Are solid objects really “solid”?

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:

[AlphaPhoenixChannel]

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.

[michaelmolter]

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.

[nashsok]

"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!

[ChaosUnit]

I never really considered how soft, flexible, and noodly steel is until I became a machinist. Now it's a constant thought and struggle in my daily life.

[GregHassler]

"All models are wrong, but some are useful"

[PracticalEngineeringChannel]

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.

[essigautomat]

The end explaining the methods and failures was even more interesting than the initial question, great video 👍

[Mathijsvanwijk]

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!

[account]

I find the fact that you ALSO explained the "failures" in the experiment, as valuable as the results themselves.

[EngRMP]

As a 66 yo electrical engineer, I've found myself going back to learn physics over the past 20 years. Your demonstrations of physics are REALLY well done... really informative. Nice job.

[MrZapper]

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

[personalsinr]

Impressive! Love hearing the behind the scenes design and testing of the experiment as well! Very cool.

[AppliedScience]

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!

[admthrawnuru]

This exact concept blew my mind about 15 years ago, but I never saw it actually demonstrated until you did it here.

[jonathanbailie]

I love your explanations of the physics. Brilliant job on the experiment, so interesting!

[FPSG]

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

[thethoughtemporium]

Love the setup. Very simple and clever. Great demo!

[NickC_]

Man, YouTube has really been slacking with my recommendations because this is the first time I've seen your channel. I've always wondered about this exact thing but never had any way to find out for myself, and never even knew quite where to start vis-a-vis googling an answer. This video was such a satisfying vindication of my curiosity. I subscribed right away!

[shikhanshu]

This video was VERY interesting to watch. I didn’t learn much of anything new as such, but how it all came together is fantastic!!

[davidn]

I'm glad somebody else has had this exact question before.

Always thought about how faster than light communication could come down to just having a really long stick, knew that it couldn't be right, and now I know exactly why it couldn't.