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If we hit the water at great speeds, we die.

This is because the water has no time to "move out of the way" and acts as a "solid" surface.

At what falling speed can we consider the water as a 'solid' surface? The speed of sound in water?

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  • $\begingroup$ The speed of sound is higher in water than in air. You will die well before hitting Mach 1 (in air or water). I guess it is a question of inelastic collision, but on top of that is the question of what causes you to actually die, which is not a physics question. $\endgroup$ – Jon Custer Sep 15 '14 at 22:27
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    $\begingroup$ The question of "at what impact speed does a liquid surface behave as a solid surface" is certainly physics though, and on-topic... as long as this is interpreted that way, it is on-topic (and, I would argue, interesting). $\endgroup$ – Kyle Oman Sep 15 '14 at 22:30
  • $\begingroup$ Water can never be considered as a solid. Indeed, it is the other way around, at high speed solids behave like liquids. $\endgroup$ – CuriousOne Sep 16 '14 at 0:21
  • $\begingroup$ right maybe I shouldn't have said that it behaves as a solid, but you get what I mean. The water does not move away quick enough. How can I get a velocity at which this happens? $\endgroup$ – SuperCiocia Sep 18 '14 at 22:03
  • $\begingroup$ If the person hits the water incorrectly, they could die from falling 10 feet to the water. $\endgroup$ – LDC3 Nov 16 '14 at 16:43
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There doesn't appear to be speed at which you can fall on water, and have it be the same as falling on a solid. The Water vs. Pavement episode of Mythbusters did tests involving falling from a variety of heights, and even at a height great enough for the human analog to achieve terminal velocity, the effect of the impact was notably greater for landing on the pavement than landing on water.

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  • $\begingroup$ This neglects all speeds higher than the terminal velocity of a human in air. Since the relevant timescale is probably fixed by the sound speed in the liquid, those higher speeds probably need to be looked into. $\endgroup$ – Kyle Oman Sep 15 '14 at 23:59
  • $\begingroup$ @Red Act, even so, if you land on water past a certain high velocity (no, I don't know the value, as that value is dependent on the angle at which your body enters the water), you will not survive the very high forces involved. $\endgroup$ – David White Apr 19 '17 at 20:12
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For all practical purposes, in the Earth's atmosphere, terminal velocity is less than the speed of sound in water by a large margin ie around 15x. So water will cushion an impact. That Mythbusters episode left something to be desired - accuracy. The world record for a shallow dive into 30cm of water is about 11m in height, meaning an impact velocity of around 14 m/s and a deceleration of at least 36G. Terminal velocity is around 56 m/s

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protected by ACuriousMind Apr 19 '17 at 18:08

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