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-   -   If humans ran fast enough, could we run across water? (https://boards.straightdope.com/sdmb/showthread.php?t=802214)

boffking 08-21-2016 07:45 PM

If humans ran fast enough, could we run across water?
 
There are small animals that can run across the surface of liquid water. I have heard people say that if we ran at some insanely high speed(I've heard both 70 and 200 mph) , we could do it too. Is there s hypothetical speed where this would happen? No, it doesn't count if the water is frozen.:p

Zach29 08-21-2016 08:07 PM

Yes, theoretically, if humans were capable of running fast enough, they could run across the surface of water.

There are videos you can find showing people running across the surface of a pool filled with water and cornstarch. If they simply try to walk across, they sink and it behaves as a liquid.

Humans have a much higher weight than small animals, and it has to do with the ratio of surface area touching the water to the amount of weight pushing down.

engineer_comp_geek 08-21-2016 08:09 PM

Quote:

Originally Posted by Popular Mechanics
Jamaican runner Usain Bolt, the current world record holder for the 100-meter sprint, ran 10.4 meters per second. But J.W. Glasheen and T.A. McMahon, two Harvard biologists who studied how the basilisk runs on water, found that in order to mimic the lizard, a human would need to run at almost 30 meters per second, "a velocity beyond human ability." A man would also need "an average power output almost 15 times greater than the maximum sustained power output for humans."

More info in the complete article here:
http://www.popularmechanics.com/cult...sics-debunked/

Zach29 08-21-2016 08:17 PM

Some Italian researchers calculated that a 145 pound person would have to run 67 miles per hour to be able to run across water—that's 15 times the force human legs are capable of producing.

(long boring complicated article here: http://journals.plos.org/plosone/art...7300-Glasheen1 )

In this video, skilled athletes with special shoes are actually able to run across the surface of water over a short distance (kind of).

https://www.youtube.com/watch?v=Oe3St1GgoHQ

Zach29 08-21-2016 08:27 PM

edit: that video was later admitted to be a hoax

Rhodes 08-22-2016 04:11 PM

Quote:

Originally Posted by Zach29 (Post 19571670)
edit: that video was later admitted to be a hoax

It was also the subject of a Mythbusters episode. They concluded that there was a solid platform just under the surface of the water.

Velocity 08-22-2016 08:58 PM

If I'm not mistaken, humans are simply too heavy, and their feet surface area too small. The basilisk lizard is light and has big feet.

Martian Bigfoot 08-22-2016 09:07 PM

Quote:

Originally Posted by Zach29 (Post 19571629)
There are videos you can find showing people running across the surface of a pool filled with water and cornstarch. If they simply try to walk across, they sink and it behaves as a liquid.

This has nothing to do with the water and everything to do with the cornstarch.

Non-Newtonian fluid.


This is a fluid that changes its viscosity when the forces on it change. Smack it, and it gets hard.

....

Sorry, that sounded dirty.

Mangetout 08-23-2016 01:50 AM

Quote:

Originally Posted by Velocity (Post 19574651)
If I'm not mistaken, humans are simply too heavy, and their feet surface area too small. The basilisk lizard is light and has big feet.

I think that only means that humans can't do it at the same speed or energy expenditure as the basilisk lizard - the only question is whether ramping up the speed (i.e. a theoretical superhuman) would mean that some other limiting factor comes into play.

watchwolf49 08-24-2016 10:56 AM

I don't know, a lizard weights a few ounces, them big paddle feet need only generate a few ounces of lift. Humans with stubs on the end of their legs need to produce well over a hundred pounds of lift. There would be a little lift, very little.

So, let's look at the OP from the other direction. If we had no lift from the footfalls, then a human would have to be traveling at around 17,000 mph to run across the water. However, with a little lift then the human could travel a little slower, say 16,975 mph. That's as close to a ballistic trajectory as to make no difference.

