Q: Why is hitting water from a great height like hitting concrete?

The original question was: I know that if you hit water at a certain speed it is supposed to crush your body like you have hit concrete. Is this the case for all liquids or is something to do with the surface tension of water and if you hit a gas in a confined space fast enough would it feel like hitting concrete?


Physicist: There’s nothing terribly special about water, and even hitting a gas fast enough would “feel like concrete”.  For example, when meteors (which are fast) hit the atmosphere they generally shatter immediately.

A good way to think about high-velocity impacts is not in terms of things (like water) acting more solid, but in terms of things (like people, rocks, Fabergé eggs) acting more fluid.  The more energy that’s involved in a collision, the less important the binding energy (the energy required to pull a thing apart) is.  A general, hand-wavy rule of thumb is: if the random kinetic energy of a piece of material is greater than the binding energy, then the material will behave like a fluid.  A bit more energy, and it will fly apart.

Whether a substance behaves more like a solid or more like a liquid depends on the energies involved.

This shows up on a much smaller scale as well.  For example, the difference between water and ice is that the random kinetic energy of water, better known as “heat”, is greater than the binding energy between the molecules in ice.

So, when you fall from a great height and land in water there’s a bunch of kinetic energy going every which way.  The water continues to behave like water, but since the kinetic energy in different parts of your body are greater than the binding energy keeping them connected, then the body as a whole will act more like a fluid.  That is; it’ll “splash” (in the grossest sense).

Clearly there’s a big difference between something breaking into chunks, and something liquifying, but that difference is mostly just a matter of energy; it takes more energy to make sawdust than wood chips, but the process is more or less the same.  The take away here is, there’s a lot of different kinds of binding energy (molecular, structural, etc.) but they all do similar things.

Professional fluid dynamicists use a value called the Reynolds number to quickly talk to each other about this property in fluids.  It essentially describes whether a fluid is more “inertial” (water-like) for values much larger than 1 or more “viscous” (honey-like) for values much lower than 1.  Being natural comedians, they’ll say things like “usually my half-and-half has a Reynolds number around 20,000, but this morning it smelled weird had a Reynolds number of 0.3!”.  It’s usually used a ballpark-estimate, short-hand-description kind of thing, but in general the Reynolds number gets larger when things are bigger, faster, and denser, and it gets smaller when things are more viscous.

So, in a very hand-wavy way, a fast moving body hitting water (or whatever) has a higher Reynolds number, and is more waterish itself.

The “solid” marshmallow picture is from here, and the Ghostbusters “liquid” marshmallow picture is from Ghostbusters.

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12 Responses to Q: Why is hitting water from a great height like hitting concrete?

  1. Kirov says:

    So in a high velocity impact with water, what’s the best way to disperse energy in a manner that involves as little fluid body parts as possible?

  2. The Physicist The Physicist says:

    In the language of the post, you’d want the movement of the water to be “less random”. It’s the shearing of forces that really messes things up. So, streamlining yourself is the best way to survive a terminal velocity fall into water (like a dart).
    Even so, don’t expect to live or stay conscious.

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  4. The water molecules that forms the surface of the water posses a surface-tension (i.e. a weak inter molecular force between the layers of molecule… The inter-molecular force remains within the surface molecules unless these molecules were disturb by an external force and thus acts as a concrete when an object falls from a great height…….!!!

  5. The Physicist The Physicist says:

    Mythbusters did a whole thing about that in the “hammer drop myth” (where did we get examples for stuff before that show?). Turns out the surface tension doesn’t factor in appreciably.

  6. Mehrdad says:

    I am again too late to comment but in my humble opinion it is easier to predict what will happen by using the drag formula P=1/2ρv^2Cd. Once you have the pressure you can compare it to compressive strength of the matter (or is it tensile? or shear?) and see if it disintegrates. But that is only good for rigid solids. For living beings and most man-made objects (cars, planes, phones, laptops etc) that have delicate internal structures you can calculate the acceleration.
    Let’s see what happens:
    Drag coef. of man=1
    Density of water=1000 kg/m^3
    Terminal velocity=50 m/s
    Cross sectional area of man=0.6 m^2
    Mass of man=75 kg
    Therefore:
    P=1.25 MPa
    F=750 kN
    a=10000 m/s^2
    About 1000 gs! Not a single bone survives. (I suspect it should have been 770 gs, the density ratio of water to air)

  7. DooMHammeRchen says:

    First of all apologies for my bad English. Neither am I a native speaker nor Mathematician or Physicist ( I just posses a Major in Computer Science) so it would be great if you could, if you like, answer my question like speaking to a child :)

    Second of all. I read your article with great interest and liked it a lot.

    To the question which bugged me since I read your article:
    Is there a point where the displacement of water factors in? If there is a point where is it and could you elaborate on this topic a little more?

    If we had a cylinder full of water and a cylinder head which we would try to push into the cylinder we would not be able to displace any water and would find heavy resistance because we would try to compress the water.

    If we had a huge object like a big solid ball of steel ( perhaps 50m in diameter ) and we would let it fall into the ocean at maximum atmospheric falling speed, there would be displacement of water. In the process of displacing the amount of water needed for our big object, which will not happen for free I suppose, my guess is that huge amounts of water (big object) would be moved fast (high impact speed). To accelerate objects with mass energy is needed. To accelerate massive objects quickly lots of energy is necessary.

    At this point I would really like to know if I made any major mistakes in my reasoning and if not at which point the energies I mentioned come into play.

  8. Travis says:

    Had a question that maybe someone here can help me out with. We are looking at training in multiple locations on a helicopter and the survivability of a person falling on a regular ground surface like a dirt road versus concrete. We conduct training with a recue hoist system and sometimes this includes hoisting people from buildings, urban enviorments and vartied terrian. The argument we are having is I believe based on false perceptions of falling or impacting something like a dirt road versus a concrete pad. Half of the group believes that falling on something like a hard packed dirt surface is more survivable that falling on concrete. Any anyone can assit me in this I would greatly appreiate it.

  9. Jason says:

    @Travis.
    Neither… My understanding is that winching operational training over a solid surface of any sort is no longer legal in Australia. You must do winch training over water. I pointed this out to some military pilots who proudly explained that civil rules don’t apply to them. About a week later a winch motor overloaded, burnt out and applied power to the explosive line cutter. I understand that they didn’t die, but they didn’t do much tangoing afterwards either. Moral, don’t fall onto anything that you’d be unhappy about jumping onto intentionally.

  10. Jason says:

    @Travis PS, they were training over the grassed parts near the airfield. Grass is softer than a “dirt road” (the softest of your options) and they really really didn’t enjoy it at all.

  11. Brian says:

    I think i saw an article somewhere that stated a fall of only 18 inches is sufficient to break a bone under certain circumstances. So any kind of winch training over a non-water surface would entail some risk. I mean, imagine your emergency room visit just jumping off the roof of your house.

  12. kris blouch says:

    So, just for fun: What is the reynolds number for a fluid that a human could fall into, traveling at terminal velocity, and survive? Lets give the premise that an average human can survive 100 ft fall and terminal velocity is 120 mph. And lets ignore body posture.

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