Wikipedia:Reference desk/Archives/Science/2015 February 7

= February 7 =

Self-assembly and creation of carbon nanotubes fibers
Hi there!

On this link it speaks about the creation (formation) of carbon nanotube fibers. Is it right to say that at first a self-assembly is necessary for having longer carbon nanotubes that they can be spinned to fibers? How does the self-assembly part exactly work and how the spinning part? I need to know all important details. Can you help me please?

Thank you in advance for your help!

Calviin 19 (talk) 11:39, 7 February 2015 (UTC):


 * Uhum... In Mork and Mindy, he was was always concatenating nano-nano-nano on the TV tube and spun it not in to fibers but a series.--Aspro (talk) 18:18, 7 February 2015 (UTC)
 * So, what do you mean?--Calviin 19 (talk) 11:21, 14 February 2015 (UTC)

Alewives
What does the word alewives mean?--Calviin 19 (talk) 14:02, 7 February 2015 (UTC)


 * The alewife is a type of fish. The word is sometimes used humorously because it looks so strange. Looie496 (talk) 15:03, 7 February 2015 (UTC)


 * So it's really a fish in the text which I've linked?--Calviin 19 (talk) 15:36, 7 February 2015 (UTC)
 * Sorry, I can't see any link. Alansplodge (talk) 18:05, 7 February 2015 (UTC)
 * In the original post there was a link, but it seems to have disappeared. It linked to a paywalled site, though, so I couldn't look at the text anyway. Looie496 (talk) 16:26, 8 February 2015 (UTC)


 * I would have thought it was British English for a "barmaid". StuRat (talk) 16:37, 7 February 2015 (UTC)


 * In Britain we often use the term 'serving wenches' - an entirely different kettle of fish- if you pardon the pun--Aspro (talk) 18:06, 7 February 2015 (UTC)


 * See alewife.—Wavelength (talk) 17:24, 7 February 2015 (UTC)


 * EO's take on it squares with what StuRat says. ←Baseball Bugs What's up, Doc? carrots→ 18:32, 8 February 2015 (UTC)


 * See also, Entwife. μηδείς (talk) 22:04, 8 February 2015 (UTC)


 * In addition to other antecedents of the British Pub, including Taverns and Inns, there was also the Alehouse or Beerhouse, which was a private dwelling whose householders were licensed to brew and serve ale/beer (let's not go into the difference for now, it can get argumentative). If the brewer was a woman she was (by definition) a brewster, and if she was the alehouse's proprietor she would often be known (colloquially if not legally) as an alewife. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 16:39, 9 February 2015 (UTC)
 * The link hasn't disappeared. It's in the paragraph above.--Calviin 19 (talk) 11:19, 14 February 2015 (UTC)

Tire radius and mileage
In a rerun of Car Talk, a charming retired math teacher asked the Tappet Brothers to resolve a dispute with her husband, who wanted to install bigger wheels to get a better mileage. They sided with him, citing that "2πr" will be larger, and hence the car will move further. Much as I respect their knowledge of cars otherwise, their conclusion doesn't sound right. I rather trust our editors Friday and Atlant, who, at, pointed out that bigger tires should waste more energy. So, who's right? &mdash; Sebastian 18:48, 7 February 2015 (UTC)


 * Using larger tires reduces the mechanical advantage of the engine, so it's just like shifting to a higher gear. For each gear there is a particular speed that will give the best fuel economy, and this would change what speed that is.  But fuel economy is a function of drag as well as engine design and I don't think there's a simple way to figure out how it would change.  If you found yourself spending more time in lower gears (according to the speed of traffic you were usually driving with) then it would be worse. --65.94.50.4 (talk) 19:39, 7 February 2015 (UTC)


