Wikipedia:Reference desk/Archives/Science/2014 May 21

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May 21

What type of tree is this?

The image is a partial screen shot of a scene of Foyle's War which is set in Hastings, UK. The scene takes place outside a manor. A closer shot of the trees shows that they have fairly smooth bark between those knots that you see on the trunks. The scene is set in February which I'm guessing is why there are no leaves. So, what type of trees are they and are they trimmed like that on purpose to be more ornamental? Thanks, Dismas|^(talk) 07:04, 21 May 2014 (UTC)

Those are Common lime (tilia × europaea) trees, called Linden trees in the US. They are deliberately cut that way partly so that they are size controlled and some people (me included) like the knobbly aged appearance when it has no leaves. You are right about the season, in summer the tree grows shoots of up to 3 feet long giving a slight lollipop appearance to the tree. The process of pruning the shoots back each year is called pollarding. Richard Avery (talk) 07:32, 21 May 2014 (UTC)

Thanks! Dismas|^(talk) 08:24, 21 May 2014 (UTC)

Pick proof lock

On my new house I would like to install a lock on the door that cannot be picked even be a professional locksmith. Does such a lock exist. I have heard about electronic locks that use a small keyboard built into it, then you make a password and then enter in your password to open the lock, and also another lock that uses your fingerprint. Can these be picked? And what happened when the battery runs out are you locked out forever? Also the one that uses your fingerprint is is possible for someone to extract your saved fingerprint from it?--Interestingusername123 (talk) 08:37, 21 May 2014 (UTC)

On Mythbusters, they managed to get into a couple of fingerprint locks quite easily. Electronic ones with no key backup require electricity to open. I don't think there's an "unpickable" lock. Keyboard locks are good, but they can wear in a pattern of the most used keys, which makes it easier to guess. 217.158.236.14 (talk) 09:07, 21 May 2014 (UTC)

Easier to guess...maybe...but not necessarily! If you have a single 4 digit code with no repeated digits then the wear pattern says what the four digits are...but not in what order - that would result in the bad guy having to try 4x3x2=24 patterns. That's way too easy! But with a 6 digit code (no repeats), he's got to go through 6x5x4x3x2=720 patterns. My wife and I each use a different 6 digit pattern - which (I suspect) cover all of the ten buttons on the pad...so all of the buttons wear out more or less equally fast and the bad guy is out of luck! If you really must have just one 4 digit code - then at least change it every few months to even out the wear on the buttons! SteveBaker (talk) 14:08, 21 May 2014 (UTC)

And after a change or three, they will forget the curent code and, wait for it, need a locksmith. Oh the irony. - ¡Ouch! (^{hurt me} / _{more pain}) 11:49, 22 May 2014 (UTC)

I once read about a door key lock which had only I, V and X keys in an attempt to avoid this problem. I can't find any online references though, so I guess it didn't catch on. 2A01:E34:EF5E:4640:E572:F28:C9E1:6456 (talk) 07:01, 22 May 2014 (UTC)

Clever! But you end up with much longer key sequences to remember...so I guess it's diminishing returns. Incidentally, last night I took a look at my door lock - and while there are actually three keys that are not in either my wife's or my key code - I can't see or feel any difference between those and the ones that get used half a dozen times a day. The buttons are all very smooth - I'm guessing they're made that way for precisely this reason. I think the "smooth key" exploit is a bust for decently made digital door locks. SteveBaker (talk) 16:47, 22 May 2014 (UTC)

I was also reminded of a key lock we had on a gate at work that used multiple simultaneous button presses (2 then 4 then 3 and 5 together). Using "chords" like that increases the number of combinations a LOT and also makes it vastly harder to use the "worn keys" trick to make brute-force searches work. SteveBaker (talk) 16:50, 22 May 2014 (UTC)

I'm reminded of this story: Ouch! William Avery (talk) 10:55, 21 May 2014 (UTC)

If this isn't just a case of "I want to start a forum chat about this topic because I'm bored", then you probably should talk to a professional to figure out what your security needs actually are. Note that most burglars aren't picking locks anyway—they're breaking a window or using a short prybar to separate the door and lock from the door frame. Making one small component harder to break doesn't magically secure your entire house; having a totally unpickable lock doesn't protect your valuable goods from a chainsaw through the wall attack. TenOfAllTrades(talk) 11:39, 21 May 2014 (UTC)

