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motch

Would You Survive This?

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Right, weird one this, but has been bugging me for a while for some strange reason....

If you was 200metres up in the air, floating in the middle on top of the surface of a huge suspended cube of water, say 50m3, and then the base was rapidly pulled away to one side....

would you survive the impact ? (onto a football field say, rather than solid concrete)

edited: I mean 50m by 50m by 50m high block of water.

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Hell no. 50m^3 is only about 3.68m deep. Even if this cube is on the ground, it would not be deep enough to stop you hitting the bottom from 10m, never mind 200m. Also, falling water would quickly expand and lose density (think waterfall), so you'd be a red smear on the ground, regardless of a grass or concrete surface.

I bet most suicide jumpers go from bridges <<200m

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Hell no. 50m^3 is only about 3.68m deep. Even if this cube is on the ground, it would not be deep enough to stop you hitting the bottom from 10m, never mind 200m. Also, falling water would quickly expand and lose density (think waterfall), so you'd be a red smear on the ground, regardless of a grass or concrete surface.

I bet most suicide jumpers go from bridges <<200m

Depends if the OP intended '50m3' to mean '50m3' or '(50m)3' ;)

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Right, weird one this, but has been bugging me for a while for some strange reason....

If you was 200metres up in the air, floating in the middle on top of the surface of a huge suspended cube of water, say 50m3, and then the base was rapidly pulled away to one side....

would you survive the impact ? (onto a football field say, rather than solid concrete)

I reckon a full answer to this question invovles fluid dynamics, which is beyond me, so I will make assumptions to make the maths easier.

Assuming the 50m cube of water remains intact as it falls, and neglecting air resistance:

The cube (and the swimmer) would be travelling at about 55 metres per second (122 mph) when the bottom of the cube hits the ground. I suspect that when the bottom of the cube touches the ground a pressure wave would propagate rapidly from the bottom of the cube upwards. As people are mostly water, the pressure wave would probably propagate through the swimmer's body in the same way it did through the water. I guess it would collapse the swimmer's lungs, and wouldn't be survivable. I don't think the type of ground that the cube lands on would make much difference.

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I reckon a full answer to this question invovles fluid dynamics, which is beyond me, so I will make assumptions to make the maths easier.

Assuming the 50m cube of water remains intact as it falls, and neglecting air resistance:

The cube (and the swimmer) would be travelling at about 55 metres per second (122 mph) when the bottom of the cube hits the ground. I suspect that when the bottom of the cube touches the ground a pressure wave would propagate rapidly from the bottom of the cube upwards. As people are mostly water, the pressure wave would probably propagate through the swimmer's body in the same way it did through the water. I guess it would collapse the swimmer's lungs, and wouldn't be survivable. I don't think the type of ground that the cube lands on would make much difference.

Interesting description.

I want to know if the water would fall slower than a collapsing WTC building falls. Apparently there is some difference of opinion about that too. ;)

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would you survive the impact ? (onto a football field say, rather than solid concrete)

What happens all depends upon the detail.

Astroturf, or real grass? Long, or recently mown?

I hope you don't think my questions are strange or pointless. B)

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Right, weird one this, but has been bugging me for a while for some strange reason....

If you was 200metres up in the air, floating in the middle on top of the surface of a huge suspended cube of water, say 50m3, and then the base was rapidly pulled away to one side....

would you survive the impact ? (onto a football field say, rather than solid concrete)

Damn dude. This is complicated enough for it to be the subject of a PhD dissertation!

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Right, weird one this, but has been bugging me for a while for some strange reason....

If you was 200metres up in the air, floating in the middle on top of the surface of a huge suspended cube of water, say 50m3, and then the base was rapidly pulled away to one side....

would you survive the impact ? (onto a football field say, rather than solid concrete)

No

edit : and may it bug you no more....