Mangetout 08-25-2016 02:58 AM

Quote:

Originally Posted by watchwolf49 (Post 19578298)
I don't know, a lizard weights a few ounces, them big paddle feet need only generate a few ounces of lift. Humans with stubs on the end of their legs need to produce well over a hundred pounds of lift. There would be a little lift, very little.

So, let's look at the OP from the other direction. If we had no lift from the footfalls, then a human would have to be traveling at around 17,000 mph to run across the water. However, with a little lift then the human could travel a little slower, say 16,975 mph. That's as close to a ballistic trajectory as to make no difference.

Where are you getting those numbers from?

It's possible to waterski barefoot at survivable velocities. I realise waterskiing is a different thing from running on water, but waterskiing *is* about lift generated from the interaction of human feet and the surface of the water - and is possible at speeds that are several orders of magnitude smaller than you're talking about.

bucketybuck 08-25-2016 03:46 AM

If you have to be moving really fast to travel across the top of the water are you actually running on the water? Or are you just being propelled by your previous momentum?

Channing Idaho Banks 08-25-2016 04:15 AM

Quote:

Originally Posted by bucketybuck (Post 19580303)
If you have to be moving really fast to travel across the top of the water are you actually running on the water? Or are you just being propelled by your previous momentum?

You've seen the lizard run on the water, right? It really does run on the water.

LSLGuy 08-25-2016 09:13 AM

Fairly recent thread on topic: http://boards.straightdope.com/sdmb/...d.php?t=784722

watchwolf49 08-25-2016 09:47 AM

Quote:

Originally Posted by Mangetout (Post 19580274)
Where are you getting those numbers from?

That's the orbital speed ... a tongue-in-cheek joke meaning humans can't run on water pre se.

Quote:

Originally Posted by Mangetout (Post 19580274)
It's possible to waterski barefoot at survivable velocities. I realise waterskiing is a different thing from running on water, but waterskiing *is* about lift generated from the interaction of human feet and the surface of the water - and is possible at speeds that are several orders of magnitude smaller than you're talking about.

We're using the foot as a hydrofoil to generate the lift ... no doubt ... but that's not "footfalls" as I had specified. This is all original research I did at the lake yesterday afternoon, at no time or under any circumstance was I able to put my foot down on water and then lift it back out without touching bottom or totally crashing into the water.

Then them brat kids started throwing big sticks out into the lake and experiment quickly ended ... [sigh]

naita 08-25-2016 09:52 AM

Quote:

Originally Posted by watchwolf49 (Post 19580662)
That's the orbital speed ... a tongue-in-cheek joke meaning humans can't run on water pre se.

It becomes a confusing joke when others have already presented calculations for an actual (if physiologically impossible) speed at which something with the outer dimensions of a human body would run on water. Which was what the OP asked about.

Omar Little 08-25-2016 03:11 PM

Jesus and Peter walked on water, well the latter only till he got scared.

Urbanredneck 08-25-2016 11:03 PM

Are we talking flat, quiet water or active water like in the oceans with the big waves?

With the waves one would need to step on each one which would be a trip hazard.

watchwolf49 08-26-2016 01:36 AM

Quote:

Originally Posted by naita (Post 19580674)
It becomes a confusing joke when others have already presented calculations for an actual (if physiologically impossible) speed at which something with the outer dimensions of a human body would run on water. Which was what the OP asked about.

The only calculations I see are for conditions of 20% gravity ... is that what the OP asked?

naita 08-26-2016 04:09 AM

Quote:

Originally Posted by watchwolf49 (Post 19582973)
The only calculations I see are for conditions of 20% gravity ... is that what the OP asked?

Fine, the actual calculations aren't available in the two links and Zach29 fails to notice the Italians are only referencing other work and are answering a totally different question. That totally invalidates post 3 and 4 quoting the 30 m/s estimate of Glasheen and McMahon.

