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#1
02-21-2016, 12:28 PM
 HoneyBadgerDC Member Join Date: Jul 2012 Location: Torrance Ca Posts: 7,210
How fast to run on water?

Intuitive vs real answer. Say a man had a size 10 d foot and weighed about 160#. My intuitive answer would be about 80 mph. How would you figure this? A close educated estimate would be great.
#2
02-21-2016, 01:01 PM
 watchwolf49 Guest Join Date: Dec 2009 Location: State of Jefferson Posts: 7,617
I would think at 80 mph one could "skip" across the water a little ways, but the OP says "run" ... I'd guess exactly one stride.
#3
02-21-2016, 01:03 PM
 HoneyBadgerDC Member Join Date: Jul 2012 Location: Torrance Ca Posts: 7,210
Quote:
 Originally Posted by watchwolf49 I would think at 80 mph one could "skip" across the water a little ways, but the OP says "run" ... I'd guess exactly one stride.
Assume you had reached a speed of 80 mph or whatever speed was needed before you entered the water.

I am also curious as to what speed others would guess intuitively with no figuring.

Last edited by HoneyBadgerDC; 02-21-2016 at 01:04 PM.
#4
02-21-2016, 01:12 PM
 harmonicamoon Guest Join Date: Sep 2012 Location: Yucatan, Mexico Posts: 2,459
I remember water skiing as a kid. At about 45 MPH you could kick off the skis and ski barefooted. Slower, and you went down.

I realize that there are other mechanics involved. And I weighed about 75% less.
#5
02-21-2016, 01:17 PM
 HoneyBadgerDC Member Join Date: Jul 2012 Location: Torrance Ca Posts: 7,210
Quote:
 Originally Posted by harmonicamoon I remember water skiing as a kid. At about 45 MPH you could kick off the skis and ski barefooted. Slower, and you went down. I realize that there are other mechanics involved. And I weighed about 75% less.
I though about that when I posted this. I was just watching a vidio of a lizard running across water. He had big feet!
#6
02-21-2016, 01:26 PM
 GusNSpot Member Join Date: Mar 1999 Location: N/W Arkansas Posts: 8,143
Lots of pictures of motorcycle & snowmobiles doing it online here & there.
#7
02-21-2016, 02:10 PM
 LSLGuy Charter Member Join Date: Sep 2003 Location: Southeast Florida USA Posts: 20,749
This https://en.wikipedia.org/wiki/Barefoot_skiing tells us you can barefoot ski at 30-45 mph. But only with two feet constantly in the water.

Ballpark you could barefoot ski on one foot at double that speed. So 60-90 mph depending on weight versus foot size. A bit slower for skinny teen guys with size 12s and much faster for 400lb former ballerinas with an ice cream addiction.

But that's assuming something else is providing the motion and we can use all the foot's reaction with the water to hold us up. Running's not like that at all.

This https://en.wikipedia.org/wiki/Running has a pretty good description of the kinematics of human running. But sadly no numbers. The rest of my Googling was equally number-free.

The key points are that when running, a hefty portion of the total time is spent with both feet in the air. In the case of running on water that means we're going to need more speed to increase the lift we get while a foot is in/on the water such that the average lift over time is >= body weight.

Lacking better numbers, if we arbitrarily assume a fast runner spends 1/3rd of the time with both feet in the air then we're gonna need about 1/3rd more speed to offset that. So now we're up around 80 to 120 mph. Plus the increased impact forces when his foot reenters the water. A 150 lb person "weighs" 300 lbs when his foot hits the pavement running. It'll be the same when the foot hits the water & needs to support him. Now we're up around 160-250mph.

In addition to the forces needed to support the runner, the water also has to provide the counterforce to push the runner forward. IOW, he's going to push backwards with his feet which in turn propels him forward.

Again for lack of numbers I'm going to assume 1/2 again additional force is required to propel the runner forward. Remember we're talking about pushing against 160-250 mph of wind drag at this point.

So now he we've accounted for most of the forces and we see a running speed of 240-375 mph would be required.

That's also assuming the standard human gait with about a 6 foot running stride. it can't get longer because our legs aren't getting any longer. The only way to get a longer stride is to jump harder & fly more between footfalls. Both of which increase the speed required even more. So 6 foot stride it is.

To make 375mph our runner needs to accomplish 105 strides per second. Good luck.

Actually, it's worse than all that. Water skiing lift is roughly proportional to the foot speed versus the water speed. When running on land, your foot stops completely each time it touches the ground. If your foot stops on the water you sink just as fast as you would trying to stand on the water. We all know how well that works. And this is true regardless of how fast your body is going.

