way of contrast the SR71 and Concorde recover almost all the energy from the inlet. I think that sets an altitude limit.

Can anyone shine some light on something that has fascinated me for a long time I have tried to research this but with no luck. OK, If you look at the horizontal fin/elevator on a 747 and on almost all large jet planes you will notice that the profile of the fin is like a wing only inverted.

That's completely right. Those tail wings push the tail downwards!!! Well observed!

The reason is because the aircraft has to be stable when moving through the air, so the centre of mass has to be as far forward as possible, so that the aircraft sort of shuttlecocks and keeps itself pointed into the airstream. In fact the CofM has to be ahead of the centre of lift. The centre of lift lines up pretty much with the wings, and the CofM has to be ahead of that. So if you think about it, the nose of the aircraft always wants to drop, and it should go into a screaming dive. This would be bad.

Instead, the empenage section has a wing (the horizontal stabilizer) which pushes the tail downwards, and balances out the torque from having the centre of mass ahead of the centre of lift. Because the tail is quite a long way back, it doesn't take a lot of downwards lift to balance the torque, and although it's not terribly efficient, it's not horribly inefficient.

Some 'tailless' aircraft designs, such as Concorde, don't do this, they sort of rely on just having a very long wing. Early tailless designs were horribly unstable, but newer ones are much better. Also, a lot of newer aircraft are actively stabilized, so that the centre of mass and the centre of lift are more or less inline and a computer is constantly fighting to keep the aircraft pointing forwards into the air stream using the control surfaces.

8

That CAN'T be true! / Re: Are electric cars environmental greenwash?

« on: 22/05/2021 00:53:37 »

Or, as has been pointed out by someone sensible, they could get people to use economy 7 style systems - just like they already do.

So your car ceases to be a personal convenience and becomes a state-monitored privilege that can be withdrawn. The communist dream: electric Trabants!

So what you're saying is that your fossil car is not licensed? Because your car already is a state-monitored privilege that can be withdrawn.

People WILL NOT be usually charging their cars at peak demand. People are using economy 7 or 10 because the electricity is half the price

Surely they will want to charge their cars as soon as they get home? My point is that this will create a new peak demand.

If they need their car to be topped up because they're making a long journey and they're willing to pay a premium, sure. Just like people pay more for petrol on motorways and A-roads. And, it's still cheaper than paying for petrol!

But most people prefer to pay less, and the car capacity is sufficiently large that it's not worth it. The whole point of Economy 7 etc. is to move demand to the early hours where electricity is plentiful and greener.

Of course the sensible thing to do would be to transport something like hydrogen or whatever- just like we do[ with oil.

Agreed.

Except that's not actually sensible for cars because hydrogen costs a lot more all round, and there's heavy conversion losses.

This seems to be roughly the state of the art, the hydrogen fuel costs four times as much per mile as electric vehicles:

https://cleantechnica.com/2019/04/26/hydrogen-cars-have-4x-annual-fuel-cost-2-70-times-the-carbon-debt-as-electric-vehicles/

9

Physics, Astronomy & Cosmology / Re: Can we communicate with civilisation in anti matter solar systems

« on: 18/05/2021 21:05:20 »

I think antimatter might be worth the hassle, boy or boy would it be a powerful rocket fuel!

A lump of anti-iron would be amazing. You could hold it with maglev and just ablate some off with a plasma when you want some energy.

10

Physiology & Medicine / Re: Is marijuana intoxication while driving just as dangerous as alcohol?

« on: 08/05/2021 00:53:39 »

This study seems to suggest that while alcohol tends to make people more reckless leading to accidents, marijuana tends to make people more cautious which compensates for the fact that they're not driving as well:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2722956/

11

That CAN'T be true! / Re: Are electric cars environmental greenwash?

« on: 02/05/2021 15:22:44 »

Oh well, I'd better tell Toyota and Audi that they are wasting their time.  Beats me how those idiots ever get to make anything that works.

Yeah, about that:

https://pressroom.toyota.com/toyota-to-debut-three-new-electrified-vehicles-for-u-s-market/

Doesn't look like their hydrogen strategy has worked out.

