Are solid objects really “solid”?
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- Опубліковано 28 вер 2024
- If you think about idealized physics scenarios, "frictionless vacuum" or "ignore air resistance" may come to mind, but another even more ubiquitous mechanical approximation is the so-called "rigid body approximation" where solid objects are said to be perfect geometric shapes that don't deform at all when force is applied. For a LOT of classical mechanics and mechanical engineering, it's a fantastic approximation, but like all approximations, eventually it breaks down. Today I'm demonstrating a failure of the rigid body approximation by asking "When you apply a force to one edge of an object and it starts moving, does the rest of the object actually lag behind? and if so, by how much?
Hope you enjoy the experiment!
Music in this video:
I Dunno by grapes is licensed under a Creative Commons Attribution license (creativecommon...)
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Frequent question replies and corrections!
Sup everybody! I'll edit this comment when common questions show up or people find errors I want to correct! (I know there will be many, but I don't know what they are yet, or I would have put them in the video!)
#0: the "model" I use to describe interatomic bonds is ludicrously oversimplified, but it's kinda close enough to make it look like a spring.
#1: at 20:02 said "poisson ratio" when the graphic clearly said "E" for Young's modulus. It's almost like filming brian can't even see the things editing Brian pastes on the screen...
#2: lots of people asked about the delay in the wires or sensor squish. if there was a noticeable delay, the plot I show at 17:26 would have had a nonzero intercept (if the spark between the hammer and bar sparked early, the intercept would be negative, and if the sensor or circuitry added a serious delay, the intercept would be positive. If both are happening and cancel out, they happen every time the same way, apparently!
#3 yes everything from water, to steel, to neutronium is somewhat compressible (the nutronium comments were great - thanks! Apparently in such a structure held up by Pauli pressure, Vs approaches the speed of light. Now I’m wondering what the refractive index of nutronium is and if it’s crystalline or not…)
#4 I've had enough people ask about hitting the bar witha hammer moving faster than the speed of sound that I actually looked into it. I'd need an ultralight projectile like railguned into the end of the bar in a big vacuum chamber - there's no reasonable way I can think of to make something go that fast but I if can think of one I'll make a video. bottom line though, as long as that impactor isn't actually penetrating the bar of steel, the wave will still pass through the steel at the speed of sound.
20:02 to 20:10 You say "poisson ratio" instead of "Young's modulus"
Dustin at smarter every day has a canon maybe a team up
Sounds like we need to get you more patreon supporters so you can buy a railgun and giant vacuum chamber 😂
amazing video!
7:14 wouldn't you actually be reading the devices inaccuracies? Most if not all have a 1-2 + - %. I've Never seen one 100%.....
This has application in structural pile testing, because a hammer impulse happens too quickly to load the entire member at once. Static tests are expensive because you usually have to build two reaction piles just to apply the force. There's a cool method called statnamic testing that uses explosives to create a slower impulse on the pile as a load test.
Which we all knew already because we watched some guy's youtube video about that just the other day :D If only I could remember who it was, probably Physics Girl or Minutephysics...
"Slower"
I way sitting at lunch one day watching your pile driving video and when you got to the bit about the loading depending on the speed of sound in the pile, I was internally doing that Leo pointing meme going “oooh ooh I recognize that!” cause I was working on this video at the time and was very excited to see it matter somewhere!
Always nice seeing your heroes are watching the same stuff as you.
I couldn’t run around the company cafeteria holding up a picture of an explosive pile test on my phone yelling “do you know how cool this is?!” Because theyd think I was actually insane, but ya know, I wanted to 😁
"The more correct a physics model is the more painful it is to use"
That's why experimental physics is so great - The universe takes care of all that figuring work for you!
In my mind experimental physics is essentially trying to find an informed equation that captures as much of a real effect as possible without just making a lookup table. Like you should be able to fit a curve but also extrapolate and use your equation to predict unseen situations.
@@AlphaPhoenixChannel Can you give us links (or a bibliography) to/of your published papers in materials science,,, so we can read about your research.
@@AlphaPhoenixChannel I liked that phrase a lot. Some theoretical physicist are always saying that physics is elegant an beautiful and they derive that "elegance" property from the simplicity of the models and equations in relation to the complexity and variety of observations they predict, but in fact those, let's call them platonists, are continuously neglecting the fact that indeed the more correct models depart from elegance quite a lot and tend to explode in complexity whenever you really want to test detailed phenomena.
@@mikip3242 I’m not sure I buy that. There are some insanely precise theories that are still extremely simple and elegant. A lot of physics is based on geometry and geometry is pretty perfect
Edit:
Look at the inverse square law - crazy simple to calculate, exists because everything that spreads out evenly in all directions scales with the surface area of a sphere. It’s more of a property of our universe, but very boil-down-able to just C/r^2
I had a professor say "we let the universe do the computation for us"
That feeling when you apply a small theoretical correction and the model snaps precisely to the empirical data is probably the single best feeling in the entire world. You don’t get it often.
I feel like that’s what I live for
I usually have the opposite happen where I'm like "If I just change that only little thing it will be perfect" and then everything is on fire, I've created trig functions out of thin air turnips, and ford 150's, and 1=the cube root of salsa
@@JohnSmith-hp9ds if 1=the cube root of salsa, wouldn't salsa just equal 1?
