Wikipedia:Reference desk/Archives/Science/2021 July 14

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July 14

Collision between cars of different mass

Suppose a 2,000-pound car runs head on into a 3,000-pound car. Which car absorbs the most energy (or are they equal)? Also, wouldn't the passengers in the 2,000-pound car experience higher g-forces? Bubba73 ^{You talkin' to me?} 04:32, 14 July 2021 (UTC)

If one car has a substantial crumple zone and the other has not, the passengers in the crumpled car will almost certainly express lower g-forces, while their car absorbs the most energy. So it depends on the build of the cars, and higher energy absorption is not per se bad for the passengers, as long as the car absorbs the energy and not the passengers. In a fully elastic collision, the colliding bodies experience infinitely high acceleration for an infinitesimal amount of time. But that is a theoretical concept – real collisions are never 100% elastic. Also, this does not say anything about what passengers experience if they can move to a certain extent within the car. If the cars are perfectly identical except for one having an extra dead weight, then the distribution of that weight over the car will make a difference. The passengers in the heavier car will then be better off if that weight is all in the front than if it is all in the back. This all is a complicated way of saying, it depends.  --Lambiam 06:37, 14 July 2021 (UTC)

According to Newton's 3rd law, the force exerted by car A on car B is of equal magnitude and opposite direction as the force exerted by car B on car A. The work (energy) absorbed by the crumple zone is equal to that force integrated over the distance the car shrinks. So the car that shinks most, probably absorbs the most energy. Not always though, as the crumple zones may be different. A soft, long crumple zone will shrink a lot at low force, absorbing little energy. The harder crumple zone of the other car only begins to contract after the soft crumple zone has been used up and may absorb more energy.

As the forces are equal (but of opposite direction), the accelerations of the cars are inversly proportional to their masses, so in this case the occupants of the lighter car experience 3/2 times the acceleration of the occupants of the heavy car.

Things change if a significant fraction of the mass is ahead of the crumple zone. PiusImpavidus (talk) 09:02, 14 July 2021 (UTC)

Thank you - that makes sense. Bubba73 ^{You talkin' to me?} 14:50, 14 July 2021 (UTC)

BTW, this came up because my wife was looking at car safety ratings, but the cars are grouped by weight. So running one into a brick wall is equivalent to hitting a car of the same weight head-on. I tried to convince her that if a light car runs into a heavy car, the passengers in the heavy car will usually be better off (all else being equal). Bubba73 ^{You talkin' to me?} 22:06, 14 July 2021 (UTC)

It is instructive to study the IIHS reports on fatalities in car accidents, which are classified by vehicle, age, and so on. here's an example. https://www.iihs.org/news/detail/driver-death-rates-remain-high-among-small-cars Basically very small cars tend to be bad news. The results are confounded by driver preferences, for example 2 door and 4 door versions of the same 4wds often have different rates, 2 particularly in rollover, as 2 doors are usually bought by younger people. https://www.youtube.com/watch?v=ExQUGk12S8U is an excellent introduction and is why I prefer that my wife drives the big car. Greglocock (talk) 00:32, 15 July 2021 (UTC)

Which leads to an arms race. In a collision the heavier car is safer for its occupants than the lighter car, so everyone wants to have a heavier car than every other person. The result is that everyone gets a heavy car, nobody has a heavier car than the other person and there's no effect on the number of car users who get killed or injured in collisions. At the same time, the noise, fuel consumption and pollution increase and traffic safety for pedestrians and cyclists decreases. Great, isn't it, that prisoner's dilemma? PiusImpavidus (talk) 09:34, 15 July 2021 (UTC)

"In a collision the heavier car is safer for its occupants than the lighter car" That's what I thought intuitively. Bubba73 ^{You talkin' to me?} 04:47, 16 July 2021 (UTC)

Assuming they're belted in? ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:54, 16 July 2021 (UTC)

Yes and no. Since people do buy small cars it would appear that not everybody follows the same logic, or places the same weighting on passive safety as I do. Greglocock (talk) 23:28, 15 July 2021 (UTC)

