physics

Movement in a solid object only travels at the speed of sound inside that object. If we assume your rod is made of steel, for example, with a speed of sound around 5960 meters / second, it would take about 35 days for your pull to propagate to the other end of the rod.

permalink embedsave reportreply

[–]LOSTandCONFUSEDinMAY

1064 points 3 days ago

As for why its becuase the rod is not a single object, its a bunch of atoms/molecules bonded by the electromagnetic force.

So what happens is you move the atoms you are holding and those atoms move the ones they are connected to and so on, each step taking a little bit of time.

permalink embedsave parentreportreply

[–]liberal_texan

568 points 3 days ago

Also, to that object the force you apply to it is no different than the force sound applies to it. The speed of sound inside that object is just the speed at which force propagates through it.

permalink embedsave parentreportreply

[–]Im_eating_that

112 points 3 days ago

Exactly the answer I was looking for. TILty

permalink embedsave parentreportreply

[–]paranoiaddict

71 points 3 days ago

Why the speed of sound tho?

permalink embedsave parentreportreply

[–]suprahelix

329 points 3 days ago

People are using “speed of sound” as shorthand for “speed of a mechanical wave”. It’s a continuous transfer of energy from atom to atom as they bump into each other.

Another good example is a wave of water. If you drop a rock into water, it will transfer to the water at the point of impact. It moves those water molecules out of the way to make room for the rock. So those atoms move away from the impact and they push outwards on the atoms next to them, and those push the atoms they’re next to and so on. That’s how you end up with a wave moving out from the impact site.

When you pull on the lever, you impart energy into the atoms you’re physically touching, and as you push on those, they push on their neighbors and ultimately that energy moves through the object at a certain rate. Depending on the composition of the lever, that force will travel at particular speed.

permalink embedsave parentreportreply

[–]paranoiaddict

45 points 3 days ago

Wow that is so cool. Thank you for explaining.

permalink embedsave parentreportreply

[–]AtreidesOne

47 points 2 days ago

We tend to think of solids as rigid objects but they are basically just really stiff springs.

permalink embedsave parentreportreply

[–]0K4M1

5 points 2 days ago

A car spring, sounds pretty solid to my barehand compression strength

permalink embedsave parentreportreply

load more comments (1 reply)

load more comments (2 replies)

[–]kermityfrog2

16 points 3 days ago

Hrm. So how come the speed of sound in water is faster than the speed of waves in water?

permalink embedsave parentreportreply

[–]thelastest

44 points 3 days ago

Water waves propagate over the surface of the water. They depend on the circular motions of individual particles. The sound travels through the water linearly.

permalink embedsave parentreportreply

load more comments (7 replies)

[–]suprahelix

2 points 2 days ago

Those are different types of waves. Sound is a compression wave as molecules squeeze against each other and then snap back whereas a water wave is physically moving molecules around in larger scale motions.

You can see an animation of how water waves work here

https://m.youtube.com/watch?v=NShUBfJQEHk&feature=youtu.bed

permalink embedsave parentreportreply

[–]dalnot

8 points 2 days ago

Watching trains start and stop is what made it click for me, and it’s the analogy I use.

There’s a little bit of wiggle room in each car, so when the engine starts moving it moves alone until it pushes the first car which moves with the engine until it hits the second car and so on down the line.

When the engine stops, the cars keep moving until the first car hits the end of it connections and stops, but the other cars keep moving until the second car hits its stop, but the other cars keep moving until the third hits its stop, and so on.

permalink embedsave parentreportreply

[–]SenatorCoffee

5 points 3 days ago

hmm, I am not really getting it still.

I mean there is supersonic mechanical movement right, like those airplanes?

If I somehow pulled on OPs rod with 3000 kmph why would it still stretch and move down at only the speed of sound?

Is it somehow the strength of the atoms or molecular bonds that is alway the same?

To me it seems it would be a question of how “springy” the Rod is and if its really rigid and fixed I should be able to pull it down at any, even supersonic, speed?

permalink embedsave parentreportreply

[–]FullHavoc

28 points 3 days ago

Perhaps think of it in this way: If you are holding a ruler on one end and push it, how does the other end of the ruler “know” to move? The force of the push travels from the end you’re pushing on down the length of the ruler until it reaches the other end. Because rulers are small, this is essentially instantaneous, but if you have a rod that is a light second long, then that force can take a while to travel.

