Water as unyielding as concrete

Ooh!
Thanks for this topic, I just learned two new words.
I was going to suggest that water could be considered as anti-thixotropic, at least for the purposes of an explanation, and upon Googling I found that the correct term for anti-thixotropic is rheopectic.
But, like thixotropy, rheopecty (one of the new words) is time-dependent, and a more nearly correct term would be dilatant (2[sup]nd[/sup] new word).

An illustration would be oobleck (corn starch and water if you want to experiment). Leave a chunk on a desktop and will turn into a puddle. Hit it with a hammer and it will shatter.

This is not suggesting that water is Non-Newtonian, but rather giving an example of how a "liquid" can act as a solid under impact.

 

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1. Do you mean "it's just one of those things" as in

a. We don't know why it behaves that way, all we can do is adjust for that behaviour in our calculations OR
b. We know why it behaves that way but you have to be a specialist to understand it.

So it has to push down to the bottom before it starts pushing up. This makes everything clear esp. for the airplane example but for bullets, I would have thought the water would be a little more forgiving - I mean because of the small mass of the bullet (?)

2. So does this mean that only the speed of the crashing/speeding object matters, not its mass?

It certainly helps to know that. I don't know why they don't state things like this explicitly in school texts.

3. So is this why a tiny high speed bullet will still come up against the same (? or at least, a seemingly disproportionately large amount of) resistance on hitting water?

4. If a bullet could meet such resistance hitting water how can there be a proper position for diving (I mean I assume proper position is to enter the water like an arrow - small/sharp end (hands pointed?) first).


Other questions -
5. How do I find out the upper limit of slow beyond which an object will be met by the water with concrete-like resistance?

6. Does the size of the container matter? If they had crashed into a very large lake (far from its edges but bottom of lake closer than it is in ocean, therefore water pushing down will get to bottom faster, and start to push up sooner? ) could they have had a better chance of survival?

 

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i was under the impression that highly salinated fluids (like the ocean) can behave non newtonian-ly. Is this incorrect? and why?

 

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The word "unyielding" describes part of the phenomenon, and shape matters.

If a very long, narrow cone "hit" the water point first at high speed, the water would yield/displace gradually over time — the length of the cone divided by its speed allows time for the transfer of kinetic energy of the cone to the water at a relatively low rate, and the cone would decelerate relatively slowly.

If the same cone hit base first at the same speed, the base of the cone would displace the full amount of water instantly, causing an extremely high rate of energy transfer, resulting in an relatively high rate of deceleration. People might refer to this high rate of deceleration as "splat".

Also, here's an example of a non-Newtonian liquid. Notice that the people make sure to "pound" the liquid's surface.

 

For instance would a large cone with a 2m wide base entering water point first, fare better than a small cylinder of 1/2 inch radius? (Assuming both enter at same very high speed and are of same length)

Somebody's bullet example really confused me.

 

Hmm, AFAIK salt solutions can exhibit this property, but I don't think oceans are salty enough for it. Wiki has a page on Non-Newtonian Fluids.

I don't know (I'm a mechanical engineer and it's not something I've had to consider before now). Maybe Trippy knows. I assume that someone, somewhere does know, but possibly not.

Ah, probably not, the two things weren't linked in my mind.

It's not the mass, I'd say, since I can (from the diving/ belly-flopping example) get both results with me being the same mass for each - a clean entry and a splat.

I'm working this out as we go along. So I'm explaining it as I understand it - which leaves room for someone equally lost on the topic to jump in and help out.

At first guess I'd say the whole thing is a function of speed and shape (and possibly density). A large area at low-ish speed is trying to move more water aside than a small area going quickly. This would equate, possibly, amount of water being moved and the speed that water is required to move aside. There's possibly a cross-over somewhere on the graph (if one were to construct such a thing), where impact speed outweighs area, but I don't know.

I have no idea. I haven't found anything on the net, and it's not something I've come across on my engineering courses. There is a magazine, the International Journal of Impact Engineering, which may help (I know for a fact that it contains studies of things like anti-tank rounds vs. tank armour and piledriver vs, earth/ soil/ rock).

