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I saw a video of a situation of parachute failure , and how to increase your chance of survival. The speaker says that while heading your body tries to land on land but never on water


Video link https://youtu.be/dy5xLVx2NGY


My simple question is land is obviously harder than water and when you land with a high velocity it should cause more damage to you but what he says is contradictory so does that mean water imparts more impulsive force . If yes then why , if no then what's the reason?

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The reason why some things are better to fall on than others is because of how fast the momentum is reduced. Force can also be written as the 'rate of change of momentum', which means force is inversely proportional to the time period it is applied for.

In other words, if your momentum is reduced from a very high number (something you'd expect if you're falling from a plane) to zero INSTANTANEOUSLY, the impact force would be extremely large. But if the momentum is reduced gradually by slowing down your descent bit-by-bit, the time period would be longer and consequently the impact force would be smaller, and that's exactly what makes some things better to fall on.

The fallacy in your comparison between 'land' and 'water' is that land is not a homogenous material. Land consists of trees, snow, stacks of hay et cetera, and all these things would be your best bet to land on if you're falling from such a great height, precisely because of the reason stated above - they slow your descent down bit-by-bit.

Water, on the other hand, is infamous for being incompressible. That means if you impact with water from such a large height, instead of giving way, it just halts your descent instantly, resulting in a very large impact force. One general misconception (which you too have fallen for) is that 'land is harder than water'. Even though that may be the case for smaller heights, for larger heights, falling on water basically means death because of the reasons stated above.

It's not just water, though. Any practically-incompressible liquid/solid would act the same. For example, if you (for some reason) decide to land on concrete, it would equal the same amount of devastation as water.

So yeah. Remember all this the next time you're falling out of a plane and your parachute fails.

(If you still have any doubts, feel free to ask!)

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  • $\begingroup$ Oh thanks I never knew about this misconception $\endgroup$
    – imposter
    Oct 15 '20 at 15:57
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According to this answer at Aviation Stack Exchange, your injuries will be similar if you land on solid ground or in water, but they key difference is that when you are badly injured or unconscious you will drown in water. Best advice seems to be to aim for trees, bushes, or a convenient haystack.

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Bare land is always going to hit you harder than water; if you've ever jumped into a swimming pool that was shallower than you thought it was, you'll know what I mean. But the video does say to aim for swamp, snow or trees, so that is going to narrow the difference, and I can imagine a landing in a tree might break your fall better than water.

The force you experience (which is what you are interested in) is inversely proportional to the distance over which you decellerate from whatever your fall speed is to zero. For a tall tree, that could be more than the time it takes to slow down in water. There are some articles online about cliff jumpers and how much water they require which might help you.

But I suspect the reason for aiming for land isn't to do with physics. If by amazining chance you surive the fall, you are going to be very badly injured, stunned, possibly unconscious and in shock whether you hit land or water. If you land in the water, you'll drown before anyone can get to you anyway, so better to hit the land and have a chance of being rescued and taken to hospital.

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The have the same impulse. The momentum of the falling object is always reduced to the same value (zero). If you are asking about force, then the object is crushed more on a solid as its momentum is going away more drastically, and the particles still drive into the ground.

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  • $\begingroup$ Your answer contradicts the statement in the video $\endgroup$
    – imposter
    Oct 15 '20 at 15:55

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