ELI5: If the purpose of physical pain is to alert us to injury and help us avoid danger, why is pain sometimes so bad that it incapacitates us?

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To prevent movement that would injure us more, if you break your leg the last thing you should attempt to do is walk on it.

MJMurcott

We get a lot of questions on this theme, like about fever and such, and it boils down to: evolution isn’t an exact science.

Evolution basically means, things change at random until one of them is slightly better than the alternative. Human bodies don’t show evidence of having been designed for perfect running.

So. We have pain, but sometimes it gets to be too much and does worse than not having pain. We get fevers, but they aren’t as effective as some simple medicines.

Basically bodies are good, but understand that our bits aren’t perfect and don’t always do what’s best for us.

Oudeis16

Because our body has limits. Sometimes the injury is so bad that not moving or passing out is the best thing our body can do to protect ourselfs

Lycantree

Doctors always tell you not to ignore pain because it’s your body’s way of telling you that something is wrong UNTIL you get diagnosed with fibromyalgia. Then they tell you that pain is just your nerves malfunctioning, and you should try to ignore it.

I’ve been to the optometrist because my eyes hurt. Just fibromyalgia. I’ve been to the dentist because my teeth hurt. Just fibromyalgia. I’ve had x-rays because my hip hurt. Just fibromyalgia. Now I have to practically be dying before I go to a doctor.

purplelikeme

This is the wrong way to think about evolution and biology, kind of. Tissue damage causes certain things to happen, like chemicals to be release and nerves to be active. That is just what happen when you break things. These processes can also do double duty, as any creature that could avoid the thing that makes the process happen would likely survive, so some kinds of warning systems were selected for.

As with any physical system, you can overwhelm it, – for example when the damage does not stop, and you cant avoid it by a behavioral choice.

PersephoneIsNotHome

ELI5: How do noise cancelling headphones/earphones work?

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There’s electronics inside that listen to the ambient noises around you, then they do what’s called phase cancellation where it makes a sound similar to it and puts it out of phase. The two noises then cancel each other out creating silence.
Phase cancellation is wild but really cool

TheTalmidim

There’s two types of noise cancelling. Active and Passive.

Passive noise cancellation just means the headphones are designed to try and *physically* block sound from getting in. Just like covering your ears with your hands when there’s a loud noise.

Active noise cancelling /u/TheTalmidim explained better than I can. But, often active noise cancelling and passive noise cancelling tech are used together for maximum isolation.

Mob_Nice

Sounds moves in waves. A wave has a peak and a trough. Basically just a big up and down squiggle in the air and the “zero” is a line right down the middle. So part of the wave is above the line and part is below it. The part of the wave above the line is trying to push air up and the part below the line is trying to push air down.

If the wave runs into another wave one of two things can happen: if the high part of one wave runs into the high part of the other wave their “up energy” combines and the peak gets even higher. So two three inch peaks become one 6 inch peak. Same if two parts of the down wave hit each other, they get even lower.

If the waves hit a little different though they’ll fight each other instead. A 6 inch peak runs into a 2 inch trough you get a 4 inch peak instead. The “noise canceling” comes in when you get identical waves. A 4 inch peak hitting a 4 inch trough just cancels each other. They’re trying to move in opposite directions at exactly the same speed so nothing happens.

What the headphones do is take in external noise with a microphone, wait a fraction of a second, and then replay it into your ear. The exact same sound is coming from the environment and the headphones but slightly different timing. If it’s all working correctly, the delay will be just right so all the highs of the environmental wave hit all the lows of the headphones wave. Since it’s the same wave then in theory they would cancel each other completely and you’d have dead silence so you would only hear the song or whatever from your headphones and no ambient noise.

In practice it isn’t flawless so you still can hear a bit usually but a good quality set will help a lot.

sharrrper

I’ll take a crack at it!

Imagine a pond. There are peaks and valleys of the water as it moves up and down. If you made an exact copy of the pond and inverted it (peaks are now valleys and valleys are now peaks)..then put the copied pond exactly on top of the original. It would be a flat calm pond with no valleys or peaks.

This is what the headphones are doing… but with sound waves (which are really just 3d versions of a pond)

Grimple409

The concept is really old and the same as any sound mixing. It’s called de-emphasis. The magic is doing it real time in yo head…don’t forget also there is a small delay and buffer so the waves can be flipped

garbage_band

ELI5: why do gas cars need a gear box and electric cars do not?