Well, some people can't pay for a big car, some have no room to park it close to their home, some care more for the environment and some care more for the safety of other road users. Still, bigger cars, altough better for the safety of the occupants, are worse for the rest of the world. BTW, I haven't got a car at all. I'm in one of those small countries along the North Sea. PiusImpavidus (talk) 08:37, 16 July 2021 (UTC)

Do you bicycle distances that Americans usually drive? I've heard you've got some of the best bike path systems in the world. Sagittarian Milky Way (talk) 15:19, 16 July 2021 (UTC)

Americans will drive from the front door to the curb to check the mail; they are hardly a useful meter stick for how to live a reasonable life without a motor vehicle. --Jayron32 15:21, 16 July 2021 (UTC)

Really? I thought that was just a NY lottery commercial. I've walked up to 2 hours in one sitting to save train fare. Some Americans do use bicycles but Europe seems more hardcore. Sagittarian Milky Way (talk) 16:15, 16 July 2021 (UTC)

I'd rather have a 1-seat 4WD car with strong brakes, slalom g's and high acceleration at street and highway speeds to try to outmaneuver out of crashes and thread the needle between lanes of cars as a last resort, but too many buyers of that wouldn't be careful enough with the power and raise insurance rates for everyone else. This may be a common response from New Yorkers who never drive and go everywhere on foot and know how clumsy most cars are at 0-10, fitting in gaps and especially seconds to turn 90°. At age 18 I taught myself to stop or corner hard from 10-12 mph by sliding on flip-flops while adjusting bank angle for friction blips — better g's than a 350(0) pickup truck (warning: you want a flip-flop sole that's not too sticky, not too slick and do not attempt if floor's too frictioned to drift or too slippery to balance (AKA keep center of mass over feet in the frame of the slanted apparent gravity)) Sagittarian Milky Way (talk) 21:09, 15 July 2021 (UTC)

We call this the organ-donors argument, commonly used by motorcyclists. Greglocock (talk) 23:28, 15 July 2021 (UTC)

If I only play with the thing on a closed course and maybe within legal limits in the day if the road is clean, dry and deserted and would otherwise be a cautious fuck like a Volvo driver why would I be an organ donor? Also find a narrow hallway with handrails where someone won't pop out of a door and crash into you to practice pedestrian drifting on, wear appropriate gloves with low outside friction and high-inside friction sturdy in case you need to grab the handrails. Or start at minimum speed and raise speed very slowly till you get the hang of balancing to stay aligned with the slanted "gravity" of deceleration. I would start sliding sideways (like a snowboarder) as it's less risky. Sagittarian Milky Way (talk) 00:15, 16 July 2021 (UTC)

Off-topic question, but if a car drives down a highway at high speed, or drives down the regular streets at a slower speed and thus taking longer time, would the gas usage be the same or different? 67.165.185.178 (talk) 04:06, 15 July 2021 (UTC).

That's going to depend on the fuel efficiency of the given vehicle. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:31, 15 July 2021 (UTC)

Air drag increases quadratically with speed, so if you travel 41% faster on the highway, you consume twice the fuel per unit distance. There are some complicating factors though. Petrol engines have a limited RPM range and large losses inside the engine. At lower speed, you need a lower gear, which increases internal losses per unit distance. Electric cars don't suffer from this effect. And on a motorway you have grade-separated crossings, so you don't have to stop at junctions, saving energy. This effect too doesn't apply to electric cars with regenerative braking. PiusImpavidus (talk) 09:34, 15 July 2021 (UTC)

Okay, I think I know where you got the 41% from, from the square root of 2, so v * 1.414. But when you say quadratically, you mean v^2 right? Where does square root of 2 come in? 67.165.185.178 (talk) 02:59, 16 July 2021 (UTC).