The speed that the push force travels through a material is essentially how long it takes for the molecules you are pushing to bump into the next set of molecules, which bumps into the next set, and so on. This cascade of molecules bumping into each other is a wave, and the wave travels at a speed dependent on the material of the object. This is the speed of mechanical force, or better known as the speed of sound, because that’s what sound IS.

permalink embedsave parentreportreply

[–]JonSnow

10 points 3 days ago

So the speed of sound changes depending on the medium through which it travels?

permalink embedsave parentreportreply

[–]ruralcricket

28 points 2 days ago

Yes. By a lot

wiki

For example, while sound travels at 343 m/s in air, it travels at 1481 m/s in water (almost 4.3 times as fast) and at 5120 m/s in iron (almost 15 times as fast). In an exceptionally stiff material such as diamond, sound travels at 12,000 m/s (39,370 ft/s)

permalink embedsave parentreportreply

[–]Raul_Coronado

9 points 2 days ago*

The speed of sound in the atmosphere even changes with altitude. At sea level its about ~760 mph but at 30000 feet its about 678 mph.

permalink embedsave parentreportreply

load more comments (3 replies)

load more comments (4 replies)

[–]Masta_Wayne

19 points 3 days ago

The difference is that the supersonic planes aren’t accelerating at the speed of sound, their velocity is what’s greater than the speed of sound, so while the plane is accelerating to supersonic speeds the forces are able to propagate through the materials in the plane fast enough to not cause any issues.

Another point is that the speed of sound is different through different mediums. For example, if OPs rod was made of steel, the speed of sound through the rod would be about 5120 m/s (compared to 343 m/s which is roughly the speed of sound through air). So, when the other commenters were referring to the “speed of sound” through an object, it’s not a constant speed, it’s a variable speed depending on the material of the object.

You could pull the rod at “supersonic speed” but if you are referring to the speed of sound through air (343 m/s) pulling the rod at supersonic speeds still wouldn’t be supersonic within the atoms of the rod (up to a certain point).

On the other hand, if you accelerate the object too fast the tensional forces might be too much and could cause you to deform or just tear the object.

permalink embedsave parentreportreply

[–]CountingMyDick

8 points 3 days ago

The “speed of sound” limit limits the propagation of changes in velocity, not the actual velocity.

It’s perfectly possible for the whole rod to get to moving at 3,000 kmph, it’ll just take a significant amount of time to accelerate it to that speed.

You can probably compute based on the speed of sound in the material and the physical strength of it what the maximum acceleration you could apply to it was before it broken. But the overall speed is not limited as long as you do not accelerate it past that limit.

Supersonic flight is an aircraft moving through the air faster than the speed of sound of the air, which means that the aerodynamics of how the air “gets out of the way” works differently.

permalink embedsave parentreportreply

[–]thelastest

5 points 3 days ago

A solid rod would break. That is something shattering. Cracks propagate at about the speed of sound.

permalink embedsave parentreportreply

load more comments (4 replies)

load more comments (10 replies)

load more comments (3 replies)

[–]thelastest

48 points 3 days ago

Everything is a spring, when you push or pull on it, the speed of sound is the speed that spring compresses.

permalink embedsave parentreportreply

[–]MalleableCurmudgeon

36 points 3 days ago

Sound is this movement through objects. What we hear and call sound is just our eardrums picking up this movement through air.

Additionally, since sound is the movement of air molecules moving air molecules, a significantly loud noise is an explosion.

permalink embedsave parentreportreply

[–]diabloplayer375

2 points 2 days ago

Sound is vibration. It’s the particles moving and bumping. Different materials/mediums can propagate vibrations/sound at different speeds because of their material properties. It’s kinda just the way things are.

permalink embedsave parentreportreply

load more comments (1 reply)

[–]CavingGrape

6 points 3 days ago

and that speed is the same no matter what hits it?

permalink embedsave parentreportreply

[–]-NotAnAstronaut-

12 points 3 days ago

Correct, the speed of sound in a medium is independent of what causes the motion (sound)

permalink embedsave parentreportreply

[–]ezekielraiden

6 points 3 days ago

It is the same for any particular material and structure/organization thereof.

A bar made of layers of iron and layers of carbon could technically have the same total constituents as a bar made of steel, but they’re arranged differently and thus have a different speed of sound. Likewise, the speed of sound in diamond is gonna be different from the speed of sound in graphite, even though both are made of pure carbon. Change the material or how it’s organized, you may change the speed of sound (=speed of mechanical wave propagation) therein.

permalink embedsave parentreportreply

[–]Mental_Cut8290

2 points 3 days ago

Damn, that just instantly made the reasoning click for me!

Although it’s also worth pointing out that sound travels faster in solid than it does in air.

permalink embedsave parentreportreply

load more comments (1 reply)

load more comments (9 replies)

[–]Greedy-Pen

56 points 3 days ago

So the rod would essentially stretch as the atoms move apart and come back together. Like a ripple of stretched rod?

permalink embedsave parentreportreply

[–]amakai

59 points 3 days ago

Yes, like a big and super heavy rubber band.

permalink embedsave parentreportreply

[–]appleciders

[🍰] 6 points 3 days ago

Would that ripple reflect off the end and come back to the beginning, like a lateral wave in such a springy object would?

permalink embedsave parentreportreply

[–]yanginatep

15 points 3 days ago

No more or less than it does in a 3 foot long rod. So technically yes, but pretty much imperceptibly without extremely accurate measurements.