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Internal combustion engines have a lot of moving parts. There’s the spinning crankshaft, the pistons going up and down, the valve cams that open and close the intake and exhaust valves for each cylinder, the timing chain or gears that keep those in sync with the crankshaft, the water pump, alternator, oil pump, fuel pump, and probably more stuff I’m not thinking of at the moment.

Because of all those moving parts, the engine has limits for how fast it can spin. If it spins too fast, the physical limits of the metal all those parts are made of is passed, and something will break.

But at the same time, internal combustion engines also don’t work very well at low RPM’s. Too low and they’ll completely stall. Because of that and the aforementioned limits, they can only work between about 1000 RPM and 4000 RPM for average cars.

So, in order to be able to go 15 MPH sometimes and 80 MPH other times, there needs to be a gearbox to allow the engine to maintain that operating RPM range despite how fast the car is going.

**Electric:**

Electric motors don’t need to maintain a certain RPM in order to deliver power. They can exert full power from a dead stop and also exert the same power at full speed at 100+ MPH. So there’s no need to have a gearbox at all. An electric car’s motor can be rotating at 10 RPM while crawling down a driveway without issue.

blahblahsdfsdfsdfsdf

Combustion engines have to run at a particular set of speeds (RPM) such that they can sustain their momentum. The pistons roll round from the previous explosion and get into place for the next, and if they fail to get there in time, the next piston won’t be ready for the next explosion and you’ll get a stall. Alternatively, they can only move so fast and do so many explosions in a certain time (heat, strain on the components, lubrication, etc.)

Thus a combustion engine has a range of RPM that it can run at before it either stalls or becomes dangerous. This RPM range does not generally match the speeds that you want the tyres to go round at, thus you have to gear it right so that the best RPM (efficiency, power, etc.) matches with a good road speed.

The problem is that the range of RPMs isn’t enough to just link it to the wheels directly. You’d be very limited in road speeds and extremely inefficient. And there’s no one fixed gearing that would be suitable – you’d either have trouble pulling away from a standing start, or the engine would be SCREAMING along trying to get you to a reasonable speed. So you use gearing to select different gears depending on what you’re currently doing. An automatic will select from those gears for you, in a manual you have to know to pull off in first, corner in second, and get up to 5th/6th gradually on a motorway, and to know when to change between gears so that the engine doesn’t hit unnecessary strain and is able to change smoothly between gearings without the RPM going too low or too high.

In an electric car, it’s just an magnetic electric motor. As you apply voltage to it, it turns. You connect it direct to the wheels. There’s almost no lubrication, there’s no mismatch in speeds to compensate for, it’s just directly driving the wheels. The more voltage you give it, the faster it goes, and it can turn slowly or very, very fast without hindrance. It literally fits all the speeds you want to do, directly, without having to match speeds between engine and wheel.

ledow

Electric motors can deliver full torque at any speed, while engines have a torque to rpm curve where they produce maximum torque at certain speeds. A gear box will allow the driver to allocate the optimal torque to different speeds.

garlopf

IC (Internal combustion) engines have a narrow range of engine revolutions per minute they work well at, if too low they stall, too high damages the engine, you then need the gear box to get either higher torque and lower speed, low gears, or lower torque and higher speed, high gears, or reverse (an idler gear is engaged), depending on the road.

An electric motor works well at a large range of revs so you can just choose a single gear that gives decent torque and always use that, for reverse you simply switch the current around.

MansfromDaVinci

Gas has a range of spin where power is made. To stay in that range, a gearbox is used to change how fast the engine spins the wheels. Electric also has a range of spin that makes power, but it’s a lot bigger. Because of that, electric cars don’t need gearboxes, but some do have a second gear.

Gas sounds better, though.

-King_Slacker

ELI5: Why do hang overs get worse the older you get?

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The toxins that makes you hungover (acetaldehyde and formaldehyde) stays in you / affects you more because the body gradually loses it´s ability to get rid of it. Another factor to make it worse can be increased alcohol intake with increased tolerance, allowing for higher levels of toxins.