@@adrycough -1/2 +/- sqrt(3)/2 i
@@JohnSmith-hp9ds you should have used a spherical cow of uniform density.
Really really interesting video (like all of yours)! I had no idea about the 1d and 3d difference of speed of sound. Thanks so much for sharing your curiosity and experiments with us!
Yo nice to see you here! Glad you liked the video! the extensional thing was new to me as well. I assumed it was actually like, pulling on a bar, "extending" it instead of compressing it and ignored that number completely at first. Feels like for large displacements, the interatomic potential IS asymmetric, so I bought my own assumption far too well... guess I should have figured out what it meant to start!
Wow, all the big names are here!
might be stupid of me to say. but would it not simply be that the "delay" would be the same on both ends? and that once you overcome the intertia both ends move at the same time after the 1 second delay?
Meaning. the speed of "push" can not be obtained on the object until the delay time which for all i know could be either light or sound speed has elapsed.
on a perfectly rigid solid object there would be a delay on both ends until that time has passed both ends will travel in the same direction is basically what my theory is.
il try one more time to explain my thought process.. You cannot overcome the inertia of the object on either end until the time delay has elapsed
My gut impression was that if the material is not constrained around the outside, then it behaves in a more elastic manner. It has more freedom to compress by expanding sideways, which is harder to do if there's more of that dense material in the way.
@@norwegiansmores811 my thoughts exactly, I still don't believe that "motion" takes such a visibe amount of time to travel in such small dimensions. Maybe there'd be some actual motion delay for lengths in the range of 10⁸. But yeah, totally agree on the inertial delay part
At first while watching the video I said to myself “I wish I could see his trial and error process for this experiment” and was pleasantly surprised when he actually did. Well done.
The details and mistakes are really the best part of these types of videos, I roughly knew the answer but I would not have guessed the problem with the piezo sensor having so much squish varriance.
"All models are wrong, but some are useful"
But some are less wrong
@@MadMan-r5sor less representative
@@MadMan-r5s Those are the ones that are useful.
Is that a model?
@@MadMan-r5s Yet, as noted in the video, the more accurate the model the harder it is to work with. As alluded to in the video, the more computational power is needed.
There's some interesting discussion about modeling wooden structures, like bridges. Just due to how things end up working it's normal to simply assign a beam x/y/z material properties. Yet said beam might actually be a laminate, and wood itself is not uniform. I might be mistaken, but I don't know of a commercial CAD simulation package that takes wood grain direction into account.
I love this video. As a machinist, we have a saying, " Everything is rubber." It's an anecdote about the difficulty of measuring things to extreme precision. There are literally calculations for the deformation of ruby on tungsten carbide . Sure you don't need it until you are measuring tens of millionths of an inch and by that time you need a climate controlled room and can't touch what your measuring for days before taking the measurement because the thermal expansion will throw it off more then the compression. They still exist
That was so cool. As a laymen, I have no idea what you're talking about but it's cool 😂
@@ainzooalgown6450 he means things are hard/tedious to measure with extreme precision when you want extremely accurate results because things ( beside water) tend to condense and take up less volume when it’s cooled, where the volume (of tungsten) would expand ever so slightly if it was heated.
Water is most dense at around 4°C at standard pressure if I remember correctly so it will contract as it cools from 4°C down to 0°C where it again expands as it turns into a solid.
"tens of millionths of an inch" The fact that you still use the imperial measure system as a machinist is amusing. I look down on you imperial peasants with my superior metric system.
My father says that! I was always confused about it 😂 , I'm a virologist and I could not be a machinist in a million years I've investment cast my own jewllery but fine machining is art form to me
Yes! Fellow engineer here. Thanks so much for explaining how you learned the real final result and why it affected your test. I learned something new today! It's such a good feeling as an engineer to see the theory match exactly with physical objects. It's like learning how to predict the future or something.
That's one of the defining qualities of a theory as opposed to a hypothesis or law for instance. A theory is predictive, it can accurately predict the result of an expirement that tests it because it is universally applicable to its subject.
Love the setup. Very simple and clever. Great demo!
I really like the inclusion of troubleshooting at the end. It humanises the experiment and the work behind it. It's inspiring in a way.
This is so much more interesting then the majority of shorts and quick videos you can find online. I wish more people took the time explaining and testing nature. Not for the views but to actually learn stuff. Great channel!
I really respect your ability to explain such concepts easily. That's a trait not everyone has and it's really valuable.
Some interesting thoughts I had about a 300,000 km long steel bar in space: If you were floating there with it, it would look like a small diameter steel bar but you wound not be able to move it by hand due to the mass of it's length. It would feel like an immovable object anchored on nothing. I guess it might bend on a large radius but snap back when let go. That would be a very weird thing to see. Also, a bar that long of the diameter shown would drift around like a soggy spaghetti noodle if other forces like gravity were there acting on it. It's too bad we can't have a very long steel bar in space to see how it would behave first hand.
edit: also want to add that the effect of heat and cold on a steel bar that long would make for massive changes in it's length. Railway operators sometimes need to heat up rails in cold weather because they shrink so much and create a dangerous gap between rail lengths. If my math is right (please correct me if I'm wrong) a 1 degree c increase in the entire length of a 300,000 km long bar would increase it's length by over 3000 km. Insane.
