# Water and High-Speed Impact

Argon_Asimov
I had an argument with a friend and fellow engineering student the other day about the properties of water in a high-speed impact.

It started when I casually mentioned that water (like the new item from thinkgeek.com that we were discussing) exibits different properties at different impact speeds, because of its polar nature; and that further, striking a body of water at sufficient speed was like striking concrete.

He refuted this theory on the grounds that striking any surface at high enough speeds would cause enough decceleration to damage the human body just as concrete would, and that it has nothing to do with the properties of water.

I originally heard this from a friend who was a Navy rescue diver, and again from one of my professors, and I've always taken it for granted, but I could be wrong here. My friend challenged me to do the research and find any studies that backed up my claims, but I'm drawing a blank so far and I'd like to see if anyone can prove this one way or another with pure physics.

Rll1969

## Answers and Replies

Sojourner01
It has to do with the 'relaxation time' of the material I believe. Most liquids will behave the same way if struck at high enough speed, though I don't know the relaxation time of water compares to other fluids.

Panda
Not sure but I think that surface tension comes into it.
There is an account from the building of the Sydney Harbour Bridge, IRC, that one of the steel workers fell off the bridge. He threw his rivet hammer into the water just before he hit thus breaking the surface tension and survived with just serious injuries rather than the more common fatal ones.

electricsheep
Wasn't this done on Mythbusters? If I remember correctly, they found that breaking the surface tension with a hammer didn't really help when the dummy was thrown from a sufficient height.

(By the way Panda, what does "IRC" stand for in the context of your post? )

Panda
"If I recall correctly" IRC... I'm not necessarily using the correct abbreviations but IMHO they are right...

Mentor
If I Remember/recall Correctly - IIRC.

Anyway, I think you may be looking at the issue backwards: with a sufficiently high impact velocity, a solid material acts like a fluid, not the other way around. Think: crater vs splash. The dynamics are very, very similar and the key is how much energy is required to fracture a material (which is constant) vs how much energy is required(is imparted on it) to push it out of the way (which depends on speed). So as the collision energy increases, the fraction of that energy used to break the objects decreases.

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Panda
What though if the surface you are impacting is already flowing? preferably away from you. More energy would be expended in moving stationary fluid than mobile fluid, and as survival is based on deceleration the less energy you expend at any instant the better.

As for it being disproved on myth busters. I haven't seen the programme, but not all the proofs on these TV shows are scientifically valid.
"Vroom Vroom" on Sky prooved that putting go faster stripes on your car actually slows it down... This was based on the highly scientific statistics of one sample in each state.

Mentor
What though if the surface you are impacting is already flowing? preferably away from you. More energy would be expended in moving stationary fluid than mobile fluid, and as survival is based on deceleration the less energy you expend at any instant the better.
The fluid would need to be moving at a significant fraction of the object's speed for that to matter.