In a lot of places, people don't even lock their doors. If the house is far enough away from any other houses where nobody would hear a window breaking, there's not much use in locking the doors. Dismas|^(talk) 12:28, 21 May 2014 (UTC)

You should make a room of your house easy to enter, such that you can't easily enter the rest of your house from that room. You put some stuff there that looks like valuable items that are clearly visible from the outside but in reality they are worth nothing. E.g. fake jewelry, fake laptops, smartphones etc. Smartphones with GPS tracking that are configured to automatically take pictures and send them to you by email are also useful. Install motion detectors in that room such that the police is automatically called when anyone enters that room. This can only be switched off from outside the room allowing you to enter the room without problems. Count Iblis (talk) 12:42, 21 May 2014 (UTC)

Not necessarily good advice. Our police Crime Prevention unit advised that internal locks were rarely worthwhile as once someone is inside they can easily break almost anything being out of site. Evidently the repair bill often outweighs the value of items taken. -- Q Chris (talk) 12:50, 21 May 2014 (UTC)

My God—that's brilliant! I should sacrifice one whole room of my house, filling it with valuable-looking but useless crap and the security system from an action movie, forever, on the off chance of attracting a burglar who might otherwise have walked right on past. This sort of nonsense is why you should demand references at the Reference Desk. TenOfAllTrades(talk) 13:12, 21 May 2014 (UTC)

Yeah - I agree. It's like the old joke about how fast you have to run to get away from a charging grizzly bear...faster than the guy standing next to you! All you really need is better security than your neighbors - and that's usually very easy. Security cameras (even fake security cameras), digital locks and good outside lighting (front and back of the house) are enough to make the bad guy look at your house, and one further down the street - and figure that the other house is an easier target. SteveBaker (talk) 14:14, 21 May 2014 (UTC)

Is that assertion based on crime statistics, though? Or is it just a hunch based on a general opinion you've formed about criminal psychology?

To confound the statistical research, the FBI's nationwide Uniform Crime Report, which is the authoritative nationwide crime statistics database in the U.S., defines burglary as unlawful entry. Many local jurisdictions define separate crimes for breaking and entering and larceny and burglary. If a door is unlocked, the FBI considers home-invasion and theft to be be "burglary", but some states do not. If a door is locked, and an entrant picks the lock,... some places consider that burglary, and others do not. American law - and therefore, crime research in the United States - is about fifty times more confusing than it seems at first glance. Nimur (talk) 15:27, 21 May 2014 (UTC)

And if they just break down the door? 24.5.122.13 (talk) 21:27, 21 May 2014 (UTC)

In my state, it depends why they broke down the door. In other words, it is burglary if they broke the door with intent to commit additional crime, specifically intent to commit larceny. Contrast, for example, a case where some fool breaks down a door and promptly leaves: they have probably committed felony vandalism. Obviously, a criminal court needs to convict a defendant, i.e. by proving intent before we consider them guilty of this type of crime. Nimur (talk) 22:21, 21 May 2014 (UTC)

That's right, but then many burglaries are committed by people in the neighborhood who are familiar with the owners of the house, they know when they are on vacation and are then not fooled by such measures. Also you explain, physical security is problematic from a collective point of view, you depend on your neighbor's home having less security. A honeypot strategy makes more sense from a collective point of view. If burglars know that they'll get busted when they succesfully steal something (the problem being caused by what they steal), they won't attempt to break into a house in the first place. Count Iblis (talk) 16:01, 21 May 2014 (UTC)

It depends on what you mean by "pickable". That term normally means opening a lock without destroying it, which is important for say, spies, who don't want to give away the fact that they were there. Making a lock that must be destroyed to open it should be possible, although the lock could be replaced by an identical one, given enough time, so cameras that send images to a remote, secure location for storage might be a better option. StuRat (talk) 13:04, 21 May 2014 (UTC)

I think a bunker would deter most criminals but it is expensive, perhaps a safe room might do the job okay? :) Dmcq (talk) 13:28, 21 May 2014 (UTC)