Cloudberrymaster

The older you get, the more effort it takes your body to filter out consumed alcohol. The left over toxins causes the feeling of a “hangover”

lsaffie

When you get old enough, your body doesn’t work as good fixing itself. Also you can tolerate more booboos before it really hurts.

This added together means it takes more damage to feel pain and the damage you get takes longer to fix.

On an non-ELI5 side, there are things you can do to help not make it not as bad:

* Drink a shit ton of water before you go to sleep
* Take an advil in the morning
* Drink better alcohol
* Buy reasonable amounts to hard limit yourself
* Nothing good comes after 2 AM
* start earlier
* drink the same amount over a longer period of time
* pet your cat until they are annoyed
* eat something in the morning (everyone has their own hangover meal)
* Build up tolerance slowly
* Befriend a genie
* keep yourself busy instead of over drinking out of boredom

fast_cherry_bomb

Alcohol dehydrates you until your liver filters it out. This is why you need to drink water first thing in the morning after drinking. Brain doesn’t do too good if it’s a bit dry…

Most of the alcohol is taken care of as you sleep, and whatever hasn’t been filtered causes a little ghost of being drunk. The combination of sleepiness, dehydration, and this tiny amount of alcohol causes the hangover. As you age, your liver isn’t as efficient at filtering your blood as it was when you’re younger, so when you wake up, your body has a little more alcohol than it used to, causing the hangover to feel much worse.

Now I got this from some rudimentary research online that didn’t really answer the question, causing me to have to fill in some blanks, so if it’s not completely accurate, sorry.

Flyaway_Prizm

Depends what you mean by getting older, while it is true that your metabolism slows down with age, it’s not to the extent that you go from no hangover to feeling half dead in a couple of decades or even a lot less.

However, there are numerous other factors, especially when combined, that can make it a lot worse.

**- Fitness**: if you’re less active, you will have more problems dealing with toxins, the impact will be bigger. Having a sedentary lifestyle does make your body feel a lot older than it actually is.

**- Smoking:** if you smoke and this has be going on for several years, it will not only have made you less fit it will also have decreased your overal health. Not to mention smoking are extra toxins, and the longer you have been smoking, the more addicted you are and the more you will be smoking.

This is also true for a lot **other drugs.**

**- Adaption to alchohol:** when you first start drinking, your receptors are very sensitive to alcohol, which is also part of the reason why you get drunk so quickly, so if you need to consume more to have the same effect, it does come with extra toxins as well.

But even if you quit drinking for years you’ll never be as sensitive anymore as in the beginning, so the more you drink, the more alcohol you need to have the same effect, regardless fo habituation and addiction.

**- Weight:** If you weigh more than you were younger, you need more alcohol to get to the same effect as well.

**- Diet:** If your diet has become richer over the years, which is the case with many people, it will also make your liver work more, and that also means this extra work will be felt when you go a night out.

**- Responsibility, sleep deprivation and stress levels:** the older you get the more responsibility you have, work for instance, which also means you will be more tired, yet you’ll sleep less than you’re younger. Stress levels will also change your hormonal balance. All this will make a lot more difficult to bounce back from a late night out.

– Last but not least **money**: Don’t underestimate how much more you drink simply because you can afford it. It’s a major difference sharing a bottle with friends, or having to go out on a budget, compared to someone that can afford what he wants in terms of buying drinks, like a working person can. You’ll drink quicker, you’ll drink more , and you’ll drink stronger stuff as well.

Masspoint

ELI5: If you have 2 of the same item, one heated to 100deg and one cooled to -100deg. Will their temperatures depreciate at the same rate or is one faster than the other?

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You are missing one component that makes it impossible to know, actually a couple. But mainly, how warm is the room they are in. If the room is at 70 degrees, the 100 degree item will cool slower than the -100 degree item heats up. Because the 70 degree air won’t be able to dissipate the heat as fast as lets say 30 degree air, or -20 degree air.

There is still other factors too. For example, not all materials transfer heat and cooling at the same rate. So depending on what material the item was made out of, it might shed heat a lot faster than heat up.

ntengineer

Heat transfer is a really difficult subject with whacky equations governing the details, so the true answer is “it depends on a lot of factors”. The temperature of each object affects the air flow around it, among other things. Heat transfer in a vacuum is not that interesting, so we can not go the typical simplifying route.