Drag increases with the square of velocity. If you increase velocity by a factor 2, drag increases by a factor 4. To increase drag by a factor 2, you have to increase velocity by a factor ${\sqrt {2}}$. PiusImpavidus (talk) 08:37, 16 July 2021 (UTC)

Resolved

How can just only magnetic field generate radio waves?

Sentence from Radio star reads: The rotation powers a magnetic field, which generates the radio emissions. But on Earth, it was generated by a transmitter and then detected by a receiver. Rizosome (talk) 17:15, 14 July 2021 (UTC)

It's not a static magnetic field. See more in Pulsar#Formation,_mechanism,_turn_off. --Amble (talk) 18:06, 14 July 2021 (UTC)

Here's a paper about using rotating permanent magnets to generate low-frequency electromagnetic waves for near-field communication: [1] --Amble (talk) 18:10, 14 July 2021 (UTC)

Hypodermic and catheter needles

As a US 60+ year old, I have had blood drawn for physicals for over 40 yrs, and @ 30 yrs of injections (flu vaccines, tetanus, etc). I've also had IV lines inserted, years and decades apart, for comparison.

My perception is that the "needles" have improved over the decades. Perhaps the diameter has decreased? They are sharper? Feature an improved angled entry point? Plastic needles? More expert phlebotomists? My husband has a severe needle phobia, and even he agrees with this "over the years" improvement perception. Does anyone have information to support our mutual perceptions? I read Hypodermic needles and Catheter, but they don't seem to address my specific questions. Tribe of Tiger ^{Let's Purrfect!} 23:47, 14 July 2021 (UTC)

In short, yes. Forgive me, I am having a bit of trouble finding more than a handful of good sources ([2], [3], [4], [5], [6], [7]) from among manufacturing and biomedical research journals for two reasons: 1) the evolution of the form and manufacturing of such needles (even over just your lifetime) has, of course, proceeded over decades, and 2) the search queries share in common many terms with other leading edge biomedical products, including microneedles, making digging up the right sources a laborious task. So I will attempt to better populate that short list later, time allowing. However, in the meantime, I think you are looking for more of a summary of the developments in this area, so I will attempt to provide a (very brief) discussion those details here, with the caveat that my understanding of them relates better to the medical applications rather than the principles under which their engineering has advanced. But how these technologies have adapted to physiological demands seems to be more the thrust of your question than the material science of the manufacturing anyway.

In any event, we can break down the improvements to needle design and engineering into a few discrete (if obviously interrelated) categories: refinement of the composition of the needles (that is, general developments in the material science relating to the metals--and more recently, to some extent, plastics--from which the needles are constituted), diameter of the tip as well as the particular geometry of the tip, diameter of the gauge of needle trunk (which can itself be understood as a function of the thickness of the tubing support itself as well as the inner volume, both as dictated by differing applications), and auxiliary features such as lubricant coating. But in some regards, it isn't so much any uni-dimensional movement in one direction (i.e. a smaller diameter or particular tip/aperture geometry), so much as increased variety of options and clinical understanding of what qualities are appropriate and necessary to particular medical applications.

These advancements in particularized roles combine with (as you speculated) a generally improved level of training for the personnel administering the relevant procedures, as well as generally improved understanding of technique for reducing force (and thus trauma) in the injection of the needles. But yes, as to the needles themselves, they have benefited substantially from purely material composition/contouring features which have reduced the necessary force required to achieve penetration, overall coefficient of friction, and likelihood of compromise or buckling of the needle under the influence of force, which have combined to improve the rate of successful and therapeutically appropriate penetration with fewer attempts on average, decrease the rate of adverse events, and generally make insertion and/or maintenance of a needle/catheter less unpleasant in a somatosensory and nociceptive sense. There has also been marked improvement over time in the standards for how many repeated uses a given needle will be allowed before disposal (and of course a substantial increase in the role of fully disposable one-use needles), addressing the factor of decreased efficacy of these products from wear, even as durability of remaining multiple-use needles has generally improved over time.