If you are in the USA, you could go to the library and look at back issues of Consumer Reports at your local library. Consumer Reports rates locks (among other things). The good part is they're impartial and about as proficient as you could expect from generalists. The bad part is that since they're generalists, they don't get into subtle differences between products that would be important for a professional in a particular field. Jc3s5h (talk) 13:52, 21 May 2014 (UTC)

The digital locks that I have (which cost ~$80 in HomeDepot) are "unpickable" by mechanical means because there is no keyhole or other means of getting to the mechanical stuff. The keypad can be programmed to use any number of codes of either 4 or 6 digits - and you can do things like creating a time-limited code that only works for one day (so if you have someone calling at the house to do some work, they can let themselves in - but their code expires). It's handy to create temporary codes for visiting friends and such too.

The thing is indeed battery operated - and when the battery dies, you're certainly locked out. The machine uses a small electromagnet to engage a clutch when you enter the correct code - then you can turn the knob to open the door. So the battery only has to power the pad and the magnet...and only then while you're actually opening or locking the door. I change the battery once a year (when I change the smoke detector and thermostat batteries) - but I know I forgot one year and the battery was still just fine after 2 years of use. When the electromagnet isn't engaged, the door knob just spins uselessly because it's not physically connected to anything.

The only way to get in (short of smashing down the door) is to enter one of the right codes...which would take a LONG time to do by trial and error if you use a 6 digit code...but maybe the bad guy could guess it if you used your birthday or something stupid like that! I suppose there might be some incredibly clever thing you could do with magnets to cause the clutch to engage without the lock's own electromagnet...but the actual mechanism is buried in a lot of metal, a couple of inches behind the faceplate...that seems unlikely.

Fingerprint locks are relatively easy to fool if the bad guy can lift one of your fingerprints (eg from the door handle!)...that's been demonstrated many times and there are lots of websites showing you exactly how to do it.

Nothing is ever going to be perfect - but since it's so insanely easy to learn to pick or destroy the standard cylinder locks that 99% of people have, anything is better than nothing. IMHO, unless someone is specifically out to get **YOU**, your locks only have to be more secure than your next-door neighbor because the bad guys will always take the softer target. But unless you have bars at the windows and a sturdier-than-normal garage door, smashing a window is always going to be easier than getting past a digital lock. SteveBaker (talk) 14:02, 21 May 2014 (UTC)

I hope you've got a good one! There's quite a lot of simple hacks for digital door locks published on the web. The best ones I've seen involve inserting a wire strategically through the space beside one of the keys so it makes a contact inside which opens the lock, they have competitions with different locks to see how quickly people can crack them. It is a bit disturbing to see how easy it is to get lock picking equipment. And without even picking the lock they have things like a flexible arm with a camera and gripper st the end you shove through a letter box and control with a joystick to open doors. Dmcq (talk) 16:24, 21 May 2014 (UTC)

My door lock has a solid sheet of deformable plastic between the buttons and the switches beneath - I think it's there to stop water from getting in - but it might also be to stop the exploit you describe. Also, US houses don't have letter boxes...we have to walk two blocks to the array of mail boxes to get our post! SteveBaker (talk) 16:41, 22 May 2014 (UTC)

Being permanently locked out of your house if the battery dies is still unacceptable, IMHO. If you have multiple doors with independent battery power, it's less of a problem. But, if you have an apartment with only one entrance, I'd expect it to work on either battery or mains power, and to beep loudly when the battery runs low (using mains power to make the loud sound, so it doesn't run the battery down even quicker or use the short, high pitched sound that smoke detectors make when their batteries are low, which is notoriously difficult to track down). StuRat (talk) 16:49, 21 May 2014 (UTC)

Why do Britishes say mains power? Mains are big. The wire going into your house isn't even thicker than your water meter pipe. By that logic you should have mains dial tone and mains cable TV. Sagittarian Milky Way (talk) 00:59, 24 May 2014 (UTC)

I'm not a Brit, but I do like the expression "mains power", as the US equivalent is ugly: "power derived from plugging into the wall outlet". And I've noticed "water main" is used to mean any clean water pipe outside of buildings, not just the "main" ones. StuRat (talk) 04:01, 24 May 2014 (UTC)