The length makes no difference, this same thing is happening any time you touch and move any physical object.

permalink embedsave parentreportreply

load more comments (1 reply)

[–]CausticSofa

3 points 3 days ago

I’m picturing how it looks when you peel off a strip of 3M tape that had been previously holding up a wall hook.

permalink embedsave parentreportreply

[–]TryndamereKing

11 points 3 days ago

Does it take equal force to stop it from moving? My basic physics knowledge says yes, but I just can’t imagine it.

permalink embedsave parentreportreply

[–]linos100

22 points 3 days ago*

Not exactly. It would take the same amount of work or energy to stop it. Work is force times the time that force is applied to an object, it has units of energy. When you apply a force to move something you are adding kinetic energy to that object, to stop it from moving you have to cancel that energy by applying the same amount of work but in the opposite direction. Yes, this is done by applying a force on it, but it can be a smaller force for more time or a bigger force for less time. Think about how when you brake a car you can do it fast and hard or slowly and soft.

permalink embedsave parentreportreply

[–]GroteKneus

6 points 3 days ago

when you break in a car you can do it fast and hard or slowly and soft.

This had me confused for a second. At first I wondered why you brought up burglary as an example and then I realized you meant ‘brake’.

permalink embedsave parentreportreply

[–]200Dachshunds

5 points 3 days ago

And I read it as ‘break in a car’ like you break in a new pair of boots. Oh, English!

permalink embedsave parentreportreply

[–]kevronwithTechron

2 points 3 days ago

I figured it was the expert dancing styles of Ray Gunn.

permalink embedsave parentreportreply

load more comments (5 replies)

[–]Bright_Brief4975

10 points 3 days ago

That brings up the question, what if you have 2 people, one at each end of the rod. They both pull the rod toward themselves at the same time. Since neither end of the rod know the other end has been pulled yet, what happens when the 2 forces meet in the middle?

permalink embedsave parentreportreply

[–]RestAromatic7511

18 points 3 days ago

Bear in mind that this is an extremely large, heavy object. In reality, a person pushing on it will not have much of an impact, and the resulting stresses will dissipate as they move towards the middle, becoming even weaker by the time they meet.

If you could push with extreme force, then you would deform it. Depending on the material and the force used, you might get elastic deformation, in which your end of the rod would eventually bounce back towards you, or you might get plastic deformation, for example, the rod might snap. It wouldn’t really be that different from pushing on both ends of a spring, except it would be much slower.

permalink embedsave parentreportreply

[–]goodmobileyes

7 points 3 days ago

The atoms on each end get pulled away from each other, until the ‘wave’ reaches the middle. If the force is sufficient, thr rod will just snap and each half will go towards each end. Otherwise the rod will hold its shape and the pullers at both ends with feel that resistance from pulling on sometjing stuck.

permalink embedsave parentreportreply

[–]Bubbly_Safety8791

6 points 3 days ago

So if they both pull their end of the rod a short, human-sized distance - like, 0.5m say - they pull the ends of the rod apart by 1m.

That takes the length of the rod from 17987547480m to 17987547481m, putting the rod under a strain of 0.00000000006

If the rod were made of steel, say, it has a Young’s modulus of about 200GPa, so once the tension in the rod evens out, the stress in the rod would be about 11Pa - 11 NNewtons per square meter of cross section.

If the rod has a cross sectional area of 1cm2 then that means the total tension in the rod would be 0.0011 Newtons. So they’d feel like the other person was pulling on the rod with the equivalent force to lifting about a tenth of a gram, or approximately one raisin.

Steel is not very stretchy per unit of length, but a light-minute is a LONG WAY, so that tiny amount of stretching caused by pulling with just the force required to lift a raisin gets multiplied out over a lot of distance.

That said, to accomplish that amount of stretching you’d have to move a lot of steel… You’d be stretching half of a bar that weighs 14,390,037,984kg by half a meter, which means you’d be moving the center of gravity of 7,195,018,992kg of steel by 0.25 m. So that’s going to take a while, even if you don’t need to apply much force to do it.

permalink embedsave parentreportreply

[–]PaulaDeenSlave

3 points 3 days ago

Or if those two people grabbed their end and began pushing for 35 days.

permalink embedsave parentreportreply

[–]robbak

2 points 3 days ago

The influence of both of them pulling would be like a compression wave moving through the steel, which would pass through each other.