You wouldn't be able to move the whole rod, sure, but you could easily shake or bend the end of it. And, as you said, it would most likely just snap back and vibrate for a long long time. You're spot on regarding the contraction/expansion due to heat. Frankly, any sudden change in temperature, like it emerging from a shadow into the light of a nearby star and that rod could literally impale an astronaut or a spacecraft, unfortunate enough to be nearby and in its path. It would be a very very weird sight indeed. I suspect, at such length, other physical effects might manifest that we haven't even thought about.
The value of 3,000 km expansion for a 300,000 km would be 1% per degree, that seemed high (100°C change would be 100%). So, I found a document listing steel's expansion from 0°C to 82°C being 1.34 mm / meter. So that's 402 km for the entire range or about 4.9 km / °C. Though apparently different types of steel can vary considerably (by about 50%).
Also depending were the bar is in space and how it moves, it may be influenced by magnetic fields and get an induced electric current on it. If you touch it you might get zapped, or the bar could be very hot, or the steel could burn and snap like a soldering stick.
@@Pixelarter Very true, I was thinking about this too. In a way, it's a giant antenna and will convert any radio signals hitting it into electric currents, zipping back and forth along the length of the rod. Getting zapped by it would indeed be a real concern, in my mind too. Then there is the magnetism of a steel rod, which could cause it to change shape and attract/repulse other magnetic objects around it.. It's a mindfield haha.
@@TheRadiastral Nanohertz communication
This exact concept blew my mind about 15 years ago, but I never saw it actually demonstrated until you did it here.
Man, this channel is one of the best channels there are. Great video.
This is exactly the kind of content I love. Questions that seem so simple, but are complex to answer and the answer is not commonly known. The kind of questions a curios child would ask, but no one knows the answer.
Questions like : why does the arrow moves forward if I let it go from the bow?
It’s really cool that you were able to measure it with such a short bar
12:00 it's interesting to thing that real springs are made of atomic electromagnetic springs
Nice! My fave thing, philosophically, that you said: "I love it
when...the underlying mathematical machinery can be recycled." A close second, "Almost every physics model...is technically wrong." That our understanding of the world is really just a collection of imperfect models, but that the models of the different phenomena share mathematical pattern, is the basis of successful engineering, and kind of astonishing. My fave practical thing is the illustration here of the impact of ratios on the validity of models. In this case the small radius to length ratio allowed for the 1-dimensionalization of the model, simplifying it and the math, and it was accurate!
This is one thing I think about a lot relative to high velocity explosives. The way they instantly shred very dense materials into tiny fragments.
I assume this is due to out-running the speed of sound in the material.
Woah wtf
When an object strikes another object, causing a spark, for an infinite moment are both objects traveling at the speed of electricity?
This is awesome. People forget "the speed of light" isn't just about light. It's the speed of causality. The quickest information can be transmitted. So no matter what, even if there were NO space between atomic structures (assumingly existing in theory only), that's the fastest possible speed.
Edit: As soon as you hear the term "wave function" to calculate any option, that should tip you off right away that it's not instantaneous. Love the ball/spring model!
It makes me wonder whether it's misleading to teach it as "speed of light" in the first place, as it should just be a fundamental maximum speed of causality like you said. Speed of light is just one manifestation of it.
I can't say I'd ever thought of this before. On some level I understood that every solid object was just individual atoms bonded tightly together, but I never really considered how applying force to one side meant that the force had to propagate through the material.
Edit: Coming back to this a year later having forgotten about this video was interesting. I've definitely internalized what I learned from this. The model I had for this situation in my head was a lot more accurate and I more or less thought it out step for step with how it's explained here. Very satisfying.
Ever think about a seesaw that is a lightyear long and what would happen if a weight were placed on one side?
Your videos are always so well researched, well thought out and your demos are top notch. All the while the editing is there, too. Thanks for all the work you put in and not cutting corners! It really makes a difference, I love learning about the basics again as too many times we think we are too smart for our own good. I just last week had to re-learn bernoullis equation to understand again how energies are distributed in fluids within closed loop systems; something I always thought of as an easy basic - living is truly constant learning.
I love this. Could it be, however, that the force transducer takes a small time to register anything, mening the delay from one end of the bar to the other might be smaller than measured?
OK, so you absolutely do not need to be told this, but the risk despite it being incredibly unlikely it’s so great that I have to say it anyway. Please always leave this type of extra stuff like you did with the math of the longitude and speed of sound.
That was so informative and so interesting and I feel like I learned more from the video just from that one piece.
Even though I came into this video knowing the answer was the speed of sound, it was still fascinating to see it experimentally demonstrated and be shown the theory behind it. It's one thing to _know_ the answer, it's another thing to *understand* why that answer is correct. I love videos that elucidate concepts like this. Great job!
this guy deserves way more subs. Kudos for the content you are making.
I agree
I regret that I have but one sub to give to this channel
I can't recall where I'd seen this question posed before, but I came to the video knowing the "speed of push" is equal to the speed of sound. And it only makes sense, sound travels at the speed in which atoms are able to push upon each other through a medium. The only difference between a sound and a push is that the sound is a vibration whereas the push is a single movement. It's really easy to visualize if you imagine it as a really thick door. You push/knock on one side but someone on the far side wouldn't feel/hear it until the sound travels all the way through.
