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comment by Devac
Devac  ·  1359 days ago  ·  link  ·    ·  parent  ·  post: Pubski: July 8, 2020  ·  

It's been a week since my procedure, and I already noticed an improvement: I don't get dizzy when I stand up.

I recently recalled this:

and in the prolonged absence of mk's data (sorry if this sounds like I'm passing blame, it's not that; I know everyone's busy and dealing with lots of things), started working on an explanation. It's a surprisingly tricky problem on both classical mechanics and the theory of elasticity, at least to someone who usually deals with electrons doing their thing. I don't yet know if/when it'll be ready to share with you, but I'm having heaps of fun and, oddly, get a lot of satisfaction from designing my own experiment to measure both mechanical properties of grapes and test their limits without a centrifuge.





mk  ·  1359 days ago  ·  link  ·  

Good news. Glad to hear it! I am in the lab tomorrow. I believe I can bring a grape.

user-inactivated  ·  1359 days ago  ·  link  ·  

Only a grape? Can you bring one or two more things to give a chance? You know, for science.

mk  ·  1359 days ago  ·  link  ·  

Perhaps, but I do have work to do. :)

OftenBen  ·  1359 days ago  ·  link  ·  

Thanks for the first bit of good news I heard today friend.

Devac  ·  1359 days ago  ·  link  ·  

My pleasure to share it. Stay strong and take care of yourself man!

user-inactivated  ·  1359 days ago  ·  link  ·  

    It's been a week since my procedure, and I already noticed an improvement: I don't get dizzy when I stand up.

That's huge! Appreciate the updates here.

Also, someone's brought a lot of traffic to Hubski.

Devac  ·  1359 days ago  ·  link  ·  

Thank you very much! I shared a personal problem, it wouldn't be OK with you if I didn't follow up. At least that's what I think which, admittedly, isn't saying much.

Also, I don't know what happened, but that's one amazing, ego-boosting bugski. :D

user-inactivated  ·  1359 days ago  ·  link  ·  
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Devac  ·  1359 days ago  ·  link  ·  

Here, the problem comes from a deformable body that changes the area where it touches the container in the centrifuge. The skin of the grape is crucial and pretty tricky, too; responsible for tensile strength and elasticity tensor, but having no appreciable depth. Waaaaay too complex model for this kind of problem, but I like the challange. I'm also curious how far I can take it before it'll force me to drop analytical formulae in favour of numerical approximations.

Deciding to share my results isn't going to be hard, but there's a question what could be learned from a failure in this case. We have quite a few physicists around here, so I think 'when' is the real queston. :)

kleinbl00  ·  1358 days ago  ·  link  ·  

Let me know if you want to weigh in on "round vs square ice cubes."

Devac  ·  1358 days ago  ·  link  ·  

For whisky cooling? Sure, if I'm able. Seems a lot harder than what I'm doing here though, at least intuitively.

kleinbl00  ·  1358 days ago  ·  link  ·  
Devac  ·  1358 days ago  ·  link  ·  

So, some user who probably never used ODE in their life refuses to see their sophomorism on a problem that's good two leagues above my master's thesis in applied maths, which boiled down to a very restrictive treatment of Budyko-Sellers models (type of an energy balance model, used in climate science). Yeah, I'll weigh in, gimme a few days for prep.

kleinbl00  ·  1358 days ago  ·  link  ·  

LOL

I mostly just figured you'd appreciate the argument around complex modeling. I don't think there's any point in actually attempting the model but I sure as shit never had the interest in attempting to build an equation to replace the one he wanted to use.

Devac  ·  1358 days ago  ·  link  ·  

And I do, perhaps sounded a touch too militant above. It is a great problem, though. It's deceptive. The statement doesn't sound that far from those found in problem sets where "an ideal sphere of magical ice that always remains at 0 C melts without ever changing the surface area at the interface which somehow always remains at 10 C..." I like poking at this kind of stuff.

kleinbl00  ·  1358 days ago  ·  link  ·  

    It is a great problem, though. It's deceptive.

Clearly.

John Gardner makes the point in On Writing that teachers don't teach the books that are the best, they teach the books that make their point as easily and clearly as possible. Scientific experimentation at the undergraduate level is intended to illustrate the bright spots, not the gray areas and convection is fucking awful. It didn't surprise me to spend a month saying "no this is hard" only to be told "no it doesn't exist" on that basis; what did surprise me was the surety with which the argument was made.

user-inactivated  ·  1358 days ago  ·  link  ·  
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Devac  ·  1357 days ago  ·  link  ·  

It's one of those problems where my intuition is an immediate 'yes', but I'm gonna think it over.

    Which is 100% useful if you want to build a bicycle but 0% satisfying if you want a deeper, intuitive answer of why the world works.

There's a wonderful book by W. Kruczek Moment Pędu w 20 Doświadczeniach (The Angular Momentum in 20 Experiments) that's aimed at Polish highschoolers. The goal of the book was to make the topic more intuitive. I don't have it, but I'm going to scan relevant pages next time I'm in the library and translate as best as I can.

kleinbl00  ·  1357 days ago  ·  link  ·  

    I'm sure you've experienced this firsthand in working in Acoustics - overconfident assholes with physics degrees who think they know everything there is to know because they know the words "turbulence" "fourier transform" and "decibel" and a couple oversimplified models.

It's one of the many reasons I hate Kate Wagner.

    She's a shithead that didn't pay attention in class and couldn't get a job with the same meaningless degree my boss decided to get through distance learning, despite the fact that a year into it she discovered she usually spent her time arguing with her professor about math.