You're only locked out from one door if the battery fails - my house has two doors and a garage door - all with digital locks. It's a stretch to imagine all three of them failing! Apartments are a different deal - I agree. But with at least a 2 year battery life, it's really not a big deal to change them once a year along with the thermostat and smoke detector batteries. SteveBaker (talk) 16:41, 22 May 2014 (UTC)

Practically speaking, I think most people want locks that can be picked so that it doesn't cost too much to lose the key or other access method. As long as the lock is harder to pick than it is to break in another way, what difference does it make? The crook will still get in. Wnt (talk) 21:28, 21 May 2014 (UTC)

The beauty of digital locks is that there are no keys to lose. You might forget the code - but it's unlikely because you use it several times every day. My door lock also has a 'master code' with a dozen or more digits which I keep carefully buried in an obscure web page somewhere online! I think I can open a regular cylinder lock using a decent set of picks a little faster than I could reliably break it open...but perhaps an expert at the "hammer a screwdriver into the keyhole" might do better. Breaking the digital lock seems like it would be insanely difficult. You could smash the outside part completely - and still be no closer to getting in. If the door itself is reasonably sturdy and you used those really long bolts to hold the striker plate to the door frame - it would be hard to break the door down by kicking it. So I think the difference between "pick-time" and "break-time" is less important than how long the "break-time" is. IMHO, digital locks are better because they are harder to break - not just because they are harder to pick. But in the end, the ultimate question is "Is it easier than smashing a window?" ...and that's where you hit the limits of what's worth doing with the door lock.

Personally, I just like not having to carry a key - and never being accidentally locked out! SteveBaker (talk) 16:41, 22 May 2014 (UTC)

I got plenty of nothing. Bus stop (talk) 17:15, 22 May 2014 (UTC)

I asked a hardware store owner about this question. He said some inexpensive widely sold locks have cylinders he could pick in a few minutes, typically, but with luck or extreme skill it might take only seconds. He charges 7 dollars to pick a lock when it is brought in and the customer wants it rekeyed but doesn't have a key. It is an every-day job. Some expensive security locks would be hard to pick, such as the Medeco. But he said he could drill out the cylinder even on such a lock in a short while. He also commented that if he wanted to open a door with an absolutely pick-proof lock, it would be little challenge to use a battery powered drill and saw to cut around the lock. Or should we posit drill- and saw- proof door and walls? Edison (talk) 04:15, 23 May 2014 (UTC)

One also has to take into account the potential problem of the lock failing and you becoming locked into your home as a result. Especially if this is prone to happening in an emergency situation like a fire when you need to get out asap. Count Iblis (talk) 10:36, 23 May 2014 (UTC)

That's a non-issue. The battery doesn't drive a motor that retracts the bolt as you might, perhaps, expect. Instead, the handle on the inside of the door is manually operated and works even if there is no battery in the lock mechanism - just like any 'conventional' lock. The way the bolt is retracted from the OUTSIDE of the door is also a rotary knob - but there is a clutch mechanism that disconnects the outside knob from the bolt mechanism unless the battery has enough charge and you entered the correct code. So opening the door from the outside requires you to enter the correct code, then the clutch engages and you can turn the knob until the door opens. The electromagnet that holds the clutch releases after a few seconds whether you open the door or not. SteveBaker (talk) 20:45, 23 May 2014 (UTC)

With all this discussion on picking locks, don't forget that punching the pins out of the hinges (with an outward opening security door) is often an easy option, and not often protected against! 122.108.177.30 (talk) 07:04, 24 May 2014 (UTC)

LASIK surgery

Is lasik surgery done when you are consious? Is it painfu Zonex shrestha (talk) 10:08, 21 May 2014 (UTC)ljonesh

[According to lasik.com|http://www.lasik.com/articles/common-lasik-fears], the patient is awake but they're given numbing eye drops and sometimes a mild sedative. They say " some patients notice mild discomfort". Olaf Davis (talk) 10:25, 21 May 2014 (UTC)

My wife recently had it done - they used sedatives - but she was conscious though it all. She said that it was a bit scarey - but not painful - although her eyes felt 'gritty' and a bit sore afterwards..."kinda like if you'd been awake all night" was how she described it. She told me that the worst part was the smell. A representative of the company that makes the equipment happened to be there and we got chatting after I mentioned that we own a couple of laser cutters at home. So I got a ringside seat and he talked me though the whole thing on a couple of large-screen TV's they have in an adjacent room with a window overlooking the Lasik suite.