Chronos 08-26-2016 10:50 AM

So, watchwolf, what you found is that if the human has almost no lift, then its trajectory will be almost the same as if it had no lift? I fail to see how that's an interesting result at all, and it's certainly not a relevant result to the question at hand, which concerns a human with a large amount of lift.

watchwolf49 08-26-2016 11:26 AM

Quote:

Originally Posted by boffking (Post 19571579)
If humans ran fast enough, could we run across water? ... [snip]

Quote:

Originally Posted by Chronos (Post 19583551)
So, watchwolf, what you found is that if the human has almost no lift, then its trajectory will be almost the same as if it had no lift? I fail to see how that's an interesting result at all, and it's certainly not a relevant result to the question at hand, which concerns a human with a large amount of lift.

Right, the Factual Answer to the OP's General Question is no, humans cannot run across water. Zack29's citation states this, it makes common sense and I've experimented up at the lake the other day.

Quote:

Notwithstanding various internet hoaxes, humans are apparently incapable of walking or running on water. In their classic study (Glasheen, McMahon; 1996) of the Basilisk lizard, Glasheen and McMahon calculate the unsurprising result that humans are far too big and weak to splash their feet hard enough to hold their weight. According to their estimates (ibid.), humans would be able to run on water only if they were able to slap water at speeds >30 m/s, which they estimate would require about 15 times a human’s available muscle power.
Humans do not have enough lift, there's no "if" about it. Bucketybuck presents the better counter-argument; it's not running, it's momentum; certainly better than humans are actually 15 times stronger than they really are.

naita 08-26-2016 12:16 PM

Quote:

Originally Posted by watchwolf49 (Post 19583656)
Right, the Factual Answer to the OP's General Question is no, humans cannot run across water.

No, that is only the factual answer to a limited reading of the OP's subject line. The full OP however includes the salient lines: "I have heard people say that if we ran at some insanely high speed(I've heard both 70 and 200 mph) , we could do it too. Is there s hypothetical speed where this would happen?"

The answer to that is "Some scientists working on a model for how Basilisk lizards run on water have estimated that humans would have to run at 67 mph to run on water, so the 70 mph figure isn't far off."

If we want to be super cautious we may include "But that is of course impossible", but that caveat doesn't seem to be necessary for the OP based on his full question for a hypothetical speed.

Cayuga 08-26-2016 02:40 PM

The Flash does it all the time.

If it's in a comic book, it must be true, right?

Chronos 08-26-2016 03:18 PM

watchwolf, the question was how fast we would need to be. All your answer amounts to is that the necessary speed is greater than what we're capable of. But everyone in the thread already knew that. We'd like a more detailed answer, like those already given by more knowledgeable posters before your answer.

Mangetout 08-26-2016 07:41 PM

Yup, the question in the OP acknowledges that it would be a superhuman feat. The question is not 'can ordinary humans do it', but 'could they do it if they were able to run fast enough, and if so, how fast is that?'

chorpler 08-26-2016 07:52 PM

Quote:

Originally Posted by Urbanredneck (Post 19582741)
Are we talking flat, quiet water or active water like in the oceans with the big waves?

With the waves one would need to step on each one which would be a trip hazard.

I don't know why, but the idea of waves as a "trip hazard" cracks me up. Thanks, Urbanredneck.

watchwolf49 08-27-2016 09:41 AM

Quote:

Originally Posted by Mangetout (Post 19584954)
Yup, the question in the OP acknowledges that it would be a superhuman feat. The question is not 'can ordinary humans do it', but 'could they do it if they were able to run fast enough, and if so, how fast is that?'

What does horizontal speed have to do with vertical lift?

The citation in post #4 had the humans wear flippers scaled to be proportional to the lizard's feet and then they suspended the humans from the ceiling to simulate 20% gravity over a pool of water. There was no lateral speed involved, the humans created enough lift by running in place, 0 mph.

I haven't read the reference concerning the lizard, but I think it fair to assume the lizard started out by running, to chase down prey and escape predators. It then adapted to running on water to better chase down prey and escape predators by evolving flipper feet and perhaps a highly specialized muscle configuration.

It's about the rate of slapping the water, which isn't dependent on forward speed for humans.