As for NASA, using ridiculous stuff like hydrogen as rocket fuel and in their fuel cells - no wonder nobody really believes they flew to the moon.

Oh I know something about that. Liquid hydrogen is MUCH denser than compressed hydrogen. It's also EXTREMELY inefficient to manufacture. First you have to make the hydrogen. This is not particularly hard. Then you have to liquefy it. This takes an ENORMOUS amount of energy. Bet you thought water had a high heat capacity, well, hydrogen, it's much worse.  And in the overwhelming majority of cases, that heat of liquefaction is wasted.

This means that liquid hydrogen is extremely inefficient to manufacture. And it has nothing to do with cars. It has to be stored in vacuum containers, and even then long-term storage requires active cooling. Even aeroplanes would have difficulty handling it and would have to store it in the fuselage, wing tanks have too much surface area, it would boil off too quickly.

So, no, not the same thing at all. Even with the extra density from liquefaction it's only marginally worth it even for rockets. The main issue is the remarkably low density of the hydrogen, makes the tankage extremely heavy for rockets. There's a sizeable fraction of the space industry that claim that hydrogen is an expensive mistake even in rocketry, still, and you'll note that the Falcon 9 doesn't use it.

As for NASA, using ridiculous stuff like hydrogen as rocket fuel

Their vehicles run about 5 inches per gallon.
Did you somehow think they were relevant?

Are you suggesting that they would fly better with an electric motor?

Electric pumps are a decent choice:

https://en.wikipedia.org/wiki/Rocket_Lab_Electron

They use lithium ion batteries for power.

12

That CAN'T be true! / Re: Are electric cars environmental greenwash?

« on: 02/05/2021 04:50:56 »

The energy density of hydrogen is much lower than gasoline all round. The mass density is multiple times lower, once you include the mass of the tank, and the volumetric density is too:

"To carry the energy equivalent of 400 L of diesel oil would require a truck to carry a 5 kL hydrogen storage tank, with a weight of 3.4 t."

https://www.sciencedirect.com/topics/engineering/compressed-hydrogen

That's quite a lot; it's a factor of ten worse than diesel and adds significantly to the axle weight.

The fuelling stations are extremely rare and tend to explode:

https://uk.motor1.com/news/354304/hydrogen-fuelling-station-explodes-norway/

Putting hydrogen through the existing UK gas pipes is likely to be a nightmare due to hydrogen embrittlement on high pressure steel pipes as well as escapes at low pressure joints:

https://www.sciencedirect.com/science/article/pii/S0360319913006800

Additionally hydrogen is the most flammable and most explosive gas there is and, charmingly, it also burns with an invisible but extremely hot flame. People walk into flames they can't see. The idea of piping it around the whole UK fills me with dread, particularly through a network that was never designed for it.

There's simply too many problems with it.

13

Physics, Astronomy & Cosmology / Re: Can you measure the one way speed of light without synchronised clocks?

« on: 25/04/2021 01:30:39 »

FWIW Veritasium has a video about this exact topic:

14

Technology / Re: Why do wind turbines shut down?

« on: 12/04/2021 19:59:54 »

Does anyone share my visceral dislike of the idea, that in the 21st century, we should be building windmills?

Doesn't it strike any scientifically-minded person, as a deplorable throw-back to the medieval ages.  When "windmills" were one of the best of the only energy-sources available.  In those pre-scientific times.

But now we have modern science.  Surely this can enable us to devise better, more efficient and reliable sources of energy than anachronistic windmills.  The deficiencies of which have been well pointed out by previous posters.

I live in Brighton, on the south coast of England.  Just off the coast, out to sea, there seems to be a whole collection of windmills.

In a  "windmill farm", as the term is.  I've glimpsed them with my naked eye, but have never cared to look at them with binoculars or a telescope, to verify whether they're really there, or just a mirage.

That's because I don't like the sight or even the thought of them.  They are repellent.

Whereas if a modern 21st-century nuclear power-station were built off the coast. I could look at that all day with admiration and delight!

Am I the only one to be thinking this way?