There is evidently more than one way to do this - so I can only describe the procedure they used for my wife - but maybe it'll be of some help. If you're squeamish - skip the next paragraph!

First they have to remove the outer layer of the eyeball so that they can get at the lens beneath. So the laser cuts out an 8mm diameter circle of the outer membrane leaving a gap at the top (this takes about 3 seconds) and then cuts out a thin channel, maybe a millimeter long off to the side. Then it switches focus so it's delivering energy beneath the surface of the membrane and scans back and forth over the circle in a series of close-spaced horizontal lines, making the fluid underneath vaporize (maybe 10 seconds) - the resulting gasses vent through the tiny channel they made earlier. The bubbles this produces gradually detach the circle of membrane from the underlying lens, leaving it connected at the top. The doctor then lifts the resulting 'flap' out of the way. Next, a different laser scans across the exposed lens surface shaping it into the new form (maybe 30 seconds) and then the flap is dropped back on top of it and gently smoothed out with a small plastic tool to remove air bubbles. Once all the bubbles are gone, the flap stays 'stuck down' by the surface tension of the fluids under there - so it doesn't move around or anything like that.

Then you're done...the whole thing was over in 10 minutes...it would have been faster but the fancy height-adjusting chair that she sat in had developed a problem and they had to reboot the chairs' computer to fix it(!!).

The moment the procedure was over, her vision was instantly hugely improved. They gave her a very quick eye-test to be sure everything worked out OK. She was given sedatives to take through the rest of the day - and some transparent goggles to wear that are just there to ensure that you don't accidentally forget and rub your eyes. (That's really important during the first 24 hours because they don't want you to displace that membrane flap until it starts to heal.)...for most of the rest of that day the sedatives kept her asleep...over the next few days, up to maybe a week, she had a complicated schedule of three different eyedrops to use at different times of the day. Throughout that time, her eyesight got slowly better and better (although it was vastly, amazingly better right after the treatment). After a week or two, she was down to just one over-the-counter eyedrop that just added lubrication. She was still using those a couple of times a day more than a month later. She had to go for checkups a half dozen times over the next few months - they just gave her an eye test and that was that.

SteveBaker (talk) 13:39, 21 May 2014 (UTC)

This description (in small font) appears to confuse a few terms and parts of the procedure, described at LASIK. The lens of the eye is not involved at all – it is the cornea that is reshaped, by ablative removal of tissue at a shallow depth inside the cornea. The initial lifting of a "flap" consisting of the surface of the cornea (along with epithelium) is done with a mechanical blade. At no stage do they affect anything more than about 30% into the thickness of the cornea, thus leaving the remainder of the depth of the cornea intact. —Quondum 14:23, 21 May 2014 (UTC)

The technique using a blade is very obsolete. These days, "bladeless LASIK" is the thing. And heck, please don't call me a liar - I was there and watched the whole thing - my description of how they removed the flap is what I saw, there was no blade involved and the entire procedure was explained to me by one of the guys who works with these machines every day. I saw the flap being cut, the bubbles being made and the membrane gently separated by the expansion of those bubbles (not "cut"). You're correct about the cornea rather than the lens being involved...it's actually forming the cornea into a corrective lens - so we need to be a little more careful than I was about that. The bladeless approach is better for many reasons - less risk of infection from the blade, less cornea being removed (because the laser-created flap is precisely the thickness of the membrane) - and less of the cornea has to be removed to form the lens because you don't have to fix the problems that cutting the cornea with a knife caused. Also, the laser is vastly more accurate than the blade - so the shape of the flap is more accurately formed and the resulting flap fits precisely back into the hole that the laser made - so recovery time is faster. SteveBaker (talk) 16:15, 22 May 2014 (UTC)