As for the effect - pulling on that rod would feel like pulling on a rod embedded in concrete. Pulling or pushing on it would have no discernable effect, but bending it sideways would create a lateral wave that you could see moving off into the distance.

permalink embedsave parentreportreply

load more comments (4 replies)

[–]duplo52

11 points 3 days ago

Someone asked a similar question about the speed of light the other day and it was explained more as the speed of causality because being faster than light would break things. Your explanation was wonderfully worded to make me remember that post and understand this the same way. They basically had said all the atoms cannot move at the same time and so you end up with this slinky type effect on the atomic level.

permalink embedsave parentreportreply

[–]eightfoldabyss

17 points 3 days ago

Light moving at the “speed of causality” is true and has a meaningful interpretation, but I worry that it confuses this example. What’s happening here is entirely governed by the speed of sound in the rod, which has to do with how quickly the particles can push or pull each other (and, importantly, how long it takes to cascade that force.)

permalink embedsave parentreportreply

[–]Expert_Journalist_59

2 points 3 days ago

Otherwise described as “the speed of causality” in this context. The force being the cause, the movement being the effect. The movement cannot propagate faster than the speed of __. It just happens to be sound in this context. Personally, it seems a perfectly reasonable way of building a mental model.

permalink embedsave parentreportreply

load more comments (2 replies)

load more comments (2 replies)

[–]NapsAreAwesome

1 point 3 days ago

So if the steel rod were one metre long, the same rule must apply, wouldn’t it? There would be some incredibly miniscule effect on that piece of steel?

permalink embedsave parentreportreply

[–]jocona

5 points 3 days ago

Yes, and it is measurable

.

permalink embedsave parentreportreply

[–]HilariousMax

1 point 3 days ago

So it becomes longer?

permalink embedsave parentreportreply

[–]TheFerricGenum

1 point 3 days ago

What happens when you get down to the level where things are a single object. When you spin one end, will the other spin before light could travel that length?

permalink embedsave parentreportreply

load more comments (1 reply)

load more comments (8 replies)

[–]Mason11987

35 points 3 days ago

It’s like if you pull on a slinky. The end will move, but not immediately.

A metal rod does the same thing, but much faster. Still not instant though.

permalink embedsave parentreportreply

[–]Hyndis

9 points 2 days ago

This is a big problem for long trains when they start or stop unexpectedly. The front engine and the rear train car don’t start and stop at the same time, which can result in derailments.

permalink embedsave parentreportreply

[–]lalala253

12 points 3 days ago

So from another perspective, the rod would look like it expands?

permalink embedsave parentreportreply

[–]SteptimusHeap

12 points 3 days ago

Yes. It would expand and contract due to the forces.

permalink embedsave parentreportreply

[–]Pentosin

7 points 3 days ago

Any object etc expands and contracts when transmitting sound. If it was truly solid, it couldnt transmit sound.

permalink embedsave parentreportreply

[–]AtreidesOne

7 points 2 days ago

I think you mean “rigid”. It is truly solid.

(Then again, I suppose when you consider the empty space inside it, nothing is anywhere near solid).

permalink embedsave parentreportreply

load more comments (3 replies)

[–]Target880

40 points 3 days ago

The speed of light is 299 792 458m/s, which is 299 792 458/5960 = 50 300 times faster the the speed of sound is steel.

So if a object is 1 light minuts long, sound in steel take ~50 000 times longe to travle the same distance.

The moon is just 1.3 light seconds away from Earth, but it still takes 50000*1.3/60/60 = 18 hours for sound in steel to travel the same distance.

Diamond is one of the stiffest materials with the highest speed of sound, around 2x the speed of steel. So half the time for a diamond rod.

permalink embedsave parentreportreply

[–]audiodrone

4 points 3 days ago

Movement in a solid object only travels at the speed of sound inside that object.

Why the speed of sound? Why is that the limit?

permalink embedsave parentreportreply

[–]LocalHyperBadger

17 points 3 days ago

It’s the other way around: the speed of sound in a material is just the speed at which forces can propagate in that medium.

permalink embedsave parentreportreply

[–]pladhoc

4 points 3 days ago

Consider a push or pull is just a really slow whack with a drumstick. When you hit the rod, the atoms bounce against each other and propagate down the rod. That bounce propagation is the speed of sound in that material.

permalink embedsave parentreportreply

[–]Thekingoflowders

11 points 3 days ago

Ok can you dumb that down a couple more levels please 😂

permalink embedsave parentreportreply

[–]rogervdf

81 points 3 days ago

Solid is not solid when object rly big

permalink embedsave parentreportreply

[–]UltimaGabe

51 points 3 days ago

Another way to put it is that every object is squishy to some degree, and the squishyness plays a pretty big role in OP’s thought experiment. No matter how hard you push/pull you aren’t able to move the whole object instantly, you can only move it as fast as the squishyness allows.

permalink embedsave parentreportreply

[–]sharrrper

39 points 3 days ago

At any size actually. You push a rod 1 foot long there is a delay before the opposite end moves.