This was truly beautiful. Thank you. I've theoretically understood this, but never quite wrapped my brain around it. This was a wonderful demonstration. I hope you have plenty more projects planned for that oscilloscope because it does wonders
Ive wanted one of my own since high school when I first played with an old analog crt scope. They’re beautiful tools
When I was a kid I used to always ponder this question as well as whether or not two objects really could “touch” each other. Fun to see a video like this
The two objects touching blew my mind I remember squeezing my fingers together and thinking if you kept zooming in as you went down they would never touch and it blew my mind and I desperately tried to discuss it with my dad and he didn’t understand. Thanks for unlocking a super weird memory for me lol
exacly the same experience here.
It also means you can bitchslap whomever in the face and get away with "I never touched him you honor!".
I once wondered what would happen if you could make a infinitely dense object into a bar or just a line, then push it, it's infinitely dense so there is no space between anything so shouldn't it instantly move on the other end?
I have no degrees, its just something I'm curious about.
@@speedy01247
Wouldn’t an infinitely dense object have a null volume? Like a point. If it’s a single you can’t really measure its ends. It’s instantaneous but the pressure wave doesn’t travel any distance.
I think it would make it’s sound speed undefined too. (0m/0s)
My opinion isn’t worth much im not great at maths or with physics.
My instinct was right in that in order for the material to move the atoms would have to get closer and then be repelled by their electrons; However, it never occurred to me that this nature of atoms getting closer and spreading apart is what sound is and that by proxy it would travel at the speed of sound. Very great video!
But isn't that force that keeps these atoms and electrons "fixed" in place enough to carry that momentum instantly? Or am I overthrowing it and not realizing it would basically be a Soundwave moving through steel?
That's quite a SOUND argument
@@busttoboostWell the force is inversely proportional to the distance between the atoms that means that a given aton would accelerate the next in line slowly and not instantly. This time it takes to accelerate and deccelerate would be the wave of sound travelling. Or in this case the wave of movement.
@@busttoboost I was thinking about the same problem here. The speed of the impacted force is traveling with a small decay, depending on its material structure. For example, hitting a sponge would take more time for the atoms to carry the momentum the the opposite side, a metal bar on the other hand would transfer the impact almost immediately, because of its dense and hard structure.
Now the theory: if an object had absolut zero plastic deformation on impact, would it transfer the information, or in our case the force, immediately? So faster than light? Of course in reality, finding such an object would take a bit of time, although im not an Physicist im quite curious if this theroy is possible in some way.
So what happens if an object hit it at say mach 2 with no deformation or energy loss between the 2 materials?
It really took me most of a-level physics (16-18 yrs) to understand the concept of what was really happening when two solids collided. Like I vaguely understood that atoms had bonds forming molecules, and molecules couldn’t occupy the same quantum state as each-other, but learning about how it’s photons that transfer kinetic energy and not just a particle bumping into another particle seemed to make so much sense when I first heard it. That’s probably one of the reasons I’m know trying to complete a degree in physics, because I finally understood a concept intuitively that really seemed convoluted at first. I think it’s very similar to the reason I first became interested in learning anything whatsoever. I will never forget my year 2 teacher miss goody (when i was 8-9 years old) who left the school but left me a note saying to never stop asking such great questions.
When I was a kid, thinking of the constancy of c, always thought that a mechanical arrangement (without any play) would easily violate that constant, being the motion transfer instantaneous.
Then thought about it in the years and I realized it couldn't be the case.
You demonstrated it in the neatest way !
Good job.
It's weird that kids have a better hold on einsteinian relativity (or quantum physics) than basic mechanics like hookes law
How many years ago did you first think of this?
@@XtremiTeez don't remember. I think I was 8 or 9. But remeber my father told me once that superluminal speeds were impossible, without a thorough explanation.
@@stesala because I posted something about this 25 years ago.
@@XtremiTeez no. Internet wasn’t a thing at the time 😀
Damn, this is really cool. I like the way you set up the experiment and could explain the concepts simply enough for a layman to understand, but also include enough detail for an engineer when needed. Definitely worth the subscription
There's a good theoretical explanation of this, relying on a very profund concept related to symmetry breaking and collective modes in a solid. In condensed matter physics, when we describe systems in terms of symmetry breaking, we find that every symmetry breaking is linked to the appearance of a collective mode called Goldstone mode. In the case of a solid, the arrangements of atoms in a discrete way breaks continuous translation symmetry (which is reduced to discrete translation symmetry).
The oscillation in this system, or the basic excitation of the ground state (the particles being still) are phonons: quantized oscillations of the atoms in the lattice. These modes have a very specific dispersion relation (for each energy given to the system, they will develop a specific wavelength of the collective wave).
Now here's the relation to compressibility. The dispersion relation of these modes tends to zero wave vector (k) linearly for small energies. What does this mean? For a very tiny energy, k tends to zero, and since k is inversely proportional to the wavelength, for very small energy the wavelength tends to infinity. What's a wave with infinite wavelength? A plane wave, in other words, the atoms moving at the same time and in the same direction, and therefore the system being rigid.
