Discussion in 'Physics & Math' started by Lilalena, May 13, 2011.
Log in or Sign up to hide all adverts.
(Sorry, the system kept freezing up everytime OP's submission so I did a test post)
Quick question - basic physics
How did the speed of the crashing plane make the ocean "unyielding as concrete" ? Didn't understand this part of article I'm reading:
"Preliminary autopsy results, summarized for reporters, found the three travelers died instantly of multiple trauma when their Piper Saratoga struck the water after descending at a rate of more than 5,000 feet per minute. That rate, combined with the plane's forward speed, would make the ocean as unyielding as concrete. "
Just guessing...maybe surface tension?
Did you ever do a belly flop off a diving board? That darn water is really hard!
The faster you go the less chance the water has to "move out of the way".
At its simplest.
Hard yes, water doesn't compress very well. However "unyielding as concrete" is not an accurate statement. If you were to crash that same plane into a concrete runway, there wouldn't be much of a splash. It would be all splat.
You get a similar result when you fire bullets into water and concrete. Or a person can survive a belly flop in water and as bad as that might hurt, he wouldn't even try it on concrete.
So surface tension takes time to break? This is a question I've always wondered about. You can jump off the high dive and survive...but if you jump off the Golden Gate Bridge...you die. What is the difference between the two? Is this due to surface tension...or due to viscosity?
I dunno, my reply was more to the Op than your post (dang! I've just admitted I read your posts: ignore that comment).
Like KilljoyKlown said: water isn't compressible (negligible for most purposes), so if you enter water slowly you can displace it. If you hit it at speed you're trying to displace it very quickly (and it is fairly dense stuff), so it comes down to trying to compress it when the aircraft (or your stomach) hits it. And compressing water doesn't work...
I've dived off 10 and 15 metre boards with no problems, provided I enter in the correct position. I've also managed to get a marvellously red belly from about three metres Please Register or Log in to view the hidden image!
IIRC Mythbusters did something about breaking the surface (with a thrown hammer?) before you hit. The dummy ended up wrecked...
Please Register or Log in to view the hidden image!
Since liquids don't compress...just from my uninformed position...this seem the more likely explanation. I hope the physics guys chime in on this.
You don't regard me as a "physics guy"?
Please Register or Log in to view the hidden image!
Yes..I respect your opinion on many different subjects including physics....but there are members here that make their living with physics...Alphanumeric, BentheMan. etc....
Don't go all pussy on me, bud...I know you know your shit....I just wanted to see a bunch of "Tex" formulas I don't understand.
Like Rpenner's posts.
I follow them 100%, right up to, and sometimes including, the first word. Please Register or Log in to view the hidden image!
At that speed, you might as well be crashing into stone. Your body will be compressed, your bones broken.
Mac the mythbusters did an episode where they fired bullets into a pool, only the slow heavy ones could penitrate (which is oposite of what you get with armor). The 45 easerly killed the target but the sniper rifle bullet bearly makes it through the surface without disintegration
I saw that program. Anyway I'm pretty sure surface tension only applies to small things, like a needle or a water skate.
However the subject of compressing water does interest me. Do we know at what pressure water does start to compress? It must be quite high, after all it still hasn't compressed at 7 miles deep and that's a lot of pressure.
Wrong!!! Please Register or Log in to view the hidden image!
See? A decrease of a whole 5.3x10[sup]-5[/sup] of the original volume. Please Register or Log in to view the hidden image!
Thanks for the info. Water is pretty amazing stuff. If anything about it was just a little different we wouldn't be here. For example if it didn't expand when it froze, life just wouldn't be the same.
It does tend to catch people out (the compressibility). Most people tend to take "incompressible for all intents and purposes" as "absolutely incompressible".
I can't think of a single occasion in 30+ of engineering (including hydraulics and fluidics) where I've had to take account of compressibility.
Why should it take much time at all , I thought molecules in water aren't as tightly packed as in solids; in addition, there's plenty of space between the air molecules above the water's surface (?) . How could it not easily make way for a tiny bullet?
If not exactly the spacing of molecules what property is it of water that makes it difficult to compress?
They aren't. But, as we've noted, water is effectively incompressible (it's just one of those things), and hitting water quickly allows less time for displacement - you have to move the water away to get the aircraft into it. And whatever water you move has to move other water, which has to move other... It's a question of diminishing returns Please Register or Log in to view the hidden image!
Above? Yep. But while you're still above the water you haven't hit it yet.
Or are you talking about splashes going upwards?
In which case you still have to get that water to move - and the aircraft is moving downwards, which will tend to push the water downwards also. (And eventually "push" against the sea bottom). Splashes upwards are a delayed reaction. They come later. (Albeit incredibly quickly to our to vision/ perceptions).
To give some figures (and bear with me here, I'm calculating from the Bulk Modulus because Google can't find me a simple chart with compressibility on it):
Call this the "squishability value"* Please Register or Log in to view the hidden image!
Air - 7042
Water - 0.454
Steel - 0.000625
So water, although 72 times more "squishable" than steel, is also 15,492 times LESS squishable than air. Anything attempting to transition from air to water is going to undergo a very sharp shock and deceleration.
* The SV (squishability Value) (if anyone's interested) is derived from the inverse of the Bulk Modulus (which I could find, and which itself is the inverse of the compressibilty - ho hum X = 1/y and all I can find Y so I do 1/Y to get damn value Y is derived from!) but multiplied by (er, I just forgot!) 10[sup]9[/sup]? 10[sup]12[/sup]? whatever... just to get rid of looooads of zeroes after the decimal point and create a number that's more easily grasped.
Extra edit: just saw Lilalena's edited question.
From the values above it should be "apparent" that, despite the fact that it flows, water is far closer to being "solid" than it's generally given credit for.
Um, extra edit Mk.2: AFAIK it's not how far apart the molecules are in water that lets it flow, but how easily they slide across each (simplified - it's years since I did this). The two things (packing of molecules and "slideability" [whatever the technical word is]) aren't directly related from what I remember.
Separate names with a comma.