At 1 foot the delay is only about .00005 seconds, which is close enough to zero to not make any difference for basically all human applications. Buy it is actually possible to measure that with sufficiently high end equipment.

permalink embedsave parentreportreply

[–]00zau

12 points 3 days ago

Everything is elastic if you look with sufficiently accurate measurement devices. When things are really big, that device can be the Mk1 eyeball.

permalink embedsave parentreportreply

[–]CorvidCuriosity

3 points 3 days ago

Or even when rly smol, but when smol the linear approximation is more accurate

permalink embedsave parentreportreply

[–]SuperMein

3 points 3 days ago

Why’d you look at me when you said that?

permalink embedsave parentreportreply

[–]CorvidCuriosity

3 points 3 days ago

You know why ;)

permalink embedsave parentreportreply

[–]Sol33t303

11 points 3 days ago

At large enough distances things act like they are elastic is what I gather

permalink embedsave parentreportreply

[–]ILookLikeKristoff

56 points 3 days ago

They always act like that, they just have to be stupid big before our dumb monkey eyes can see the effects

permalink embedsave parentreportreply

[–]sharrrper

12 points 3 days ago

At any size is the wacky part. It’s just such a tiny flex we can’t perceive it with our squishy eyeballs.

If you push on a 1 foot steel rod, there is in fact a delay between when the front of the rod starts to move and when the back actually does.

At that scale, the delay is only about .00005 or 1/20,000 of a second. This can be treated as zero for almost any human activity, but you can actually measure this phenomenon with sufficiently sensitive gear.

permalink embedsave parentreportreply

load more comments (3 replies)

[–]AlienEngine

14 points 3 days ago

Nah they act like that all the way down it’s just less pronounced in non-thought-experiment scenarios

permalink embedsave parentreportreply

[–]-King_Slacker

9 points 3 days ago

Basically, everything stretches. All of the things we touch, even scissors, stretch. At such small scale, it’s not worth mentioning, similar to how liquids don’t compress under pressure. They do, but it’s by so little that it doesn’t matter an overwhelming majority of the time. However, a light minute is about 11 million miles long. At those much larger scales, we can notice that stretch because it’s significant for how comparatively small we are.

The reason that stretch moves as fast as sound is because sound is molecules transferring energy. When something is pulled or pushed, the transfer isn’t immediate, but it takes so little time on our scale that we don’t bother with it. On the scale of 11 million miles, sound isn’t very fast, so we can theoretically notice the delay, since one end would move and the other wouldn’t for at least a few days, while we can communicate with a minute delay.

permalink embedsave parentreportreply

load more comments (1 reply)

[–]PlaneswalkerHuxley

8 points 3 days ago

Everything is a slinky.

permalink embedsave parentreportreply

[–]bullevard

6 points 3 days ago

Imagine pulling a slinky. The other side doesn’t move as soon as you pull. It takes time for that “pull” to reach the end.

Everything around us are slinkies. When you pull or push one side it takes time for each piece to pull/push the next and the next and the next.

permalink embedsave parentreportreply

[–]Thekingoflowders

2 points 3 days ago

Wow. That’s pretty mind-blowing

permalink embedsave parentreportreply

load more comments (1 reply)

[–]jimmythefly

5 points 3 days ago

Solid materials are just a bunch of atoms next to each other, held together by atomic bonds. Imagine a long train sitting on the tracks, each train car is like an atom, each coupler is the bond between them.

On a train, there is actually some slack between the cars, when the locomotive starts pulling from a dead stop, the first train car moves a bit and takes up the slack, then the next car moves, and so on all the way to the caboose which is the last car to start rolling.

For this super long solid rod example the same thing happens. When you pull on the atoms on one end of the rod, there is a bit of “slack” until they pull on the atoms next to them, all the way to the end. As it turns out, that delay or slack works out to be the speed of sound in that material.

You can test this with a nice long section of railroad track and a friend. If they are sufficiently far away, you will see them hit the rail with a hammer before you hear the sound in the rail itself. Because the speed of light is faster than the speed of sound, even though the speed of sound through steel is much faster than through air.

permalink embedsave parentreportreply

load more comments (1 reply)

[–]cakeandale

3 points 3 days ago

u/LOSTandCONFUSEDinMAY

does a good job explaining why, but basically sound is physical movement passing through an object. If you tap on the end of a metal rod you send sound waves through it, moving each atom back and forth. That moving back and forth causes the first atoms to push and pull on the atoms next to them, which causes those atoms push and pull on the atoms next to them, which causes those atoms… etc etc.

If you push or pull on the rod instead of tapping it, though, to the metal atoms themselves the action is basically the same - it’s just a single wave of each atom pushing or pulling the next atoms in one direction instead of them all pushing back back and forth like they would with sound. The overall effect travels through the rod the same way, causing it to flex slightly as one side moves before that wave of motion can get to the other side.

permalink embedsave parentreportreply

[–]BigGuyWhoKills

2 points 3 days ago

Molecular bonds can push and pull nearby molecules only so fast. Their limit is the speed of light, but usually much lower. They can compress or stretch to some degree, and will crumble or crack if you exceed those limits.