Now the interesting point is that we always put some energy that, even if small, is finite, meaning that the wavelength may be long but not infinite. That means that there will be a wave displacing atoms through the lattice. The actual slope of the dispersion relation is what we call the speed of this wave. This speed is the speed of sound in the solid. And these phonons are called acoustic phonons.
As a mechanical engineering student I can confirm that compressible fluids are one of the hellholes of fluid mechanics and thermodynamics
Do it bring a lot of variables that come with it?
Also as a FEA analyst Speed of sound through metal is the bread and butter of all the computations and basically make all the modern stuff around us work
@@vlogcity1111 not only that, but the relationship between them (the equations) is more complicated too
@@vlogcity1111 Yeah but the real problem is that is where the additional terms in the equations drop off into infinity and often cannot be solved even numerically.
@@ianallen738 yes this. I hope someone comes out in future to solve this problem
Years and years ago I had to explain basically this to a guy in a youtube comment who was trying to imply that physics (maybe just astrophysics) is bullshit. His example was a very long tube with golf balls lined up inside it - that pushing in a new one would mean that the one at the other end of the tube would fall out instantly, and therefore the speed of light being a cosmic speed limit was bullshit. I reasoned that, first of all, you'd need to apply a shitload of force to push millions of stationary golf balls - and then since you're applying so much force, that will clearly compress the golf balls, and you'll have a pressure wave moving through the balls which would likely be very fast, but decidedly slower than the speed of light.
I think he then got mad.
Sounds like a SJW. Did they try censor and cancel you after they got triggered?
WHAT?!! ARE YOU TELLING ME PHYSICS ARE RACIST?
im joking ofc
@@obsideonyx7604 Lolz, how easily triggered do you have to be to imagine SJWs in scenarios with nothing to suggest one might exist. You're not even a snowflake, you came to the table pre-melted.
@@QuesoCookies observation = triggered
Nice logic
@@obsideonyx7604 Speculation =/= observation
Logic 101
What an amazing educator you are. Knowledge, along with the personality. As well as the straightforwardness of your presentation. Amazing!
Very interesting. I had actually been wondering this exact question like a year ago. Glad to know the answer now. Never figured it would have to do with the speed of sound through the object. Neat.
the pressure sensor is not that fast, 180 ms
Subbed. This is some high-quality content and I love the enthusiasm. I also really appreciate the thorough explanations of your "failures" at the end of the video--I love to hear how smart people like yourself problem solve when faced with unexpected results.
I actually performed an experiment similar to this during my freshman year, but the setup was different. We used an RC circuit that only discharged when the bar touched a metalic base. We then dropped the bar from a small height and measured the difference in the charge in the capacitor after one bounce with a voltimeter. Since the bounce means an impulse traveling twice the lenght of the bar we used the exponential decay of the capacitor as the means to measure the time elapsed during the travel of the pulse. With the travel distance and time we got the speed of sound in the bar. The osciloscope makes for a better presentation though ;-)
Impressive! Love hearing the behind the scenes design and testing of the experiment as well! Very cool.
Just ran across your channel... You do an amazing job of explaining/teaching physics with relatable models and analogies. Like and subscribed. My favorite part of this video is the FIRST Robotics clock hanging on the wall. Ive been a FIRST mentor and coach for 10 years now.
This channel is too underrated, you explain complex things amazingly well
This reminds me of the time i accounted for relativity in a physics experiment where the electrons were moving at something like 0.1% c - the sheer satisfaction of all the data perfectly clicking into place was the stuff of legend
I'm about to graduate from university with a materials engineering degree. Keep up the great work, videos like this will really get more people interested in our field :)
I am new to material science and I did not think that I might find it so interessting, but after watching this video and the one where you bend copper (and do cool bubble experiments) I really start to like it. Thank you for the enthusiasm and for the entertaining videos. I am exited to see the next UA-cam play button.
Measurement delay a big factor. Current through wires, sensors, pcb and so on. Great video love the channel!
Solving the speed of sound equations in 3D crystals is a very well documented process and yet it still remains very challenging to solve for the elastic constants in the materials I study... Very interesting how the crystal symmetries play a part in speeds of sound.
it should be faster right?
Does it depend on the angle? Also what about pressure vs shear waves? Very complicated, moreso than in fluids.
This video and the km long wire video are so so good. I am in 3rd year of physics and I love this.
Sometimes we can learn as much, or even more, from failures and the process to achieve results than the results themselves. Really glad you included them in your video. Makes it even better than it would've already been!
I hope I can teach my children your tenacity to follow through on a difficult problem until you figure it out. That was inspiring to watch. You deserve your high viewing number for such efforts. Congrats. I enjoyed your video so I like and subscribed.
Shouldn’t you be the one teaching your knowledge instead of someone on UA-cam ? 😂
@@JeuneF If one does not have the knowledge, there is nothing wrong on having someone who has it teach them. Or someone who knows how to explain it better
Weirdly enough, I had this as a thought experiment a few weeks ago, specifically a light-second length of metal pressing a button. After mulling it over for a few days, I couldn't come to a conclusion. I thought it was likely to be the material speed of sound, maybe the speed of light, but without a practical way (or inclination) for testing I figured that was that.