 * The story that I am about to tell is not directly relevant, but it has to do with the measurement of the wheels. In the early 1970s, steel-belted radial tires were new.  It was said that installing steel-belted radial tires, in place of bias-belted tires, would improve gas mileage.  I installed steel-belted radial tires, and was unable to detect any improvement in fuel economy.  However, what I learned is that the saving could not be measured using the usual measurement technique of the odometer.  What steel-belted radial tires did was to reduce tire slippage against the road.  The odometer had been calibrated at the factory based on an assumption about what type of tires were being used and so how many times the wheels would rotate in a mile.  There was a saving in fuel, but it wasn't reported, because the odometer was "off" by the amount of the saving.  I think that installing larger wheels will likewise cause the odometer to misreport.  Robert McClenon (talk) 20:10, 7 February 2015 (UTC)


 * That is interesting. My 1973 car came with pre-steel-belted wheels (Goodyear Tigerpaw, to be exact).  From the first set of new tires, I used steel-belted ones.  So the speedometer was probably reporting a speed that was lower than the actual speed with steel-belted tires, right?  Do you have any idea how big this factor was?  Bubba73 You talkin' to me? 01:38, 8 February 2015 (UTC)


 * This is segment 7 of today's show (#1506), starting at about 29:45. I listened to it because I couldn't believe they would really claim on the air that fuel efficiency is linearly proportional to tire circumference. But they did. They said she could calculate the mileage improvement exactly that way, even using the word "exactly". They're wrong, of course, as 65.94.50.4 already said. -- BenRG (talk) 22:04, 7 February 2015 (UTC)


 * Excellent replies, everyone, thanks a lot! I wish I had thought of Anonymous' way of putting it so simply! &mdash; Sebastian 22:15, 7 February 2015 (UTC)


 * There's also the weight to consider. The more a car weighs, the worse it's mileage will be, and larger tires, and especially wheels, add to that total.  Another factor is that larger and heavier tires/wheels will have more rotational inertia, and all that inertia is wasted every time you hit the brakes (unless you have regenerative braking). StuRat (talk) 22:23, 7 February 2015 (UTC)


 * The increased weight and rotational inertia of the larger wheels and tires are surely negligible, since the change is tiny in proportion to the total weight and inertia of the car. --Srleffler (talk) 02:12, 8 February 2015 (UTC)


 * The tires are rotating a lot faster than the car, so have a lot more rotational inertia. If this wasn't the case, nobody would have bothered inventing regenerative braking.  As for weight, I can imagine the wheels and tires being maybe 5% the weight of some cars.  So, doubling their weight would have a significant effect on the overall weight. StuRat (talk) 02:19, 8 February 2015 (UTC)


 * Your reasoning about regenerative braking doesn't sound quite right. Regenerative braking would still be beneficial even if wheels had negligible moment of inertia. --173.49.18.95 (talk) 08:32, 8 February 2015 (UTC)


 * True, in that the regenerative braking also converts the linear inertia of the car into electricity (or possibly rotational inertia, in the case of a flywheel system). StuRat (talk) 16:31, 8 February 2015 (UTC)


 * Your wording suggests that you're assuming that the rotational kinetic energy of the wheels is more significant than the translational kinetic energy of the rest of the vehicle. I haven't done a sample calculation but I don't think that's true. The mass of the wheels is only a small fraction of that of a moving vehicle (which has at least one occupant and possibly cargo). Don't forget that rotational kinetic energy is just translational kinetic energy of mass moving about an axis. If you ignore the compressive deformation of the tires, the linear speed of any parts of a wheel around its axis is no greater than the linear speed of the vehicle as a whole. From this it'd seem impossible for the wheels, considering their total mass, to carry more rotational kinetic energy than the translational kinetic energy in the mass of the rest of the vehicle. (The wheels also have translational kinetic energy, but I'm ignoring that in this discussion.) --173.49.18.95 (talk) 18:07, 8 February 2015 (UTC)


 * So why not put tiny wheels on every car ? Well, big wheels give you more ground clearance, can last longer, may be less likely to slip on snow and ice, and do better going over road hazards like potholes. StuRat (talk) 22:23, 7 February 2015 (UTC)