Polls have been done on that, the overwhelming majority of people disagree with you. They either really like them as I do, I think they're really elegant and cool looking, majestic and inspiring, or they think it doesn't matter how they look. Only a very small minority hate wind turbines like you do. And that INCLUDES people that live near them.

Where I am now, there's several less than a mile away, and numerous wind farms about 5 miles away. If I look in the right direction from the living room, there's one I can just see one through the trees. They're absolutely no trouble to us at all. They did turn one down for planning permission because it was on a ridge and might have caused TV interference for the village, but that's all. And they don't kill huge numbers of birds either, they kill about as many birds as a single house per year, and far less than the cats from inside the house.

Very few people think nuclear power plants are cool-looking. They're just squat buildings most of the time.

15

That CAN'T be true! / Re: Are electric cars environmental greenwash?

« on: 11/04/2021 01:19:18 »

Arguing that electric cars are more environmentally friendly because they are electric as many do while ignoring the real environment damage that is comming more from the mining of the materials to make the cars and the system of overall production, beside the fact that the energy source that powers the vehicle can also be highly damaging to the environment, seems to me rather rediculas.

Hence I consider the suggestion that electric cars are more environmentally friendly as merely an expression of green wash.

This isn't true at all though. The amount of materials that go into the fabric of an electric car is only slightly more than that of a fossil car, and most of it is, or can be, just as recyclable as a fossil car.

But that's missing the enormous mineral elephant in the room; the huge pile of mineral oil that the fossil car burns over its life! That pile is an order of magnitude bigger than the car and cannot be recycled.

Meanwhile the electric car can overwhelmingly be recycled, even a lot of the battery.

For electric vehicles, we're talking a factor of over 300 more recyclable.

Source:

https://www.transportenvironment.org/sites/te/files/publications/2021_02_Battery_raw_materials_report_final.pdf

16

Technology / Re: Microwave side door gap, is it safe?

« on: 01/04/2021 19:11:21 »

The fan is used to cool the magnetron. It's more like a computer fan, it's removing heat from a relatively small component. Those types of fans generally are quite noisy.

17

Technology / Re: Could a special window maximise sunlight and vitamin D production?

« on: 21/03/2021 18:06:52 »

Quartz does let UV-B through, but it's very expensive and not very transparent to normal light:

<commercial link removed>

You'd be a lot better off just taking vitamin D pills! They don't cause cancer.

18

Physiology & Medicine / Re: Can measles erase the body's antibody memory?

« on: 21/03/2021 17:49:25 »

Yeah, measles creates immune amnesia. It came to light because when measles jabs were given in the third world, childhood mortality went down massively, lagging by about two years- whereas the measles vaccine acted almost immediately. Turned out, having had measles in the last two years left the immune system as a smoking ruin that took a while to get working again, and that leaves people susceptible to getting diseases that can kill them.

Allowing for that effect, it turns out that measles was indirectly killing up to 1% of the people it infected- making it even worse a disease than people thought it was.

19

Technology / Re: Could regenerative braking help with air pollution from car brakes?

« on: 31/01/2020 22:56:45 »

Brake dust apparently makes up 20% of pm2.5 pollution:

https://www.dailymail.co.uk/health/article-7864855/Metal-particles-vehicle-brake-dust-damage-immune-diesel-pollution.html

Certainly brakes on vehicles with regenerative braking tend to last many fold longer, so you would expect a lot less pollution to be released.

20

Technology / Re: Would it be possible to 3D print housing using waste plastic?

« on: 29/01/2020 02:21:01 »

I mixed up some thermoplastic polymer concrete a year or two ago. To make it I found all you have to do is just heat a thermoplastic, stir in some sharp sand and then let it cool. I tried a few different polymers and they seemed to all more or less work. It seemed to be potentially 3D printable, although it looked like it was going to be quite abrasive on nozzles. Polymer concretes are actually quite strong.

If the waste plastic is cheaper than portland cement, then it could be quite good; normally plastic is relatively expensive. What kind of cost per kg can you get?

The upsides are that polymer concrete is usually waterproof, but the downsides of concrete is that it's a good thermal conductor; in most cases you want poor thermal conductors for structures, and the flammability is an issue, but with the right additives or polymers might not be an issue.