Generally speaking, the factor that matters most for conductive heat transfer is the temperature of the item relative to its surroundings. In this case, reading your other posts, that is the same. So, their temperature would change at the same rate, and both items will reach equilibrium temperature at the same time.

However, the hot item can also cool down due to radiative heat transfer. An object emits more radiative energy if it has a higher absolute temperature. This number scales by T^4. This number may not be that big at 100°, but it is there.

tldr; there are too many factors to give a definite answer, but I’d put my money on cooling being very, very marginally faster in most cases.

lamiscaea

Also, are they wet? What is the humidity? The cool one might get covered with frost, which would insulate it. The hot one would cool faster when wet if the air isn’t saturated.

What color are they? Is the airflow unrestricted, or is the object on a surface?

Not enough info, bud

autoposting_system

That depends on what the temperature of the room is and if the objects are being exposed to any light.

If the objects are dark and being exposed to light then they will both be absorbing energy from that light and so the hot thing will cool down slower than the cold one heats up.

But, all else being equal, if the room is at zero degrees then they should reach equilibrium in practically the same time.

aragorn18

So im going to make a couple of assumptions here first. I am going to assume that these objects are metallic in nature and are placed in a nearly perfect vaccum and the “temperature” of this near perfect vaccum is 0deg. In this case, if the material of the bodies are metallic in nature, much of the temperature can be attributed to the kinetic energy of the electrons in the electron kernel. Heat can be transferred, if you ignore quantum mech for now, via conduction, convection and radiation. Since its a near perfect vaccum, the conduction factor is lower. However if you were to consider it, then the electrons with more KE can have more collissions with the particles in the free vaccum to transfer energy, but when you consider the element at – 100deg, the number of collissions between the particles in the vaccum and the ones on the plate are substantially lesser so the heat exchange doesnt happen more. When you take radiation into account, depending upon the reflectivity, emissivity and absorbtivity, the factors change but generally speaking, since objects at a lower temperature (considering a black body approx) radiates and absorbs at a lower rate/higher wavelength, the heat loss from the one at the lower temperature will be lesser.

A similar concept also applies for non metallic plates.

So technically, the one at the higher temp cools faster than the one at the lower temperature heating up.

anaghsoman

ELI5: How do those walk-through scanners in libraries know if you’re taking a book without borrowing it, especially if those books don’t have metal inserts or anything like that?

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> especially if those books don’t have metal inserts or anything like that?

Because it DOES have metal inserts. They are called RFID chips, and they are very small and very cheap. You can even buy them. I suggest google image: “rfid chips label and tags”

BeatriceBernardo

I work in a fairly poor urban library and we do use RFID chips, which are great for self checkin and checkout. They’re on everything, including wee DVDs.

Every few years we have a run on toilet paper (in one series of instances, a lady would bring a bunch of kids in a couple times a week and they would break the paper holders and conceal massive amounts in the kids’ backpacks). A bunch of RFID tags inside the TP rolls, and you’ve found the culprits.

Moral: Don’t steal from libraries. We’re here to help you borrow things for free.

Travelgrrl

The most well-known anti-theft device for books is a tape of magnetic metal strip. The tapes are applied between the spine and the binding of a book (for hard covers) or deep inside in between some pages (for paperbacks). It’s called **tattle-tape**.

Zhinnosuke

Do you know how if you hold a tuning fork near your mouth and sing the right note, the fork will sing back? The books contain very thin “tuning forks” that look like stickers. The scanner you walk through emits a tone that causes any near by “tuning forks” to ring at that same frequency. The scanner then listens for the resonating forks and sounds an alarm if it hears any. You can’t actually hear the frequency though because it is done with radio frequencies.

Edit: It occurred to me that I didn’t actually answer ops question. Basically when you check in or out the book it runs over a large magnet what wacks the tuning fork into and out of tune.

ShiftAlpha

A lot of books have the metal inserts in their spine or in their cover where you can not see them. But it is not unlikely that the scanners are unable to pick up on every book, just the most valuable ones which have the metal inserts. The way these work is tha the metal inserts are in a perfect length to create a magnetic resonance that precisely matches that of the scanner. So when you walk through the scanner the magenetic field makes the metal inserts resonate which can be detected by the scanner. However during the checkout they use a magnet to change the magnetic characteristics of the metal insert and therefore its resonance frequency. This prevents it from triggering the scanners. This is also a reversable action so they can reset the books when you return them.