 * All other things being equal a larger radius tire may have better actual rolling resistance as the contact patch flexes less when it contacts the ground. this is offset by the increase in aerodynamic drag and weight. Also, all things are not equal, usually. Greglocock (talk) 01:18, 8 February 2015 (UTC)


 * The key fact here is that changing the diameter of the wheels is precisely the same thing as changing the gear ratios in the transmission - which is more or less the same as shifting gears at some different point. On a modern automatic transmission, the computer does the shifting - and generally, they do it as a compromise between performance and fuel consumptiun...and flipping into "SPORT" or "ECONOMY" mode (assuming your car has that feature) - is pretty much the same as changing out the wheels.  So what you're doing by changing the wheel diameter is saying "Those idiots at Ford/BMW/Kia/whatever are incapable of picking the optimum gear ratios and shift points - so I'm going to use this kludge to improve on that."...and unless you know a LOT about cars - you're very, very unlikely to be right.


 * Of course, your speedometer will read incorrectly, and your odometer will mis-read - and tampering with the function of that device is illegal in most countries.


 * There is a deeper issue - larger wheels generally weigh more - that's bad because you need more energy to accelerate a heavier car - but it's especially bad for handling...the ability to keep the wheels planted onto the road depends on "unsprung weight" - the total weight of everything between road and springs. Adding more unsprung weight can dramatically worsen handling - so you won't be able to throw your car into corners like you could with smaller wheels.


 * So there are many ramifications here...but fuel improvement is unlikely to be one of them unless you have a car that's heavily optimised for performance at the cost of fuel economy...and that's a rare thing for normal road cars.


 * SteveBaker (talk) 00:10, 9 February 2015 (UTC)

Related Question

 * The above discussion touches on something I was wondering about (again) just the other day.
 * Tyres are permitted a certain amount of wear before the tread depth falls below a legally permitted depth (which doubtless varies between different jurisdictions). Presuming that one's speedometer and odometer are calibrated to brand-new tyres, then tyres that are legally worn to their maximally legal lesser diameter will presumably result in over-reporting of both distance travelled and speed. Roughly what percentage will the difference be? {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 15:58, 9 February 2015 (UTC)
 * It's easy to calculate - the circumference of the tire is 2.pi.R - if the radius of the tire changes by 1% then the circumference will also change by 1% and so will the odometer and speedometer readings. For a typical 30cm radius wheel, every millimeter of tread wear represents a third of a percent difference in speedometer/odometer readings.   Since speedometers in almost all modern cars are deliberately set to read around 5% high, an extra couple of percent either way won't make much difference.  SteveBaker (talk) 17:28, 10 February 2015 (UTC)

Can a rotating black hole be a pulsar?
Like for example, it was previously a pulsar as a rapidly rotating neutron star, and then gained mass and became a black hole, couldn't the pulsar effect continue? thanksRich (talk) 19:38, 7 February 2015 (UTC)


 * See pulsar. The pulsar effect is due to the radiation beam that is pointed in the direction of the sun.  If the neutron star gains mass and becomes a stellar-mass black hole, then the beam can no longer escape from the gravitational field of the black hole.  So the answer is no.  Robert McClenon (talk) 19:43, 7 February 2015 (UTC)


 * That's incorrect; black holes can and do have strong external magnetic fields and emit energetic jets. But to qualify as a pulsar, the magnetic poles (and jet direction) have to be misaligned with the rotational axis, and as far as I know that's impossible for a black hole (it would be hair). -- BenRG (talk) 20:17, 7 February 2015 (UTC)


 * Agreed. To clarify a bit more, the beam can escape when matter is swallowed from the accretion disk, outside the event horizon.  Only matter inside the escape horizon is unable to escape (except for Hawking radiation). StuRat (talk) 22:30, 7 February 2015 (UTC)