So I'd like to thank you for settling that particular loose shower thought; as well as explaining why the extensional speed of sound applied instead of the longitudal.
A few questions I have are: Around about how wide would the bar have to be for the longitudal speed of sound to apply? I assume it would have to do with the wavelength or amplitude, but it was implied that it was due to the dimensional ratios. Could the longitudal speed apply if the bar was struck across the diameter as opposed from end the end or is there a minimum dimension for it to apply?
Again, thanks for the video.
I was asking myself the same question 😅
This topic is why earthquake warning systems electrical signal, can beat the physical force waves, plural that travel at different speeds themselves.
But the thought experiment about the apparent instant motion of most objects is pretty cool.
2:00 I'm calling it now, it's going to be C, because the mechanics we're dealing with here isn't dissimilar to how sound waves propagate though air. The bar doesn't all move as a single object; the bar experiences a compressive force that moves through the object in form of a wave. The first row of atoms experience the force at the point of impact, they move forward, then transfer that force to the next row of atoms, and so forth until the entire body has been carried forward. The propagation is only faster in the solid because we're dealing with smaller distances between the atoms, as they are packed more densely. Other than that, it works exactly the same as it would in a fluid or gas.
I know it’s 2yrs later but for some reason I was recommended this vid again and I’m so happy to see that it has 5.6 million views as opposed to some bottom feeding gossip trash / challenge / unboxing / Top 10 / brain rot vid with a mouth agape thumbnail.
This is a thought I've been mulling over for like 20 years; I've had trouble putting it into words and trouble finding a way to test it. This video is GREAT. I hope you do a similar one with different shaped/sized objects (spheres, cubes, super-thin poles) and graphs like the one at 17:39 to delve into the longitudinal/extensional speeds of sound.
'Like 20 years'? Do you mean 20 years or not?
First truly informative video on youtube. I appreciate the care of the presentor and his humility in approaching these experiments, versus the typical rhetoric usually dumped upon the observer as if all truth was encased and questioning the results was heresy. Thank you.
I knew it would be in relation to the speed of sound through the material, but I was not expecting that short of a bar to not even register movement by the time the hammer had been removed
Regarding the 'weird' negative dip before the actual reading, I believe this is consistent with how hydraulics engineering describe pressure wave transmission.
A positive pressure impulse creates a depression ahead of the front, similarly to what a negative impulse does by creating a positive spike ahead of the negative impulse.
Think of what happens on the beach when a tsunami hits: the water recedes before the actual impulse is seen.
Look at 'water hammer' for more information, that is pretty interesting stuff!
3:51 I tought it was in slow mation lol
As an electrician I could have told you that movement across an object is instantaneous. There’s a reason, aside from resistance of the conductor, that we use fiber optics to transmit data across extremely long distances instead of bigger conductors, light is faster than electricity, despite the fact that theoretically, electrons should transmit across a conductor instantaneously. If light wasn’t an incredibly inefficient means of transferring energy we’d use fiber optics for power transmission too.
Lag is introduced due to compression and release of matter in the bar, and it's proportional to density of it. Density also is proportional to speed of sound.
But even if we had a theoretical perfectly solid material that cannot be compressed or bend, it still could not transmit the force instantly. At that point it comes down to the speed of light and no force being capable to be faster than it. So the force is at most traveling at the speed of light. ( or to say it even rougher, the speed of light also is the speed of causality)
@@theexchipmunk FORCE is not an object. It cannot move. What moves is object itself. Speed of light is unrelated.
@@AllExistence I nowhere stated that force is an object. And no, force has a speed. Like everything in the damn universe. And as everything else, nothing travels faster than light. And force spreading though an object could at most move at that speed. If you had a pole long enough to be affected by light speed delay, pushing it would not instantly move the other end, because the force would travel along the pole faster than the speed of light, wich is impossible.
i totally love these kinds of videos. Seeing analyses of stuff we encounter every day through the lens of simple physics models is so cool! I love how physics is totally ridiculously complicated but can be pretty accurately simplified with just weird though experiments, and then these weird thought experiments can actually work as real experiments!
more Kinematics stuff pls 👉👈 (idk if this counts as kinematics or not, but my request still stands ((someone enlighten me please)))
Too many brackets. You don't need one set around "someone.....". There- enlightened! :-)
I always "knew" that it would be at the speed of sound in the material, I was really glad to see your very well done demonstration. But, it would have been better if you had done it with a 1 light second length bar!!
I had a *DUH* moment where I metaphorically slapped my forehead and went DUH... it's a COMPRESSION wave, of course it's the speed of sound.
As a blacksmith, welder, and inspector, I love this explanation of the "elastic" connection between iron and carbon atoms. Plus, I learned about a third sound wave measurement that I had not run across in ultrasonic testing. (UT)
A bit too late to the party, but my two cents on the negative reading on the pressure sensor at the first section.
My hypothesis it: I believe the ZERO reading was not actually zero, but the rod's tip had some preload on the sensor. The impact from the hammmer sent a sine wave shaped elastic deformation along the rod (exactly like an arrow going off of a bow) and when the sinewave reached the sensor, it momentarily pulled the tip away from the sensor before ramming itself back onto it
I really love your videos. They are so educational and entertaining.