Gnonthgol

ELI5: Why do plane and helicopter pilots have to pysically fight with their control stick when flying and something goes wrong?

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As others have said, that’s largely theatrics in movies and TV.

There are essentially three systems in use:

* Fly-by-wire is what you will predominantly see in modern airliners and military aircraft. Here, your stick isn’t actually physically linked to any control surface – instead, your inputs send signals to a computer which then positions flight control surfaces to do what you are asking the computer to do. The computers are, in relaxed stability aircraft (like fighter jets), actually continuously sending signals to the flight controls to keep the jet flying stable. In some aircraft, if you turn off the flight control computers entirely, your jet is no longer able to maintain controlled flight.

In this case, the fighting control stick does absolutely nothing. In fact, you won’t even feel the actual feedback from flight control surfaces on aircraft because the stick isn’t directly linked to them.

* Hydromechanical. This is used in older fighter jets and in airliners/aircraft with big control surfaces. Basically, when flying at faster speeds (which creates larger pressure/air loads on control surfaces), human power isn’t enough so the control stick is *mechanically* linked to hydraulic systems that move the control surfaces for you. These hydraulic circuits operate in the thousands of psi. For instance, if you pull the stick back, you are mechanically telling the servos and actuators to move the stabilator (or elevators) to pitch the aircraft up.

In this case, if you did have something go wrong, fighting the stick doesn’t do much either. Most likely, if something went wrong, it’s because your hydraulic line or mechanical linkage broke, or you lost a control surface. In which case, fighting the controls won’t do you anything.

* Direct linkage. This is what you commonly see in older aircraft/lighter aircraft/general aviation like in your Cessna. Here your control surfaces are directly linked to your control stick/rudder via wires and pulleys. You will directly feel the loads on the control surfaces.

Here is where you could, like in the movies, perhaps try to fight for control via physically fighting the stick more. A jammed linkage or connection might require more force to fight through. But even then, you risk breaking something even worse (sudden snapping of control surfaces can overwhelm mechanical limits) OR getting into a PIO (pilot induced oscillation).

MORE likely to happen is if you have a failure in a control surface (e.g. an aileron fails), you have to put in some input like rudder or opposite aileron to keep the plane flying straight and level. In that case, you are “fighting the controls” by keeping some force on the stick to maintain the flight attitude you want. But you aren’t “fighting the stick” like in the movies – instead, you’re precisely and finely putting your control inputs in (or trimming the aircraft) to offset what was lost.

nice_big_cup_of_stfu

The controls are often (especially in older aircraft designs) physically linked to the control surfaces by steel cables. If the force of air is pushing on the ailerons/elevator/rudder it’s also moving the stick around.

rhomboidus

Helicopter pilot, answer is you don’t unless the hydraulic system has failed. This is less likely the larger the helicopter as they have multiple independent hydraulic systems so one failing has no effect at all. Smaller helicopters like Jetrangers or Astars are harder to control with a hydraulic failure but not even that bad, we train to land with the hydraulics off by flying real aircraft with the hydraulics turned off, it isn’t considered dangerous to do so. For a large helicopter if you somehow had all hydraulics fail at the same time depending on the type it is a major emergency and possibly unrecoverable.

CryOfTheWind

One thing I haven’t seen directly mentioned is World War II era fighters/bombers were all cables and pulley rigging for the control surfaces. If those planes dive at the ground during dogfights or attack runs and get going really fast, the forces to move the controls becomes extreme. Thus, you could get into a dive you couldn’t physically pull out of.

So, this can be quite realistic for some movies but for modern aircraft the biggest issue where a pilot is straining against the controls is something like runaway trim/autopilot. For most everything else you’re not fighting with the controls.

tehmightyengineer

Another pilot here. All the controls in my plane are directly connected to the yoke and pedals (manual). When airflow is low, especially during slow flight such as during landings, controls require exaggerated expression. They don’t have much lift being generated to cause a change.
Alternatively, very strong winds in lighter aircraft can definitely cause you to fight. They can quickly push you and change your pitch, yaw, and roll (these are the axis of motion). In this case you have to counter the effects of the wind.