FYI, on a side note, the main perceived benefit of plastic over metal needles at this time relates to their potential mass production for disposable applications, which are seen as useful for avoiding contamination in some clinical settings where effective sterilization of needles is difficult or problematic. I can't say with any research authority that there are no plastic polymer needles of a given geometry which are considered superior in terms of force-exerted-per-effect-achieved and/or buckling rate relative to a similarly shaped metal needle, but to the best of my knowledge that is the case. While plastic sounds intuitively like a "softer" option, remember that what you are ultimately looking for here is how much force is required to get the needle in place to do its particular job, with a minimum amount of trauma to the tissue being compromised for this purpose and a minimum chance of buckling, torsion, or other compromise of the needle itself. Snow ^{let's rap} 01:21, 15 July 2021 (UTC)

Snow Rise Thanks for this thorough and detailed reply! I copied it offline, for further consideration and follow up per the sources. Your phrase " they have benefited substantially from purely material composition/contouring features which have reduced the necessary force required to achieve penetration, overall coefficient of friction..." seems to address the comfort factor very well. Less force required, less "drag" as skin and tissue are penetrated...also coupled with a (possibly) smaller outer diameter, would decrease the perception of pain.

You state "There has also been marked improvement over time in the standards for how many repeated uses a given needle will be allowed before disposal..." I thought that all human use "needles" were required to be one use only, although I seem to remember my veterinarian stating that they sterilized & reused needles and syringes.

Plastic needles: Hazy memory of these being used for elbow IVs for me or my mother, in 2014, or after. Much better than the immovable spike, which hampered movement, and caused one to worry that it would poke back upwards, out of the arm. Again, I thank you for taking the time to answer my question in such detail! Tribe of Tiger ^{Let's Purrfect!} 21:08, 15 July 2021 (UTC)

You're quite welcome, Tribe of Tiger: I'm glad I could be of some help in responding to your curiosity here. :) Regarding your follow-up comment, your observation is correct that (at least in most contemporary clinical settings), the trend is overwhelmingly towards disposable needles and catheters, particularly where it comes to blood draws and short-term intravenous lines for fluids and IV medication. However, there remain many types of needles (which are not technically called hypodermic needles but which have the arguably defining features you would associate with one: possessing a contoured bevel and cannula with inner lumen for the passage or capture of fluids or tissue) with niche surgical applications and interventionist applications, and some of these, for various reasons, are not practical as disposable equipment, or continue to come in disposable and multi-use variants. Further, some clinical settings in remote locations or with inconsistent resources/supply may continue to use multi-use sterilized needles and catheters, although the disadvantages of these products are such that these practices are increasingly marginalized. But you actually already identified the one major area where re-usable hypodermics remain most prevalent: veterinary medicine: although here too the tide has turned. As to plastic needles, their use is certainly building, but hasn't proceeded quite as fast as was perhaps first expected when there was an explosion in interest in them starting in the naughties. Issues with ease of application and concerns about buckling with it's implications for poor passthrough, as well as potential catheter failure/IV infiltration remain issues that are still being researched and worked out. Oh, I also remembered this, which I think you might find intriguing: [8]--though it's not really relevant to the kind of clinical context in which your inquiry arises, at least not at the present time. Snow ^{let's rap} 01:00, 16 July 2021 (UTC)

I remember when the advent of HIV/AIDS spurred a move towards disposables. Specialized "needles" for niche use must have a cost variable comparison...depending on whether they are sufficiently sturdy and expensive enough to autoclave. That makes sense. The new source you provided states: "the injector senses the loss-of-resistance on encountering a softer tissue or a cavity". Not sure if this is in use now? This year, my 89 yr old mother has begun receiving eye injections, to preserve her sight. She relates that the physician said " ten years ago, you would have faced blindness". A sensitive injector, as described, is an obvious boon, when working with the eyes. I can imagine the eventual everyday uses/adaptations of such a device. Fascinating! Thanks once again. Tribe of Tiger ^{Let's Purrfect!} 02:39, 16 July 2021 (UTC)