Hey, great shows, I just subscribed and am binging. Have you thought of using sound waves in the bar to double check it? Run a wave through and pick it up on the other side? Also, sine the bar isn't really 1 dimensional but is far more x than y and z, there is a ratio there between the steel block and the bar, between the variations of y and z. It seems there might be a standard hiding in there somewhere. Great to see someone doing real experimentation with natural philosophy/physics... you're a century out of time!
I’m eternally gratefully for all the time and energy put into this amazing video. You’re such a fun teacher. Thank you, I learned SOOooo much
I don't get this debate, speed of sound is litteraly the speed of mouvement. I mean what do people think sound is?
Peopel Are DUmber By The Year, Im Telling You !
But Electric Waves Are MUCH Faster Then Sound, This Guy Thinks Its Oposite :D I Just Can`t :D
The interesting thing is, that if it moves instantly or at the speed of light, there would be literally no way to tell, as the way you’d know when the signal is sent would take the same amount of time as receiving the signal
This is, unless I am missing something, like maybe starting a timer from a center point and telling both when the signal should be sent
have one person send the signal from one side and then as soon as its received have someone on the other side send the signal back. bing bong
This blows my mind. Not because of the topic of the video, but because some opperations in code take nanoseconds to perform. And this bar takes microseconds to move when hit... It's really shocking and awesome how fast some computers are. They can do high-school maths faster than this bar can move.
I question myself this exact same question since I was 7 I think. Like, the metal bar and all... in space. It's very difficult to think that the movement wouldn't be instantaneous, as material would have to disapear or shrink somewhere in the way.
Nice job. I was thoroughly engrossed the entire time. I also appreciate your honesty and integrity in trying to find the error in your thinking, and telling us what you found.
Can I theorize that the denser the material the fewer free electrons?
If there is a compression wave traveling through the rod, as soon as it reaches the end wouldn't the rod pull away just a tiny bit when the wave reaches the end? The compressed part of the wave eventually has to reach the end of the rod, right?
Yes, it's reflected as a tensile wave from an open end. You can show this with a slinky
Reaction time on a mass of material depends on the speed of sound within that material iirc. So, uh, C?
Edit: Hah, knew it.
Edit2: It does make me wonder.
What is the speed of sound in superdense materials, like "neutronium" as it's lovingly called in Sci-Fi?
There isn't really any space between the particles if I understand correctly, so what would happen if you pushed a hypothetical bar of neutronium that's 1ly in length? (One that doesn't instantly collapse into a black hole or fresh neutron star, anyway)
Edit3: Ah, "The Internet" (mainly Physics StackExchange) has this to say: "recent observations of neutron star masses and radii suggest that the speed of sound near the center is close to c"
Good to know.
I am by no means an expert, but I guess that there should still be a speed of sound in neutronium, since it is bound by the strong nuclear force, with quarks and gluons acting as the points and springs.
I’m so glad you looked it up and edited with the answer cause otherwise I was going to have to go find it… fascinating thought!
This was very interesting. Almost seems like it started touching on one of my very crazy hypothetical questions like: if you had an extremely long third class lever when, where, why, and how would physics(at least our current and as of yet incomplete models of it) begin to deviate from the generally expected results.
They are called the Laws of Physics for a reason, they dont deviate.
@@id10t98 That's true, but again as he said in the video that fully depends on what model of physics you choose to use. The more correct ones tending to be more computationally expensive (Newtonian vs. quantum physics.) So an oversimplified model may break down in certain cases therefore it's laws are broken or at least inconsistent with reality. A better model must then be chosen. As of right now the Standard Model of physics is also incomplete so there might never be a totally correct solution, just a good enough approximation.
@@id10t98 someone didn’t do their required reading in methods.
_’Science’_ does not deal in truth claims. It’s about developing models, or tools, with predictive utility.
Newton’s physics are not absolute truths.
@@-John-Doe- someone else didnt do their required reading comprehension...I didnt mention Newton, I said they are called the laws of physics for a reason and it's because 99% of things behave according to the laws of physics as we currently know them.
To be honest I didn't really argue the fact that physics would actually break down just the models used (because it won't). I should have been less ambiguous with the first comment. ID10T has a point that physics itself will not break down but it's the methods we use to describe it that do. This is a language and math issue. Laws are based on an observation and used as a ground assumption for everything else so a law is only as good as the most accurate/precise observation is.
Excellent video. I hope this becomes more popular.
And even though steel seems to be very hard and dense it’s actually pretty soft when it comes to the molecules in the atoms and when you hit it with the hammer those molecules get compressed and have ripples most likely the same as dropping a table into Stillwater it will take time for the ripples to get to the other side
3:00 The comparison between water and electricity is something that strikes home for me. I'm from Québec. Most of our power comes from gigantic dams located a thousand kilometers north. We also have a series of smaller dams down south as well as a number of wind turbines. One thing that most peoples don't understand is that on an electrical network, all of the turbines need to be in phase. Operating a turbine out of phase can result in serious damage to the equipment. There is enough distance between the dams up north and the smaller ones down south that it is impractical to try and keep the dams to the north and south in phase. Hence, our electrical grid is split into smaller grids that can operate on their own phase.
the pushing person also has to be heavy. just being infinetely strong makes him just good enough to push himself away from the rod more effectively
Possibly the most irrelevant comment you could’ve made
18:16 you took my comment idea! I was about to suggest it could be something where you had a different alloy than the website was using since I imagine that (since so many properties of steel vary based on the amount of carbon) the speed of sound must vary a good bit depending on the type of steel.