Most of this is experienced extensively by all pilots in training. But it can take real physical effort (without much return from the controls). Usually however, you fly with “two fingers”. A light touch will do it 9 times out of 10 if you’re trimmed in (tuned controls to stable). Remember, flight is across long distances and you generally navigate on 10° increments (eg 010° – 360°) or smaller so planes must fly on small movements and corrections not grant turns like you see on movies.

The only times I’ve ever done movement like that when not training and with passengers was during some landings where the wind goes dead on me or once with an engine out on takeoff with about 400 feet below me to return to runway.

Garret_Pp

ELI5 what causes trees to split? Do they just decide at 5 feet to say fuck it Imma head out?

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It’s because trees want to take a shape that works for them.

Basically trees have five main things in mind.

Keep anchored (which is the job of the roots), suck up water and fertilizer (also the roots), don’t fall over (the roots plus the trunk), reach more sunlight (the branches and the leaves) and suck up sun for energy (the leaves).

There’s a lot of different ways that a tree can do all five, but it really can’t do the fifth one very well unless it spreads out sideways a little (yes, there are exceptions like palm trees). The reason for this is the leaves get in each others way, blocking the sun.

So as it grows, it splits, and this lets more leaves be in more sun because the tree starts getting more sideways as it gets taller. Now the tree COULD just keep trying to keep all its leaves and grow taller and taller without branching… but this starts making “don’t fall over” a lot tougher at some point.

So the tree branches to get more light once it gets to a certain size, and keeps branching after that to collect even more.

the_original_Retro

The top of the tree is in charge. It grows like crazy to get more sun and make more side branches. More sun and more leaves mean more sugar and trees like sugar too.

Eventually the side branches can’t hear that bossy top – they grow a little more and soon they’re telling the buds behind them to grow more side branches while they chase the sun and make more sugar.

Douglas Fir and Sequoia and Red Alder branches have really good hearing and only rarely subordinate.

Lilacs and Hornbeam and Willow never listen.

Trees that are topped go crazy – there’s nothing to listen to.

Likesdirt

At least for some trees, there are special cells (apical meristem) at the end of a stem. These cells produce a chemical (auxin) that stops other stems from branching. As the stem grows long, there is less of that chemical so other stems start to branch off.

The first stem a tree has is the trunk. So after the tree is tall enough, a new stem branches off and the tree starts to get wider at the top. This happens again and again over time.

As it grows taller, the older stems get bigger around to help support the weight. This is able to happen because the tree keeps its food and water movement cells just under the bark. Over time, these cells are made more rigid with other chemicals and new food and water movement cells grow on top of those.

This is why we see rings when we cut into a tree also. This process usually happens on a yearly cycle, so we can often count those rings to know how old the tree is.

This is important to your original question because those special chemicals from the ends of stems are pushed around by the food and water cells. That affects how much of the chemical is in any part of the tree and isn’t a perfect process.

Since this is ELI5, I’ve definitely oversimplified. Someone linked a post in r/askscience that has the more technical names.

ebookish1234

I don’t remember much of the technical lingo from my Aboriculture class (Ecology, Evolution, Natural Resources Degree), but the simple answer is that usually one tree stem is dominant and this becomes what you know as the trunk. What will sometimes happen is a second stem will compete for dominance and both will grow around the same rate. This causes the split that you see. Sometimes those split trunks will even fuse together and form one larger trunk, which is better for the tree.

Edit: still have my text and it confirms that whether or not a tree will actually split is due to genetics, but the reasoning above is accurate.

Edit2: I’m not suggesting 2 trunks are better than one. I was just making a point that it’s better for the tree if they fuse together, which is not likely. One trunk is always better.

Arboriculture 4th edition -Integrated Management of Landscape Trees, Shrubs, and Vines by Richard Harris, James Clark, and Nelda Matheny.

ap1095

The most common cause of trees splitting is a defect inherent in a lot of tree species known as included bark.

Basically, in a healthy branch union, the bark from one branch meets the bark from another branch and grows outward, forming the branch bark ridge. This normally happens in wide or U-shaped unions.

When the union is too tight, or V-shaped, the bark from both branches meet and grows inward. Eventually, this will cause weakness or a split in the timber below.

You can mitigate this by bracing, but it’s rarely successful. The maintenance of bracing (as with most long-term plans for managing trees) needs to be multi-generational for the owners. More often than not, the brace is installed then forgotten about.