If they cared so much they would stop being barbarians and ask if you want lidocaine or not before taking blood. The fuck is wrong with them if they don't even ask and expect to stab a minor's hand or elbow crook? Smallest needle ever made my ass. Doubt even space elevator material could be small enough, the minor vein pressure has to be able to pump a few test tubes through the needle before it clots, and the size of a blood cell is only 2 orders of magnitude less than a needle. And give a can of stuff to numb your DPT vaccine for a day or two, fucking hurts like a baseball bruise or something. Sagittarian Milky Way (talk) 08:27, 15 July 2021 (UTC)

:) I feel for you. I have exceptionally delicate veins that present difficulties for insertion, so any time I need any venipuncture for purpose of a catheter, it quickly becomes a massacre. Thankfully I have been lucky enough in my personal health that hospital stays for my own needs have been relatively few and far between over my life. But invariably when I have needed to be on the receiving end of the needle, they ultimately have to call in the ringer (the nurse on the shift who is best at placing lines).

Luckily for me, I have several things going for me to weather these situations: even as a child, I had a fixation with physiology, as with all naturalistic phenomena that I could directly observe, so I had a degree of fascination with the needle process when younger, and more of a vague clinical detachment to the process these days. As to the pain itself, I've always been the type to want to put people at ease if they feel guilty about something they shouldn't, and because I know the difficulty of the task, I'm inclined to brush off the pain of the repeated stabs and put an understanding, good-humored face on, absent being very ill or under the influence of a more significant sort of pain from the condition that brought me there in the first place. And because one's feigned disposition can actually have a certain impact on the perception of pain at lower thresholds, via both cognitive and biofeedback mechanisms, I generally get through the experience without feeling very traumatized. Sounds like you have a pretty high pain differential by comparison, though--I'm sorry you have to deal with that! Snow ^{let's rap} 19:06, 15 July 2021 (UTC)

I had my blood drawn with a butterfly needle where they pricked many times to get blood to come out and the pain was bearable cause they slathered 2% lidocaine (actually they put a long shallow smear on the faintest indication of forearm vein and left and I stole some to cover much more area and thickness just in case, cause I did't know how anesthetic it is or at what point is the end of diminishing returns and can't take any chances) and left it on for 10 minutes first. Sagittarian Milky Way (talk) 23:06, 15 July 2021 (UTC)

Sagittarian Milky Way, IMO, being "pricked many times" is likely a fault of the "phlebotomist", not the particular needle. Doesn't help you at the time! If possible, be sure you are well hydrated prior to a blood draw. I experienced more difficulties in early morning appts, because I was mildly dehydrated. Learned to take water with me to drink, prior to/while waiting for blood draws. Tribe of Tiger ^{Let's Purrfect!} 01:27, 16 July 2021 (UTC)

Tribe of Tiger, it's probably because I put so much lidocaine, a medicine which usually has vasoconstrictor added to make it vasoconstrictive instead of vasodilative to circulate it away from the numbed area slower. And veins are deep in the parts of the skin that hurt less. Sagittarian Milky Way (talk) 02:06, 16 July 2021 (UTC)

One factor to consider: If you're afraid of needles, look away while they're doing the injection. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:39, 15 July 2021 (UTC)

Baseball Bugs, good advice, I always look away. I don't even like to see injections on film/TV. Alien 3 has a pretty horrible injection scene, more comfortable with the beast! My husband passes out, if he not lying down...."you can put me on a table, or pick me up from the floor". They put him on a table, 180 lbs of passed-out man is hard to manage. Tribe of Tiger ^{Let's Purrfect!} 22:13, 15 July 2021 (UTC)

That would not work if you've been tortured as a child and teen by bad blood drawers before, and cannot ever trust someone to not bump it accidentally again or stab you 0.2 seconds too slow no matter how good they are, cause a sub-second mistake would be so unbearable and you have genes where a ~50-100μm wide black ceramic glaze splinter can cause a very strong stab pain and stop moving this instant reflex as soon as you flex the finger and it's not even near the fingertip, but rather the finger base, non-nail side. It barely pierced epidermis and I still had to grab the top microns by the fingernail edge to remove it. It could've just been partway cutting through an unusually amount of pain dendrites for its size and location though, still a very high amount of pain per pain neuron for me. Sagittarian Milky Way (talk) 08:27, 15 July 2021 (UTC)