Mangetout 08-27-2016 12:06 PM

It's a good point that forward speed isn't the same as vertical lift, but I think it's implied that a super-human able to run at 70mph would be doing so precisely because they were able to exert more force using their legs.
More force imparted on the water provides more reaction.

There's another thought experiment that I think works here:
When I was younger and fitter, I could tread water and exert enough force doing so that I could raise my upper body above the water surface, for a short (but sustained) time - to a level where the water line was somewhere below my nipples, but above my navel.
If I was endowed with greater strength, I would have been able to rise and remain further out of the water.
If I had superhuman strength, I would have been able to go further, and so on - like a dolphin tail-swimming. It is a game of diminishing returns, as the more of your body is out of the water, the less is able to act upon the water.

Now, I know humans are not dolphins - and the area of a dolphin's tail fluke is greater than the surface area of human feet, but these are all just scale factors - if you have smaller area, you need more force (or if you have more force available, you need less area).

MacLir 08-29-2016 10:06 AM

Quote:

Originally Posted by Omar Little (Post 19581642)
Jesus and Peter walked on water, well the latter only till he got scared.

I've heard a (humorous?) theory that this is how he came to be called Peter (Well Cephas, actually - means the same thing). His name was Simon, and Peter was a nickname.

Peter / Cephas means "Rock" :smack:

Galeiam 08-29-2016 03:18 PM

The Basilisk lizard isn't running on water.

It's just swimming really damn fast while using a method and calculations that wont mean a thing for a humans physiology.

The Key to actual running on water is steps per minute, where steps replace rotations.

Roll anything fast enough and it'll travel across water fairly easily. The problem then becomes a matter of consistent force against water tension, motion, and oscillation.

Mangetout 08-29-2016 04:46 PM

Quote:

Originally Posted by Galeiam (Post 19590752)
The Basilisk lizard isn't running on water.

It's just swimming really damn fast while using a method and calculations that wont mean a thing for a humans physiology.

I get what you're saying, but functionally, there isn't any difference between 'running on water' and 'swimming fast enough that only your feet touch the water'.

Quote:

Roll anything fast enough and it'll travel across water fairly easily. The problem then becomes a matter of consistent force against water tension, motion, and oscillation.
Nitpick: surface tension is a tiny, insignificant factor in any of this. The mass of water makes it something that forces can act upon. Viscosity also plays a part.

Galeiam 08-29-2016 09:08 PM

Quote:

Originally Posted by Mangetout (Post 19591034)
I get what you're saying, but functionally, there isn't any difference between 'running on water' and 'swimming fast enough that only your feet touch the water'.

Eh, best analogy is one used earlier.

One is Dolphin tail skimming, the other is The Flash.

Which actually ties into this bit.
Quote:

Originally Posted by Mangetout (Post 19591034)
Nitpick: surface tension is a tiny, insignificant factor in any of this. The mass of water makes it something that forces can act upon. Viscosity also plays a part.

I actually had written out a larger segment to explain why I added surface tension to the reasons buy deleted in favor of simplification.

In this instance viscosity would be attributed by surface tension and friction. As the high speed staccato would in effect create platforms in the water on which to move on.

We don't really need to explain why it would work for the OP to be satisfied, just how fast they'd need to be striking the water in order to achieve that result.

A calculation which I in no way, shape, nor form can start on.

Galeiam 08-29-2016 09:14 PM

Note that the reason running speed would not necessarily matter is because it is more a matter of contact amount due to surface tension than anything.

A person could be running 80mph and still sink if it took them too long to transition their steps.

countrarian 09-06-2016 10:18 AM

Quote:

Originally Posted by engineer_comp_geek (Post 19571634)
More info in the complete article here:
http://www.popularmechanics.com/cult...sics-debunked/

The popular science article includes the following quote:
"It means each shoe should be a mile long and a mile wide."

That seems a rather dubious assertion. Even shoes have some displacement so a much lesser size would be needed. Any idea what they were trying to convey?


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