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What happens when you fall at 100 mph into a flat water lake

  1. May 15, 2014 #1
    I'm just wonder what would happen for a physics project. I have looked everywhere and can't find anything helpful on the internet. Theoretically, I just need to find what would happen to a large humanoid object reaching a terminal velocity of 100-125 mph and falling down around 4km into said flat water lake.
     
  2. jcsd
  3. May 15, 2014 #2
    You would die :-)
     
  4. May 15, 2014 #3

    A.T.

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    What do you mean by "flat"? The water surface is usually flat. Do you mean shallow? If yes how shallow? Do you mean no waves or sprinkling disturbances?

    People have survived very high falls into water, and even into trees and snow.
     
  5. May 15, 2014 #4
    I mean no wind blowing just calm, would it be possible and if so how. If not what conditions i.e.. liquid, and other variables could be changed for it to work.
     
  6. May 15, 2014 #5

    TumblingDice

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    Water can be considered incompressible. It would be about the same as hitting concrete if the humanoid landed horizontally. :eek:
     
  7. May 15, 2014 #6

    PAllen

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    Water may be incompressible but its viscosity is a little different than concrete :tongue:, which is somewhat relevant to the question. How you hit the water is critical. Landing vertically is much more survivable than horizontally. While disputing the = concrete analogy, landing horizontally is almost certainly fatal. If falling from a plane without a chute, spread your arms and legs while falling, aim for water, then go into a vertical dive at the end - you have some real chance of survival. The water better be deep.
     
  8. May 15, 2014 #7

    TumblingDice

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    I didn't think anyone on PF would suggest a perfect dive could be accomplished. If so, you'd think some dare-devil would have certainly tried this already for posting on YouTube. :wink:

    I'd like to explore this further. We can allow "perfect" form to fall spread-eagle for the first 3.9 kilometers or so and translate to vertical. Should be able to calc approximate spread-eagle velocity and use that to calc best height to move to vertical and assume free-fall for rest of "dive". Do you think viscosity will allow flesh and bones to get a mass of water equal to your volume moving as quickly as your body needs to displace it? I'm not expecting anyone to fill three chalkboards with math. Could we just ballpark an entry speed and volume/mass of water to displace, and thereby determine the pressure shock wave the body would need to survive?

    (EDIT: I was expecting a body would undergo immediate, acute, fatal compression.)
     
  9. May 16, 2014 #8

    PAllen

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    The terminal speed of a spread eagled human body is 100-125 mph as correctly mentioned in the first post. (The jacknifed terminal velocity of a human body in air is much higher; if memory serves it could reach closer to 250 mph). 100 mph is not a supersonic (let alone relativistic) speed. Survivability in different configurations gets into a quantitative calculations about rate of displacement of water (thus very different than concrete - displacement of concrete doesn't occur; displacement of water by a falling body does - the issue is at what rate, and what damage the corresponding reaction force does on the human body), and time of slowing down. Jacknifed into water, at 100 mph, you will slow over a span of over 10 feet, and the rate of displacement of water will be much less than horizontal.

    Independent of calculation, it is a fact that such falls have been survived as AT pointed out #3. The possibility of doing so is thus not in dispute - only the likelihood of achieving such survival on demand.

    [edit: crude calculation indicates a rate of displacement of 100 cu. ft./second would suffice for a jacknifed entry; this leads to a force that is not - to me - obviously unsurvivable; painful, damaging, yes, but not clearly unsurvivable].
     
    Last edited: May 16, 2014
  10. May 16, 2014 #9

    CWatters

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    Last edited by a moderator: Sep 25, 2014
  11. May 16, 2014 #10

    CWatters

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  12. May 18, 2014 #11

    PAllen

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    It doesn't matter how many times it is repeated on the web that 'hitting water' falling from a plane is the same as hitting concrete, that does not make it true. The physics is completely different. The fundamental point is that 100 - 125 mph (terminal velocity falling from a plane unless you really try to go faster) is no where near fast enough to make the fluidity of water irrelevant.

    Even for high velocity bullets at 2000 mph, there are major differences between hitting water versus concrete. On the other hand, arrows (which may range from 100 to 185 mph), hitting water versus concrete is totally different, and there is a whole sport of fishing using arrows fired at speeds similar to human terminal velocity into water to spear fish.

    I am not claiming a human jacknifing into water feet first at 100-125 mph is 'equivalent to an arrow' or has a high likelihood of survival, but I do claim that viscosity still matters at this speed, and that incompressibility is only a secondary factor, and that survivability is much greater than for concrete.
     
  13. May 18, 2014 #12

    phinds

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    A fellow sometime in the last couple of years reached just a hair over supersonic speed (800+ mph) from a balloon at about 30miles high. HERE it is:

     
    Last edited by a moderator: Sep 25, 2014
  14. May 18, 2014 #13

    PAllen

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    Yes, but this was at a high altitude. Terminal velocity (for a person) normally refers to maximum free fall rate at standard temperature and pressure. I am quite familiar with this exploit, and also with estimations about how much he SLOWED DOWN as he fell further from the peak speed.
     
    Last edited by a moderator: Sep 25, 2014
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