Sagittarian Milky Way, I am so sorry you had such bad experiences. For a child or teen, a blood draw is an act of intent, and having a stranger " purposely hurt " you, obviously creates mistrust and fear. My younger sibling had unfortunate experiences with physicians as a child, which has extended into a deep wariness, decades later. Best wishes to you. Tribe of Tiger ^{Let's Purrfect!} 22:32, 15 July 2021 (UTC)

Tribe of Tiger, I agree, completely unnecessary. In today's vaccine- and expert-sceptical climate any little bit would help. Sagittarian Milky Way (talk) 23:17, 15 July 2021 (UTC)

Sagittarian Milky Way, from my perspective and experience (first surgery & IV catheter in 1971) things have improved greatly, which was why I posed the original question. Over this 50 year period, I have noticed improvements. In 2014/2015 I had two back-to-back life-threatening health episodes, which caused me to be "stuck" on a regular basis. So much better than prior decades. My recent blood draws & the IV on 7/13/21 were marvels of painlessness, even compared to 6/7 yrs ago. Although I've had some bad experiences, over a period of decades my perception is that things are better, in the US, at least. Tribe of Tiger ^{Let's Purrfect!} 23:52, 15 July 2021 (UTC)

contains content you might not want to read while eating Sagittarian Milky Way (talk) 19:31, 16 July 2021 (UTC)|

Extended content

I'm amazed, how did they expect everyone to take the thicker and unsophisticatedly manufactured needles of the past without anesthesia? Not everyone is blessed with those genes where they hardly feel syringes. When they IV'ed boy me for an operation they sprayed something on the area first to numb it and would've even done it with nothing if I had answered option B, I don't remember if that hurt or how much. I had so much post-operative pain and it was a red mess under the bell end plastic that held painkillers and it fell off too soon. Sagittarian Milky Way (talk) 01:05, 16 July 2021 (UTC) Sagittarian Milky Way, well, it's all we knew, shots hurt, IVs hurt. I am amazed because things have improved, over 50 yrs!. Consider childbirth prior to epidurals, etc.! The human condition is one unending search for the alleviation of bodily pain. Still, we suffer. Could you explain "it was a red mess under the bell end plastic that held painkillers and it fell off too soon."? What is the bell end plastic that fell off, etc.? You've had a bad time of it, I think. Tribe of Tiger Let's Purrfect! 02:56, 16 July 2021 (UTC) I've had one operation and it was a circumcision. Sagittarian Milky Way (talk) 13:58, 16 July 2021 (UTC) Ohhh! That is dreadful! I read the WP article to understand the "bell". Words fail me. My very kindness sympathy for your trauma and suffering. Tribe of Tiger Let's Purrfect! 17:43, 16 July 2021 (UTC) Thanks. At least it came off in deep sleep. It was designed to loosen around when the painkillers run out and it's semi-healed but I woke up with it, clothing and the organ glued to each other with drying blood plasma and unglued what I could with water soak. The stuff in the sheath must've been pretty anesthesic for something so long-lasting and non-addictive, I wonder what it was. Still didn't stop plasma from leaking till it glued the urethra opening, which causes a blink of full urination blockage and intense pain then urination and some bearable afterpain. This is why it hurts less and is more like 0.1-0.2 seconds than 0.2-0.3 when the bladder is super full and you don't try to throttle the urine pressure. If that ever happens dip a needle or appropriately shaped nail in a shot glass of pharmacy alcohol till it's sterilized, let it dry and try to poke a decent sized hole in the dried plasma. Hopefully the urethra is glued in an open slit shape, not closed. Sagittarian Milky Way (talk) 19:31, 16 July 2021 (UTC)