The article mentions both methods of creating a flap; my own experience is a little dated. I opted for PRK to avoid the corneal flap and consequent weakening of the cornea. There are many variations on this general procedure; your use of the term membrane suggests a variant of the Epi-LASIK procedure, in which only the epithelium (i.e. a very thin flap) is temporarily lifted before laser ablation. —Quondum 21:30, 24 May 2014 (UTC)

Personally, I would stick with spectacles rather than risk an unnecessary surgical procedure see: http://www.lasikcomplications.com/ Richerman (talk) 21:52, 21 May 2014 (UTC)

Yeah - my wife and I worried about that - but her eyesight before the procedure was truly terrible. Glasses did a poor job of correcting it because when lenses are powerful enough to correct her vision, the distance of the lens from the eye becomes super-critical and she never got good results with glasses. She was badly astigmatic too - and contact lenses that correct astigmatism tend to gradually rotate and when they do, everything goes blurry until you blink a few times and everything goes sharp again. Her contacts needed to be so asymmetrical that even blinking didn't always get them rotated back the right way. That's really a bad thing when you're driving! For her, LASIK was a truly amazing thing. Well worth the (very, very small) risk of problems with modern LASIK systems. My eyesight is reasonably well corrected with a combination of driving glasses and reading glasses - so I won't be getting LASIK anytime soon. SteveBaker (talk) 16:15, 22 May 2014 (UTC)

Synovial fluid agonist?

Can one raise it's levels of Synovial fluid by Nutrition for example? thanks Ben-Natan (talk) 16:23, 21 May 2014 (UTC)

Hyaluronan, a major component of synovial fluid, is a potential supplement. Apparently it is such a large molecule that the body has a hard time absorbing it. Injections into the joint are recommended. There is also chondroitin and glucosamine, although I don't believe these supplements directly increase synovial fluid. Justin15w (talk) 22:33, 21 May 2014 (UTC)

Identification of an Electronic Componenet

Various forms of Variable capacitor showing a compression trimmer at bottom center.

the following component was sold to me as 50 picoFarad Variable capacitor. but i am not sure about its variable & fixed legs. i also do not know how to connect it to the circuit for varying capacitance. front & back picture is attached. please help. picture link: https://drive.google.com/file/d/0B6hyDm8QXr34M2tnaS1hVDJPTkU/edit?usp=sharing https://drive.google.com/file/d/0B6hyDm8QXr34NXhTOEtmVUU4Rjg/edit?usp=sharing https://drive.google.com/file/d/0B6hyDm8QXr34dVNmMWJGcms3emc/edit?usp=sharing — Preceding unsigned comment added by 223.223.151.164 (talk) 19:07, 21 May 2014 (UTC)

Your pictures are unsharp but it looks like two compression trimmer capacitors side by side. Such components are used for one time settings in oscillator and tuning circuits. The tuning screw should be connected to rf ground; this is so the effective capacity does not change when you touch it with a screwdriver. A weakness of the compression trimmer is that there is no easy way to show the variation in capacity on a calibrated scale such as for manual tuning of radio stations. 84.209.89.214 (talk) 13:43, 22 May 2014 (UTC)

Eye scream [stolen eyes and retinal scans]

There's a news story linked above about a man who had part of his finger chopped off by criminals as a result of owning a car with a fingerprint-activated ignition.

This got me thinking about those retinal/iris scanners that seem to have gone beyond the realms of sci-fi in recent years. Has there yet been a confirmed real-world case of someone having their eye gouged out, or even being decapitated in order that criminals might gain access to <whatever> by fooling the scanner?