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Damn dude. This is complicated enough for it to be the subject of a PhD dissertation!

as long as i'm duly noted as raising the original question on the finished paper i'm happy! :D

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I reckon a full answer to this question invovles fluid dynamics, which is beyond me, so I will make assumptions to make the maths easier.

^ This. Fluid dynamics is horrendously complex.

Thought experiments are fun, and this is a good mental exercise.

Einstein came up with, and explained, his Theories of Relativity to a great extent through thought experiments.

I think the original question might be a bit more manageable if we are allowed that the floor supporting the cube of water disappears instantly.

There would be some minor effects of air resistance, surface tension, and interaction between the sdes of the cube and the water, but these would all be negligible.

At the time just before the floor disappears, the water at the bottom of the slug * would be under high pressure. Liquids are essentially incompressible. The moment the floor disappears, the entire body of water would surely all fall and accelerate at the same rate. The pressure of the water throughout would surely fall to the ambient (air) pressure. i have no idea what the implications of this are.

When the slug of water hits the ground, the lowest part would only be able to move sideways. It can move downwards no further, and neither can it occupy the space already occupied by the water above it. Each water particle must therefore then move sideways. Thus each particle of water would be subject to increasing horizontal forces.

The upper water would still be under the accelerating force of gravity. However, the potential energy of each particle (due to it having been high in the earths gravitational field) would still be translating into kinetic energy in the vertical direction (acceleration due to gravity.)

Now the water particles might decelerate instantly, when, and only when, they each in turn hit the ground. However, my gut feeling is that the upper part of the water decelerates gradually due to some sort of interaction with the water below it, resulting in all of the vertical kinetic energy being translated progressively into horizontal movement. I haven't the faintest idea how the details would work, though.

Common sense tells us that finally, all of the water must be moving entirely sideways (i.e. spreading out to become a puddle of negligible depth.)

So, the OP, in the middle of the top of the water, might experience gradual, cushioning deceleration, and thus survive.

* the one thing I do know about fluid dynamics: a finite volume of fluid under consideration in a fluid system is called a slug. :D

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Guest eight

Right, weird one this, but has been bugging me for a while for some strange reason....

If you was 200metres up in the air, floating in the middle on top of the surface of a huge suspended cube of water, say 50m3, and then the base was rapidly pulled away to one side....

would you survive the impact ? (onto a football field say, rather than solid concrete)

edited: I mean 50m by 50m by 50m high block of water.

I have occasionally wondered if you could be lifted up and down on a geyser of water - like when cartoons get ejected through the blowhole of a whale, that sort of thing.

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I have occasionally wondered if you could be lifted up and down on a geyser of water - like when cartoons get ejected through the blowhole of a whale, that sort of thing.

I don't see why not although balancing on top of it might be rather harder. You're probably more likely to get blown upwards and sideways but moving water could easily exert enough force. Whales' blowholes though, probably not, and geysers are probably uncomfortably hot.

As for the OP's question working it out is way beyond me but my suspicion is a splat on the ground.

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I have occasionally wondered if you could be lifted up and down on a geyser of water - like when cartoons get ejected through the blowhole of a whale, that sort of thing.

Yes, theoretically. But since people aren't uniform or aerodynamically shaped, and geysers aren't uniformly straight or constant in flow, then you would quickly be dumped out to the side. But if you have a small hollow plastic ball and a bendy straw, you can keep that ball levitating in the airstream for as long as you choose to... I presume a similar effect would happen in a jet of water?!

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Yes, theoretically. But since people aren't uniform or aerodynamically shaped, and geysers aren't uniformly straight or constant in flow, then you would quickly be dumped out to the side. But if you have a small hollow plastic ball and a bendy straw, you can keep that ball levitating in the airstream for as long as you choose to... I presume a similar effect would happen in a jet of water?!

That's the kind of thing I was thinking of, maybe if you had lots of individually controlled jets and could create a "hollow" at the top? Do you think my ten year old PC will be up to running CFD? :o

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That's the kind of thing I was thinking of, maybe if you had lots of individually controlled jets and could create a "hollow" at the top? Do you think my ten year old PC will be up to running CFD? :o

Believe it or not, it may well be.