So shen you push on a 1 light-minute rod it will compress at first. After at least one minute the far end will move.

But the molecular bonds will break long before there is any chance of breaking the speed of light.

permalink embedsave parentreportreply

[–]Not_an_okama

2 points 3 days ago

To preface: everything can be represented as a spring. Sound is a vibration, matter between the sounds origin and our ears vibrates to deliver the sound to us. (We hear through literal drums in our ears that sense this vibration, the term eardrum is pretty literal)

When you throw a rock into a calm pond you can see the ripples propogate over the water. Imagine that this is your sound wave. If i repeat this experiment and toss the rock to the same exact spot everytime it should always take the same ammount of time for the ripples to reach the shore. BUT if i were to replace the water with a different liquid like peanut butter or oil, it will take a different length of time for the ripples to reach the shore. In this example, the ripple represent sound. This holds true for solids liquids and gases (i cant speak on more exotic matter).

Now, back to the main question. First let me restate, everything is a spring. When one end of the rod is pulled, the rest pf the rod will extend and contract until each point along the rod has experienced the same displacement in space. This happens at the speed of sound within the material, so if i know that the speed of sound in the material is X m/s and the rod is 100x long, i know that it will take 100s for the the other end to catch up, and in the mean time the stuff in the middle is being pulled on by the material at the ends.

Going on alittle tangent: Everything being a spring is how FEA is calculated. The geometry is broken down to very small springs and then the results of each spring are determined. How small and how many springs depends on whats being modeled. For example the columns on a typical freeway overpass can probably be modeled as as few as one element each due to their simple shape and loading conditions, but something like a diving board may require magnitudes more. In school, i had to use roughly 1.5 million elements to get a decent result while modeling a gearbox for our schools baja car team which was only about 6”x8” and had 4 gears in it.

permalink embedsave parentreportreply

[–]Thekingoflowders

2 points 3 days ago

Thank you, I am starting to grasp it now. I think

permalink embedsave parentreportreply

[–]goodmobileyes

2 points 3 days ago

Imagine atoms are like people linking hands. When you pull on people at one end, there is a minor lag as they pull on the next person and the next, until the last person is pulled. With a short chain of people (like most objects in daily life) this stretchy effect is negligible. But if you have a super fking long chain of people, like millions and billions, when you pull on one end it can take hours or days to reach the other end, simply because how long it is and the limitation of how fast the pull is passed along the people in thr chain. The speed of the pull being passed is called the ‘speed of sound’ of that particular material.

permalink embedsave parentreportreply

load more comments (4 replies)

[–]tarkinlarson

3 points 3 days ago

What happens if I pull the rod faster than the speed of sound in the rod.?

permalink embedsave parentreportreply

[–]halfajack

6 points 3 days ago

It breaks

permalink embedsave parentreportreply

[–]blackhorse15A

3 points 3 days ago*

Maybe.

What happens is you will create a shock front that propagates through the material. This can break material, but not necessarily.

permalink embedsave parentreportreply

[–]tarkinlarson

2 points 2 days ago

We can move through air faster than sound. That causes some weird effects and it’s not really that you can break the atmosphere on that scale? Or maybe you do? I don’t know.

permalink embedsave parentreportreply

[–]blackhorse15A

5 points 2 days ago

When you break a solid what that means is that, at some location(s) you got the intermolecular bonds holding it together as a solid to release and stop holding those adjacent molecules together. Air is a gas. It already doesn’t have any of those intermolecular bonds. The air was already, and maximally, “broken” from the beginning. That’s why it’s not possible to mechanically break it any further.

permalink embedsave parentreportreply

[–]SlickMcFav0rit3

5 points 3 days ago

I’m not doubting you’re correct, but I would have expected that the movement would propagate at the speed of light since the object is held together with electromagnetic forces

permalink embedsave parentreportreply

[–]HopeFox

9 points 3 days ago

The forces are electromagnetic, but the impulse is carried by layers of atoms impacting against other layers of atoms. The molecular bonds are able to bend and compress slightly before they’re forced to transfer the force to the next layer. Their ability to absorb that force is what determines the material’s “modulus of elasticity”, which determines the speed of sound in that material.

permalink embedsave parentreportreply

[–]SteptimusHeap

2 points 3 days ago

The change in the force between individual atoms moves at the speed of light, but that new force needs time to accelerate the object.

The speed of sound is essentially an inverse measure of how long it takes for one atom to get to the same state as a nearby one. So if an atom accelerates slower (higher atomic mass) the speed of sound would be slower.

Likewise, if the force between these atoms is higher (stiffer material), the speed of sound must be higher. If the atoms are closer together, the force rises (remember electromagnetic force drops off with the square of distance) so the speed of sound increases.

permalink embedsave parentreportreply

[–]MR-rozek

2 points 3 days ago

what happens when i accelerate an object faster than its sound speed?

permalink embedsave parentreportreply

[–]HopeFox

8 points 3 days ago

The object will deform, bending or squashing around the point that you’re pushing. Your hand will reach the interior of the object before the impulse does, so it won’t have started to move out of the way before you reach it.