Maybe I shouldn't have mentioned the water reaching the bottom of the sea (since that was simply an illustration of how much water has to move overall), but I'[d guess (from something vaguely remembered at the back of my mind) that you're worse off in shallow water since there's less water directly below you to be shifted and the effect (or you!) is transferred that much quicker to solid land - solid land which is less forgiving of impact that water.

See my previous comment regarding mass, for a start. The bullet example would be confusing since the two examples given are so different. A .45 bullet is (surprise!) roughly [sup]1[/sup] 11.5 mm in diameter and travels at ~350 metres/ sec [sup]2[/sup], while a sniper round will be about 7.5 mm in diameter and moving at ~900 meters/ sec. I.e. one is 3 times faster, at nearly Mach 3.

Notes:
1: Despite bullets being named as a particular calibre they very (very, very) rarely actually are that size. From memory a 5.56 bullet actually 5.66 mm, a 7.62 mm round (typical sniper bullet) is 7.82 mm. Hence I have given a near-enough-for-illustration value.

2: Speeds given are muzzle velocities, naturally they slow down and that's a function of range and ballistic coefficient - it's easier to work from the base "nominal standard" value. Oh, and the muzzle velocity will vary with barrel length, type (and amount) of propellant used in the cartridge AND the actual type (and therefore weight) of the bullet. A "standard" NATO 7.62 round comes in at least 3 variants - which will have differing masses and muzzle velocities.

 

What kind of concrete?

 

How would the temperature of the water affect it's apparent impact hardness?

At 1 atm water is most dense a 4 degrees C, but when it boiling at 100 degrees C (least dense and still liquid), it also has less surface tension because of the bubbling action of the boiling water.

I could be wrong again, but I would guess it's less hard when boiling.

 

Hitting boiling water wouldn't be survivable either I presume due to burn wounds.

 

From the Wiki link I gave on the first page.
That's a variation of about 15%, dunno how much difference it would make between "splat!" and "ouch!"

 

Yes, but it would be a very unique way to go and you would find out if the lobster does feel pain when dropped into boiling water.

 

But unable to write it up for the scientific journals, which means the rest of us still wouldn't know.

 

That's not enough of a difference to risk getting burned.

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But when you aerate the water with lots of bubbles, that does lessen the density quite a bit. I've heard that if you could aerate enough water you could sink a battleship.

 

Yeah, boiling water isn't necessarily full of bubbles - they depend mostly, IIRC, on imperfections in the (inner) surface of the container.
Bubbles on top is one thing, but aerated by bubbles in the water is another.

Dunno how we'd go about getting information on that.
WTF happened to the "information age"? Google never has a link to "stuff I want/ need to know now, and don't give me any of this "refine search terms" crap". :bawl:

 

Not sure this will help, but is Google the only search tool you use? I've been under the impression that some search tools are better at certain type searches than others? This might be a good topic on it's own.

 

I think it is. But it was more a comment on accessing information in general. I own a total (hard-copy and pdf/ various other computer-only formats) of around 20,000 books. But when I want to look something up it's never in a book entitled "Exactly what you're looking for at this moment", I have to wade through the indices/ contents of 3 or more books before I find the specific piece of data I'm after.

I wouldn't doubt the first sentence and totally agree on the second. I use Google mainly, I suppose, through default: it was recommended to me when I started using the 'net a decade or so back, and it just never occurred to me to use anything else (probably because the ones suggested by various pieces of software I've installed later were usually rubbish).

Hmm, I wonder if I should Google for "Best search engine"?

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I have heard that from about 40 meters and more, falling into water is like falling onto concrete.

 

Pfft, even from 10 metres I used to get a headache after a couple of poor dives (head not tucked in correctly).
Falling (and landing badly) is one thing, (and 40 metres doesn't surprise me). The world record for a dive though is 53.94 metres, and someone got only minor injuries from a badly-executed impact from a height of 38 metres.

 

I am sure that once an average person with little or no jumping skills (which is most people) finds themselves falling into water from great height, the first thing they will think and do is to correct their position so as to enter the water in a safe position!