Decay fungi can also cause failure, but this is more snapping than splitting.

Garbarrage

ELI5 why do DJs sometimes hold one of their earcups?

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Headphones off, they’re listening to current music. Headphones on, listening to what to play next. One earphone up, matching beat and tempo/speed of both songs to mix well.

Once upon a time I was a (very) small time aspiring DJ

NoReallyLetsBeFriend

Djs need to hear both what you hear, and the music they are preparing on you headphones. They can do this electronically, but many quickly put down an earcup for convenience.

Taki_Yaki_Naki

So in order to prepare the next song, the DJ need to set the transition. That’s what he is doing most of the time, listening to the next version to the mix in advance in order to launch it at the right time.

gr0gui

To make sure the beat of the next song matches the beat of the currently playing song. One ear to hear what is currently playing and the other to hear the one that will be played next

obiwanjacobi

To add to what’s already been said, as to what they are listening for, with EDM like house and trance, the music is matched by way of the kick drum. So by listening closely they can determine if one track is too fast or too slow and adjust pitch or tempo accordingly. For things like breakbeats and drum and bass, the music is matched on the snare, but the idea is the same.

jp_taylor

ELI5: How can themeparks snap a crystal clear picture of you on a rollercoaster going 70mph

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Motion blur occurs when the camera takes too long to generate the image. By the time the last bit of information is taken, the image has changed noticeably. So if all the information is taken in a very short moment, that difference is very small and the image looks sharp. However since the camera has less time to collect the light for the picture, shorter exposure times need a brighter scene. So the flash needs to be stronger or the scene just needs to be very well illuminated.

lungshenli

To further the other comments – if the location where they choose to take the photo is lit primarily by the flash (and I’ve not seen a roller coaster photo where that is not the case) then the duration of the flash effectively becomes the shutter speed (because the image is lit by the flash – and the camera can only capture an image while the flash is “on”).

Flashes can have *extremely* short durations (1/1000 down to 1/20000 of a second) this means that they are really good at stopping motion.

reluctantly_human

Three things:

1. They use fast shutter speeds. That means that the shutter is only open for a very short period of time to gather data. And during that short period of time, you don’t move much. Imagine you took a video at 3200 frames per second, then advanced it frame by frame. The movement would be almost imperceptible. That is how much you move while the shutter is open. They probably shoot at 1/3200th or even 1/6400th. The down side to having the shutter open for such a short period is that there is not much light that gets through in such a short period of time, so…

2. They use bright, fast flashes. That flash will pulse with each shutter click and it will be timed to illuminate the riders for the same length of time. So the riders are getting hit with a very bright light for only 1/3200th of a second and anything that happens outside of that window is too dark to really process.

3. They use angles to their advantage. Look out the side window of your car on the highway and things will be zooming by and you can’t really focus on them. But look out the windshield and you can easily focus on approaching items.

Cameras have a depth of focus. These particular cameras are set to focus on a point in space that is let’s say…3 feet deep. So as the riders are approaching the camera from the background toward the foreground, they will remain in focus for 3 feet as they pass through the camera’s view. In 1/3200th of a second, they will probably move less than a quarter of an inch. So there is plenty of time to capture them in focus.

But if that camera were placed perpendicular to the coaster train and shooting from the side, the riders would zoom through the view much faster. It’s the difference of trying to catch a baseball thrown toward your face and trying to catch a baseball thrown *past* your face from the side. It’s much easier to see it coming straight on.

B0h1c4

A blurry pictures is normally due to one of two things:
1. Unfocused lens
2. Moving subject

In normal cases a camera needs to find the subject, focus on it and take a picture before these settings need to be changed to keep a clear picture.

In a rollercoaster camera you place the camera in a set place and calculate the focus of the lens and the position of the subject before the picture is taken.

When the train reaches a camera trigger the camera takes a picture of this already decided place. Anything there will be sharp.

The trigger sensor helps the camera to know when the subject is at the “right” spot.

This in combination with a decent flash is what results in a clear picture.

unofficial_mc

They can use a fast shutter speed because they have the flash lights. If the lights didnt flash when you went past there wouldn’t be enough light hitting the lense in the time you were there, so the image would be dark. If the shutter speed is lower you get motion blur.

chummypuddle08