Water as unyielding as concrete

That hadn't occurred to me.
I'd seen it more as symptom of (at least) two things:
1) not everything is actually on the web (I know I'm waiting for the day when the British National Archives are fully digitised - er, and an unlimited budget).
2) the information that is available prompts people to ask evermore "obscure" questions. Maybe we're moving toward a genuine synthesis of knowledge...

 

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I see no reason why it wouldn't, even more so than water. (Ie. it would be like concrete at a lesser velocity than water.)

 

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I thought along those lines at first, but I've been suspicious of Google's business model ever since I saw a youtube video of their offices.

 

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Dough almost certainly. That stiffens on impact doesn't it?
Jello/ jelly (yes I'm English, sue me). Hmm I have no idea. And I can't think of any engineering manuals that would have the bulk modulus of Jell[o/y] to check. It's not something that's in much demand as structural component.

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Just Googled: I've found the Shear Modulus but I don't think that helps.
http://wug.physics.uiuc.edu/cc/IASt...ecture 26 - Stress, strain and elasticity.pdf

 

But does it really "stiffen" - or does it just seem so, because of the force?

 

If someone throws a piece of dough at you, and you feel a painful impact:
1. it hurts because of the force with which the dough was thrusted/landed on your face
2. it hurts because the dough is hard
3. other
?

If you would throw a piece of cottonwool with enough force (in a vacuum), would it be the same as throwing a rock?

 

Define "stiffen".
I may be wrong about dough, per se, but some substances do. Measurably so. That's why they're being investigated (sold already?) as constituents of body armour.

 

You used "stiffen" first.

 

The answer would tend to option 2.
As illustrated by the cotton wool example.
That is sufficiently pliable to crush in on itself to absorb the energy of impact, thus transferring little of it to you.
(Um, ignoring the possibility of the cotton wool freezing solid in the vacuum of course, and letting it retain its "cottonwoolness").

 

I did. The engineering term.
If you're using the same definition as me then yes, it does stiffen.

 

Hi Dywyddyr,

Friction is a high level phenomenon, like a generality.

But the idea of impact looks to me to be bottom level, nuts and bolts specific,
where you want to look at many F=ma collisions.


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If you want to look at that level you have to analyze it into stages. Let’s imagine a sphere hitting the water:

The first impact would be the osculating surfaces, and only last an instant.

Then ever widening rings of penetration would occur. Each could be broken down into orthoganal components, vertical impacts like the first one, and sliding between sphere and water. Here the friction approach would again simplify the approach. I think it would be sphere-water friction and water-water friction.

All of these stages would be ultra sensitive to the velocity.

I think I remember that the square of the velocity in friction comes from the square of the velocity in kinetic energy.


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Yikes!

I just notices a whole lot of other replies after yours, Dywyddyr, so I’ll have to spend some time reading them. I didn’t think this subject would generate such interest.

I’ll also go back and look better at how friction was discussed previously.

MikeO

 

Yes!

Slow bullets have far less friction to deal with after the initial penetration occurs.

 

Huh?

Why "many"?

Nope. Why do you need to look at osculating surfaces?

After the thing has broken up, no?

Apart from that being a slightly incoherent sentence, is that what you remember?

Hmm:

Any more? In 30+ years of engineering not once have I had to account for velocity when using friction in a calculation.
Maybe you're thinking of skin friction (drag) in aerodynamics, in which case you're only partially correct about "square of the velocity" -it depends on what the velocity is: at low speed it's proportional to velocity, at high speed it's proportional to the square of the velocity.

 

Nothing to do with it.
Friction wouldn't break up a bullet - it's surface phenomenon.

 

I agree, friction is not what would break up a bullet.

Friction does significantly lower the velocity of the front portion of the bullet, though.

Then it's the momentum and higher velocity of the rear portion that breaks up the bulet.

 

:roflmao:

Are you serious?
Friction applies when the bullet is submerged (if it hasn't already broken up).
If it IS submerged then the friction applies over the whole body.

 

With most gizmos and engineering situatiojns great efforts are put into minimizing the friction, so this square function harldy comes to play. You have to look at a wide range of friction and velocity. In most gizmos it's low.

As to your questions about the relative levels where friction is high level and F=ma is low level, it is the similar to thermodynamics.

In thermodynamics you can look at individual low-level collissions, or generalize by using tools of thermo and look at things like temperature and emtropy.

 

What?
Do you speak English?
First you claim that friction is related to the square of the velocity and now you're saying that "minimising friction" stops the square of velocity coming into play.
:wtf:

In most gizmos where low friction is desired it is, otherwise no.

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Er, right. Do you know what you're talking about? 'cos I sure don't.
What's your engineering discipline? How often have you dealt with sort of question?

PS with regard to your first sentence: you don't think designing supersonic aircraft or, er, bullets is engineering?

 

I'm talking about the earlier stages of impact.

When the bullet first hits, the front slows down and the rear end keeps moving.


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When I was a kid we loved to go tobogganing. A tobogaan is a long sled and five guys could sit in-line on it.

One day we selected the wrong hill to go down. It had a sharp landing at the bottom and we hit it hard. The toboggan pretty much stopped dead. I was in front and went flying into the snow head first. I was the luckiest.

The guy behind me instantly slid forward and hit the edge of the toboggan. The other three guys continued on at amost maximum speed into his back and the front of the tobbogan. All were hurt bad, one hospitalized.

I see that toboggan scene in the early moments of impact for a bullet in water, or anything that can significantly slow down the encoutered surfaces (and down to some depth) of said bullet.

 

Uh, duh. And I've already explained that.

Ah! There's your problem then. Incorrect application of an example. A bullet is an homogeneous structure (at least front-to-back it is). Three separate guys on a toboggan aren't. There's gaps between each guy and they aren't structurally tied to each other.

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