What happens after you stop pushing will depend on the elasticity of the object. It might spring back into its original shape, or it might permanently deform or break. Given how hard you’d need to push to exceed the speed of sound, most objects will deform or break.

permalink embedsave parentreportreply

[–]notHooptieJ

5 points 3 days ago*

in gases you get a shockwave and the material will split (into a mach-vee), in liquids and solids you get a detonation flash, then a shockwave.

in a solid rod, it would explode where the shockwave crossed the motionwave front and detonated. (farther down the rod the slower you accelerated)

If you hit it with a super-sonic push, it would explode at the impact point.

if you accelerated up to super-sonic, the shockwave at the push point when it passes supersonic will travel faster than the motion wavefront, and when that wave hits the slower motion wave front you get a detonation.

You my friend have discovered the sound-barrier in solids.

permalink embedsave parentreportreply

[–]ColKrismiss

2 points 3 days ago

The object “sound speed” isn’t about how fast that object can move or how fast the sound it makes goes. In this case the object is its own reference frame, and the sound moving through it is unaffected by its own velocity.

permalink embedsave parentreportreply

[–]MR-rozek

2 points 3 days ago

yeah, but if i have a long stick, and push it from one end faster than its speed of sound wouldnt this end get to the other faster than the propagation of movement?

permalink embedsave parentreportreply

[–]ColKrismiss

3 points 3 days ago

Oh I see what you mean. My best guess is that it would break the object, no matter the material strength, but I don’t know for sure.

permalink embedsave parentreportreply

[–]pancakespanky

2 points 3 days ago

When aircraft move faster than the speed of sound in air the molecules in front get stacked up into shockwaves and create a sonic boom

permalink embedsave parentreportreply

[–]CaptainReginaldLong

2 points 3 days ago

That’s what breaking things is.

permalink embedsave parentreportreply

[–]Alanox

2 points 3 days ago

I’m no physicist but I imagine if you pull something that hard it may rip apart.

permalink embedsave parentreportreply

[–]NetDork

2 points 3 days ago

Are there any applications on earth where that movement propagation delay has to be taken into account?

permalink embedsave parentreportreply

[–]blackhorse15A

3 points 3 days ago

It comes up a lot in ballistics. You need to consider it as a projectile accelerates to ensure the projectile doesn’t break during launch. You need to consider how the projectile behaves once it stop accelerating and expands back out to make sure it doesn’t break and also consider how this may affect its flight. You definitely need to consider these effects when the projectile hits its target to understand how energy is transfered into the target and to analyze what breaks when- ideally breaking the target before the projectile breaks (well typically ideally, but that’s a whole other long discussion). Then how those same effects happen inside the target material itself to figure out how the target material gets broken.

Ballistics is really weird and really interesting. The forces involved are so high and the entire environment so extreme you run into these situations normal engineers typically ignore as being so tiny they are irrelevant. But in ballistics, like the OPs space example, it is very significant. We are talking about forces so high you have to treat solids as fluids. Not because they become liquid due to heat or something. Just that the forces involved are so high that the intermolecular bonds holding the molecules together as a solid are so weak by comparison they become negligible and can be ignore– and a bunch of molecules not bound together…is a fluid. The solid is still solid, but it behaves like a fluid under those forces.

permalink embedsave parentreportreply

[–]HopeFox

2 points 3 days ago

The finite propagation speed of impulses in water and other liquids can matter for big oil pipelines, and for pumping water around in general.

In solids? My best guess is no - if you push or pull a solid object at anything close to the speed of sound, you’re just going to break things.

permalink embedsave parentreportreply

[–]Lazlowi

1 point 3 days ago

Is there a scaled down experiment showing this?

permalink embedsave parentreportreply

[–]scriminal

1 point 3 days ago

What if they pulled in the dead center of the rod

permalink embedsave parentreportreply

[–]CorvidCuriosity

3 points 3 days ago*

Then the force would take half the time to propagate to either end of the rod

permalink embedsave parentreportreply

load more comments (1 reply)

load more comments (1 reply)

[–]ColKrismiss

1 point 3 days ago

Ok so I understand this and accept it, but there is a part of this I always get hung up on. What if the other side has a block? For example let’s say I am on earth with one end of the rod, and the other end is at the moon, and for simplicity’s sake let’s keep the notion that I am strong enough to lift the rod. What happens if I push the rod against the moon? Another comment said that sound through steel would take 18 hours to get to the moon, am I just free to lift for 36 hours before the information of the blockage gets back to me? How would my end of the rod “know” that the other end is stopped?

permalink embedsave parentreportreply

[–]blackhorse15A

5 points 3 days ago

You will be compressing the rod, making it shorter, as you push against it. That will happen even if it isn’t against the moon, because the rod itself is so massive, it’s own inertia is resisting your push.