That's really cool how the speed of sound varies based on the "dimension" of the object that you're compressing and that a really thin bar will have a different speed of sound than one that would squish outwards a little bit.
This was a flawless experiment in my opinion. Every question I had in my mind you gave answers to them. Thank you sir for the knowledge.
Nice demonstration!
The impulse through the bar is a compression wave, just like sounds we hear, a sonar ping from a submarine or the "P" wave from an earthquake. So compression waves.
We've all experienced compression waves when driving on a crowded expressway. These are the unexplained traffic slowdowns, or stops, followed by resumed travel and we see no accident or construction area after moving forward. You can imagine viewing this from over head and watching the area of compressed cars propagating up the line of cars.
I wonder if a nail bulges as the hammer impulse travels from the nail head to the point?
@@brandyballoon start - stop driving wrecks mileage for semi trucks. They cruise OK, but dick fuel when accelerating. So, stretching out the wave saves fuel and their left leg (clutch)
I have questioned this for YEARS.
I had wondered "what if". What if to bypass the wait time for information to travel from Earth to Mars and back. In therory, what if there was a very long metal rod that went from Earth to Mars. Then using some form of code, tap one end of the rod and have it received on the other end. I had always wonder which would be faster and by God this video answered a mind numbing question I've had since I was a kid. I had assumed that the answer was waiting on the chain reaction of atoms to reach the other end. But I was uncertain. Can't thank you enough for this. Now I have to question something else for years to come.
I think you should have measured both the hammer impact and the force on the other end using the same type of pressure sensors. Also repeat the experiment with the sensors on either end swapped and averaged in order to get a fair observation.
I know this is way late and I'm sure other mechanical/material engineers have mentioned it but the stress strain response of a material can be measured in a split Hopkinson bar test apparatus. The time differential is measured by strain gauge. In your case, the bar is not split but the technique is applicable.
For a correct experiment setup, you should also evaluate the reaction delay time of the sensor itself, by hitting the sensor directly or through a very short piece of metal.
around 17:00 - 17:40 he talks about testing the sensor delay and how it is proportional to the bar. For the most part the sensor should be faster as electricity travels a lot faster than the speed of sound. roughly around 90% of the speed of light says google.
Cool stuff!
I have a question about the extensional (3d ) speed of sound vs. the longitudinal (1d):
Why is the extensional speed higher than the longitudinal speed?
My guess would be that in the extensional case, the surrounding material keeps the pressure wave "on-track", while in the longitudinal case, energy is dissipated into empty space around the bar.
Is my mental model of this somewhat accurate or is there another reason for the different speeds? (I am not a material scientist, so I am probably wrong.)
im new to your channel and the intro is the coolest idea i've seen
Excellent experiment. It also illustrates how the formulas are based on experimental results. And the formulas are used to make predictions, which inspire experiments, which produce new experimental results, which allows creating new formulas.
Hammer speed on impact: 6,4 m/s
6,4 m/s = 0,0064 mm/µs
In 180 µs, the hammer/rod will overcome a distance of 1,152 mm.
For some reason, the author considers the time interval between the contact of the hammer with the stick and the stick with the sensor as a delay in movement. Not considering that both the hammer and the stick need to overcome the gap distance between the stick and the sensor.
The fact that his results match up with the extensional speed of sound in the material tells me this is an issue that he accounted for already.
@@sorayaprotera It is always convenient to attract indicators to some known values, you can ignore any evidence, the results have converged)
You can use this to measure the thickness of really thin metal films in semiconductors by using a pulsed laser to heat the surface of the film in a very short but high energy burst. It initiates a pulse of strain that propagates through the film, bounces off the substrate and comes back up to the surface. If you can measure the change in strain at the surface over time and you know the speed of sound in the material you can measure the film thickness.
An EXTREMELY tiny electric current was created by hammer at one end, and reached another end faster than sound because of electric induction. However, it is unlikely that you'll detect that (because this electric wave quickly dissipated creating a directional heat adding up into a sound wave). The sound wave was the mathematical integral of these electric waves (quantum) created by the hammer. So, the electrons on the other end started moving after d/(light speed), but atoms started moving after 1/(sound speed in metal).
That negative blip on the sensor is really cool to see. It’s a perfect indication of a pressure wave, similar to how a tsunami causes water to recede from the shore before the tsunami hits.
Do you think the steel being in any other type of structure would have any effect on this experiment? Like when it's in a martensitic state (face centered tetrahedron)just curious.
It would, because the speed of sound in an object is directly related to the hardness, so harder materials like martensite would have higher speeds of sound. (Incidentally, thermal conductivity is also related!)
First of thanks for getting people to thinking! A pressure transducer is inherently going to have a delay converting pressure to an electrical signal, as well as your other equipment to read, interpret and graph that signal. On the other end you stated you had a resistor, that alone would create a major delay would it not? You stated there is a measurable delay in electricity through wire. How much is that delay magnified due to resistor, capacitors, circuit boards, fuses and microchips that are within your oscilloscope looking device?