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Yes, theoretically. But since people aren't uniform or aerodynamically shaped, and geysers aren't uniformly straight or constant in flow, then you would quickly be dumped out to the side. But if you have a small hollow plastic ball and a bendy straw, you can keep that ball levitating in the airstream for as long as you choose to... I presume a similar effect would happen in a jet of water?!

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I reckon a full answer to this question invovles fluid dynamics, which is beyond me, so I will make assumptions to make the maths easier.

Assuming the 50m cube of water remains intact as it falls, and neglecting air resistance:

The cube (and the swimmer) would be travelling at about 55ms-1 (122 mph) when the bottom of the cube hits the ground.

If during the deceleration of the cube the ground deflects 1m, and the water does not compress at all, that gives a deceleration of the water at the bottom of the cube of 1624.5 ms-2.

Using F=ma and the fact that each square metre of the bottom of the cube must support a 50m column of water, I think the pressure at the bottom of the cube during deceleration will be about 81MPa. That's about 800 times atmospheric pressure, so would bad for any air-filled organs inside the swimmer, if the swimmer was near the bottom of the cube.

Having the swimmer at the top of the cube makes the calculation more complicated. Assuming the pressure wave travels at the speed of sound in water (about 1497ms-1) it would take about 30milliseconds to reach the swimmer. During this time, the sides of the cube would travel outwards a bit. I don't know how to calculate whether this would reduce the pressure to safe levels - the calculation might require water to be treated as compressible.

In any case, I think the swimmer would remain in the top layer of water when the cube hits the ground, because the swimmer is falling at the same speed as the water, and is about the same density. I don't think the swimmer would sink into the water like a diver does when diving from air into water.

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I don't see why not although balancing on top of it might be rather harder. You're probably more likely to get blown upwards and sideways but moving water could easily exert enough force. Whales' blowholes though, probably not, and geysers are probably uncomfortably hot.

Why not? You wouldn't weigh any more than the water being ejected by the whale.

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I think there might be something in this.

Lets take the 200m in the air out of this for the moment, a fall from 50m is easily high enough to kill.

If you were on top of a column of water 1m x 1m x 50m high and the sides were suddenly removed then the water would make no difference at all.

If that column was instead 1,000m x 1,000m x 50m then the water would not disapear instantly and would presumably provide enough resistance to slow you down and make the fall survivable.

There must therefore be a critical volume of water between the 2 above that would make the fall just survivable.

Bringing the 200m back in I can also imagine that there might be a volume of water that would disapate the energy at a rate that would also make that fall survivable.

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Another thought on this.

If you were on top of a column of water 1 x 1 x 50 and it were dropped from 200m (i.e. you start at a height 250m) then you'd die when you hit the ground or height 0m.

If the column were 1,000 x 1,000 x 50 then you'd die when the water hit the ground, you'd still be at the top of the column of water and would die at a height of 50m.

In the first case the column is too small to provide any resistance and you die at the bottom of the column.

In the second case the column provides too much resistance and you die at the top of the column.

There must be a zone between the two extremes that provides just enough resistance to decelerate you safely.

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My thinking to this is along the lines that high divers have jumped from incredible heights (probably not near maximum velocity though) and if this block of water did hit the ground as a big mass you'd get a fair bit of deceleration before hitting the floor.

Perhaps if the swimmer was bolt upright near the surface with hands in the air to reduce some of the huge forces when the block (assuming it's intact) hits the floor ?

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Why not? You wouldn't weigh any more than the water being ejected by the whale.

Is there much water blown out by a whale? It's exhaling so will be mostly air I would've thought. And it's not really about the total weight of water but the speed and power of the jet and exactly where it hits.

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  • 238 Brexit, House prices and Summer 2020

    1. 1. Including the effects Brexit, where do you think average UK house prices will be relative to now in June 2020?


      • down 5% +
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