    And my boss's degree was in interior design.

user-inactivated  ·  1357 days ago  ·  link  ·  
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b_b  ·  1357 days ago  ·  link  ·  

    It may seem like I'm dunking on half of Hubski here including myself...

I'm sure I'm guilty of it, too, but in the spirit of dunking:

    Can "headers" in soccer/football cause concussions, based on the maximum force delivered to the players head when it collides with the ball? You can actually solve it with just calculus if you make the right assumptions...

Not actually a physics problem (but even if it were, I'm sure it could only be solved numerically when you actually try to use physiologically meaningful assumptions about the brain, which is very complicatedly anisotropic). It's a medical problem, and can't be modeled theoretically. If it could, then it would be easy to engineer a concussion-proof helmet, which apparently can't be done. A concussion isn't actually a physical phenomenon, it's a medical definition, which basically boils down to: 1. Have you been hit on the head recently? 2. If yes, are you experiencing headache, dizziness, amnesia, etc, etc. If yes, Congrats! you have a concussion.

I have no doubt you can solve the problem if a spherical object of mass M and spring constant K bumps into another rigid spherical object filled with a fluid of mass m and sheer strength V while traveling at speed V, do you reach a threshold compression of C? Unfortunately, that's not what a concussion is, and nobody actually really knows what causes one.

There's definitely some interesting physics there, but as far as i know, no one has figured it out satisfactorily. One of the most popular animal models of traumatic brain injury (it's also in fashion these days to refer to concussion as a mild TBI) is call the "lateral fluid percussion" model. In this model, you open up the animal's skull and use a tube to direct a compression wave of about a couple atmospheres (I think...can't remember the exact number off the top of my head) to the direct top of the brain. The injury it creates is a very focal lesion far from the site of the blast, but always in the same spot (lateral to the blast, hence the name). I suppose that suggests that it creates a travelling wave that bounces around the brain and two or more of the rebounds create a superposition at the site of the lesion, but I'm not really sure about that. I suggested it once in a meeting with a bunch of TBI experts and they just looked at me blankly then kept talking about something else (because they basically don't know anything about physics--I know about enough to be dangerous).

There's something really important there, but no one really know what yet, because the actual cause of injury in a concussion isn't known. The most popular speculations right now are that it either shears some axons ("diffuse axonal injury") or some micro blood vessels (causing very small "petechial hemorrhages"--small enough that they can't be resolved on a CT). Obviously each has a very different cross section and physical properties, so modeling either is different..or maybe both or neither are relevant.

Anyway, nice arguing about something else for a change :D

user-inactivated  ·  1357 days ago  ·  link  ·  
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b_b  ·  1357 days ago  ·  link  ·  

Anytime I see the title "neuropsychologist" I turn away in disgust! But I'm mostly just needling you about physics supremacy here, so don't take me too seriously (even though in reality I do know a shit load about this topic).

    the spring constant of a soccer ball doesn't tend to be a regulated quantity that's published anywhere

I have no idea what the regulations around a soccer ball are, but obviously even with a given material this is going to change with the inflation of the ball. My guess is that if you really wanted to calculate the force, you have to account for the vibration of the ball, since obviously that's going to greatly affect the conservation of momentum. Given that if you drop a soccer ball just to see how much energy is lost to the world during a collision, it's probably greater than 50%. I don't know, but that's where my gut goes.

But the much bigger problem is the shape of the head, which matters a lot, and also the strength of one's neck muscles, which also matter a lot. The latter because obviously the g force experienced by a head is going to be dependent, ultimately, on how fast the head is whipped about. Stronger neck muscles are going to limit that. It's one of the hypotheses for why women soccer players appear to have more concussions than men. The former, because the chance of getting a concussion varies pretty significantly depending where on the head the force is applied and in what direction. Man now that I'm thinking about it, you could make a really good qualifying exam question around this.

user-inactivated  ·  1357 days ago  ·  link  ·  
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user-inactivated  ·  1357 days ago  ·  link  ·  

    There's definitely some interesting physics there, but as far as i know, no one has figured it out satisfactorily. One of the most popular animal models of traumatic brain injury (it's also in fashion these days to refer to concussion as a mild TBI) is call the "lateral fluid percussion" model. In this model, you open up the animal's skull and use a tube to direct a compression wave of about a couple atmospheres (I think...can't remember the exact number off the top of my head) to the direct top of the brain. The injury it creates is a very focal lesion far from the site of the blast, but always in the same spot (lateral to the blast, hence the name). I suppose that suggests that it creates a travelling wave that bounces around the brain and two or more of the rebounds create a superposition at the site of the lesion, but I'm not really sure about that.

Is that similar to, different from, or complimentary of the diagrams and videos that show the cause of a concussion of the brain just bouncing back and forth in the skull?

b_b  ·  1357 days ago  ·  link  ·  

I guess it falls under the old adage that all models are wrong, but some are useful. Showing a cartoon of a brain rattling around in the soft space between the brain and the skull sort of gets the point across that some applied force is the genesis of the injury. Beyond that it's sort of worthless. When you think about the types of injuries that can result from a blow, it's starts getting complicated, because they could be compression, torsional, shear, etc. It all depends where on the head the force was applied and with what impulse, etc. For example, it's really easy to get a concussion from getting elbowed in the chin. You see it in sports all the time where a harmless looking bump on the chin keeps a guy sidelined for weeks. On the other hand, it's probably a lot harder to get concussed if you get hit in an area of your skull that is thick and also doesn't lead to a severe torque on your neck. Applied force is a prerequisite, but it's not even close to the whole story. In fact it's a small part when you look how many times people get hit in the head really hard and don't suffer a concussion or TBI. I've had two in my life and I've been playing ice hockey for more than 30 years. Force is a starting point, not an ending point.