I suppose that there are more people in the world who'd be prepared to cut off someone's finger than would take someone's eye, with regards to the squeamishness factor of the thing... --Kurt Shaped Box (talk) 20:33, 21 May 2014 (UTC)

Violent criminals are NOT known to be squeamish about much of anything -- but the obstacle here is more practical, in that an eye is more likely to be damaged in the process of being gouged out, which would make it unusable. 24.5.122.13 (talk) 21:23, 21 May 2014 (UTC)

Plus of course the fact that it takes more effort to gouge out an eye without completely destroying it than to cut off an intact finger. 24.5.122.13 (talk) 08:46, 22 May 2014 (UTC)

Trouble is, they could have gotten much the same result by pushing the persons finger into a gummy-bear and using that to start the car...but hey, if they were smart, they'd be rocket-scientists and not criminals. Selling the car, complete with the dismembered finger needed to start it, is going to be difficult! It's really ridiculous to use fingerprint scanners - they are so easily fooled, and they cause problems like this one. Retinal scanners are a little better - but their usability is a major hassle. Biometrics in general are not a great way to do security. SteveBaker (talk) 16:02, 22 May 2014 (UTC)

Wouldn't the fingerprint on the gummy bear be backwards though? Would that still work? --Kurt Shaped Box (talk) 22:28, 22 May 2014 (UTC)

Hammering a nail

Okay, a fairly simple question, yet one I still cannot understand. According to Newton's laws of physics, if I hammer a nail with a force, the nail will also produce an equal and opposite force to oppose mine. I understand that since the forces are on two separate objects the forces do not simply "cancel out;" however, if the magnitude of the force I apply is equal to the magnitude of the force applied by the nail, why then am I still able to hammer a nail into a piece of wood? I was not sure if the answer was simply "my force is greater than the force required to puncture the wood," or if it was a more scientifically-based answer. Thanks for any input. 23:29, 21 May 2014 (UTC) — Preceding unsigned comment added by 74.15.7.68 (talk)

When the hammer strikes the nail, the nail exerts a force on the hammer and the hammer exerts a force on the nail. ( Note: these two forces are indeed equal in magnitude and opposite in direction. However, they are applied to two different bodies, and can not cancel each other ). The force exerted by the hammer on the nail is not initially cancelled by anything: it simply accelerates the nail. Indeed, the nail was stationary before, and now it's moving forward, relative to the observer. The force exerted by the nail on the hammer decelerates the hammer, slowing it down a bit. After a while, the velocities of the nail and the hammer equalize, and they now move together. As the nail progresses into the wood, the nail exerts a force on the wood, and the wood exerts the force on the nail. This force is then (mostly) passed by the nail onto the hammer, decelerating both the nail and the hammer further, until they both stop. Does this help? --Dr Dima (talk) 01:02, 22 May 2014 (UTC)

Well, think of it this way: What would happen if you tried to use a wooden board to hammer the blunt end of a nail into a steel hammer? The forces are equal, but what the force does to the adjacent object can vary. Wnt (talk) 02:13, 22 May 2014 (UTC)

The forces are equal, but the same force has a greater effect on the nail than it does on you because:

1. You're much heavier than the nail, and F=ma

2. The nail is narrower, so the force per unit area (pressure) exerted on the wood is much greater than the force exerted on your hand. --Bowlhover (talk) 05:07, 22 May 2014 (UTC)

Newton's laws obviously apply here - but they don't really help very much in the explanation of what's going on.

If you hit a piece of wood directly with a hammer, it'll leave a shallow dent about the same size as the diameter of the hammer head. When you hit a nail into wood, it too makes a "dent" in the wood - this time it's only just the diameter of the nail - but much deeper. In both cases, the energy that you applied to the hammer is absorbed by the wood as it is compressed, distorted and torn...all that really changed was the shape of the hole and how the wood fibers had to be bent and broken in order to conform them to the shape of the impacting object.

A nail is a clever little "machine" that takes a force that's distributed over a broad, flat surface and concentrates it at the tip to drive a very thin object deeply into the wood. What actually makes the hole in the wood is the amount of energy applied to each little wood fibre. The hammer head by itself has to share the available energy between a heck of a lot of little fibers - but the tip of the nail only touches a very few of them - so the amount of energy applied to each fiber is HUGE. Consider the head of a typical hammer is probably 4 square centimeters - the tip of a nail might be maybe one square millimeter...so the amount of energy applied to each wood fiber is maybe 400 times more! In fact, the nail mostly pushes the wood off to the sides of the hole rather than bending them downwards as the hammer head would - and that stretches the fibers and the resulting extra tension is what allows them to grip the nail and stop it from falling out again.

SteveBaker (talk) 15:56, 22 May 2014 (UTC)