So, two possibilities here. A) let’s say you push momentarily - just tap the end but hold the new position (like 0.1mm deep) and hold that. The compression of that will travel through the rod. All the molecules try to move forward that small amount and move the molecules in front of them forward that amount. If the rod is against nothing, eventually that tap reaches the other end and the end finally moves out the 0.1mm. You moved the rod the distance you pushed it, it just took 18 hrs to complete. But, if the end of the rod is against a very massive object we can treat as immovable - such as the moon - then when the compression finally reaches the moon, you will have compressed the rod to be 0.1mm shorter than it was.

Now case B) what if you push against the rod with constant force and keep applying force, trying to keep moving the rod. Basically the same thing happens. It will take 18 hrs for those forces to propegated to the other end. When you start pushing you will be increasing the internal compression near your end, but the far end near the moon will still be as if you weren’t pushing at all. Eventually the push will catch up. If the rod isn’t against anything the far end will eventually move forward, like before, but now it can keep moving out since you keep pushing. You will be accelerating the whole rod. Except your end will be at a higher velocity than the far end- compressing the rod more and more. What happens next really depends on the limit of how much force you can apply to it. Being just a human pushing, it’s likely you will hit your limits before you exceed what the steel can handle. It will reach steady state of compression at some shorter length. Now, if it’s up against the moon- our effectively immovable object- well, ever take a thin long rod, like a coat hanger or welding rod, stood it against the floor and pushed and keep pushing? Push a little and it just shortens a bit, with all the tension inside. Push more and it eventually ends and buckles. You’ll see things a bit different because of the 18 hr time delay for effects to reach the far end.

permalink embedsave parentreportreply

[–]HopeFox

3 points 3 days ago

For those 36 hours, you have been compressing or bending the rod. If you do it with enough force, you’ll break the rod regardless of what’s at the other end - the rest of the rod between you and the moon has enough mass that you’ll know you’re pushing against something.

permalink embedsave parentreportreply

[–]TyhmensAndSaperstein

1 point 3 days ago

So if you moved it sideways it would curve?

permalink embedsave parentreportreply

load more comments (2 replies)

[–]DirtyWriterDPP

1 point 3 days ago

Ok so what happens if I move my end at a speed greater than the speed of sound. Assuming I don’t shatter the whole thing? Could end X pass the starting point of end Y before it has had a chance to move out of the way?

Like what happens in supersonic or hyper sonic collisions? Like say a gun shot or military weapon? The front edge of the armor could most past the back edge before the back edge even has a chance to move out of the way?

All this make my brain hurt but also smile at the wonder of the universe.

permalink embedsave parentreportreply

[–]Y-27632

3 points 3 days ago

It does shatter or deform. (so long as it’s not infinitely strong, and maybe also impossibly light so you can move it)

For projectiles hitting armor, the shockwave will start to propagate at up to the speed of sound in whatever material the armor is made of. (which is typically way higher than the speed of any “normal” ballistic round, it’s over 5000 meters per second in iron vs 340 m/s in air)

The total amount of material that will get displaced by the impact is limited by the momentum of the round. (but will usually be lower than that if the round can just break through without dumping all its energy)

At much higher speeds weird things which I’m not prepared to try to explain start to happen. (sabot rounds hitting metal result in metal behaving like liquid without actually turning to liquid, and if you go even faster things vaporizing on impact…)

permalink embedsave parentreportreply

[–]SteptimusHeap

3 points 3 days ago

so long as it's not infinitely strong

An infinitely stiff material would have an infinite speed of sound. Stiffness and speed of sound are linearly correlated.

permalink embedsave parentreportreply

[–]malenkylizards

1 point 3 days ago

Which means, of course, that relativity doesn’t apply and it’s a purely classical question. And as far as “does it stretch,” yes, because all of the atoms are bound stretchily to each other. It’s not a very big stretch at all when you’re pulling on a two meter long rod, but even over lengths as short as a bridge (a few light microseconds), steel is surprisingly soft and stretchy.

Also, for fun, let’s say it’s a square rod 1 mm across. It would be 18000 cubic meters, and weigh 141,000 tonnes. Which sounds like a lot, but to be fair, it’s barely the mass of a skyscraper.

permalink embedsave parentreportreply

[–]filya

1 point 3 days ago

Thank you!! What if, and I know it’s an impossible if, the rod was completely rigid?

permalink embedsave parentreportreply

[–]WisconsinHoosierZwei

1 point 3 days ago

So, this is probably a dumb question, but how do you calculate the speed of sound in the vacuum of space? At least for these purposes.

permalink embedsave parentreportreply

[–]drigamcu

3 points 3 days ago

It is the speed of sound (or elastic waves) through the rod. Sound might not be able to move through vacuum, but it can move through matter, and that rod contains matter.