The way you thrust yourself in water is to accelerate the water backwards to as close to the speed of your foot as possible. IOW, the more effectively you make forward thrust, the less effectively you make lift.

Fun to think about, but bottom line: not gonna work. Not even a little bit.

One of my neighbors is a barefootin' fool. Despite being 70-something. I'll ask him about one foot barefooting behind a boat. If it's possible, he's done it. If it's impossible, he's tried it. Quite the character.

Last edited by LSLGuy; 02-21-2016 at 02:13 PM.
#8
02-21-2016, 02:40 PM
 HoneyBadgerDC Member Join Date: Jul 2012 Location: Torrance Ca Posts: 7,210
[.

Fun to think about, but bottom line: not gonna work. Not even a little bit.

The physics are interesting. I hit the invisible like button!
#9
02-21-2016, 02:41 PM
 glee Guest Join Date: Aug 1999 Location: Obama country Posts: 14,965
You can walk on water ... provided you add a lot of custard powder first!

(the walk starts at about 2:45...)
#10
02-21-2016, 02:43 PM
 Wesley Clark Guest Join Date: Aug 2003 Posts: 18,274

http://sploid.gizmodo.com/how-fast-d...ate-1599455890
#11
02-21-2016, 05:51 PM
 AskNott Charter Member Join Date: Mar 1999 Location: Anderson, IN,USA Posts: 14,584
I guess we'll have to wait until a human can run that fast.
#12
02-21-2016, 05:59 PM
 Morgenstern Guest Join Date: Jun 2007 Location: Southern California Posts: 9,836
Pro tip: Use frozen water.
#13
02-21-2016, 07:14 PM
 LSLGuy Charter Member Join Date: Sep 2003 Location: Southeast Florida USA Posts: 20,749
It also works good in very shallow water, say 1/4" or so.
#14
02-21-2016, 08:24 PM
 PoppaSan Guest Join Date: Mar 2011 Location: Up nort' Posts: 1,656
Have your dad be the creator of all that is, or at least be the creator's kid's bestest buddy and you can just walk.
#15
02-21-2016, 08:39 PM
 Urbanredneck Guest Join Date: Mar 2014 Posts: 5,680
In comic books the Flash could run on water. Even Wally West who could only run the speed of sound.

I would add into the problem that rarely is water "flat". There are waves and very big ones in the ocean so no way could anyone run across those.
#16
02-21-2016, 11:36 PM
 Leo Bloom Member Join Date: Jun 2009 Location: Here Posts: 11,584
OP, tell me about your intuition. I cannot fathom (heh, not intended) how you came up with 80 MPH.
#17
02-21-2016, 11:57 PM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 73,152
I think that the difficulty here is that it depends on how you run. And you can't just say "run the same way that a human does", because the way that a human runs is "much slower, and not at all on liquid".
#18
02-22-2016, 03:54 AM
 Ranger Jeff Guest Join Date: Apr 2013 Posts: 6,608
Mythbusters tried it with college track and field athletes. It didn't work. This was fairly fresh still water. No corn starch or whatever it is you can add to water to make it, um, that weird stuff.
#19
02-22-2016, 04:05 AM
 Mijin Guest Join Date: Feb 2006 Location: Shanghai Posts: 7,815
Quote:
 Originally Posted by Ranger Jeff Mythbusters tried it with college track and field athletes. It didn't work. This was fairly fresh still water. No corn starch or whatever it is you can add to water to make it, um, that weird stuff.
What?!
Did they think it was going to work and just no-one had tried it before (as well as everyone's understanding of basic physics being flawed)?

Last edited by Mijin; 02-22-2016 at 04:05 AM.
#20
02-22-2016, 07:40 AM
 Telemark Charter Member Join Date: Apr 2000 Location: Again, Titletown Posts: 20,105
Quote:
 Originally Posted by Mijin What?! Did they think it was going to work and just no-one had tried it before (as well as everyone's understanding of basic physics being flawed)?
No, they often do myths that clearly have no chance of working, then they try to push it to the illogical extreme.

Then they blow shit up.

On "Outrageous Acts of Science" they went over a hoax video of guys running on water (it was a platform just under the surface, of course). During the discussion they mention that you'd have to run 4x as fast as Usain Bolt to generate enough force to stay on top of water.
#21
02-22-2016, 02:44 PM
 Really Not All That Bright Guest Join Date: May 2003 Location: Florida Posts: 67,149
Quote:
 Originally Posted by AskNott I guess we'll have to wait until a human can run that fast.
The Messiah is coming... quickly.
Quote:
 Originally Posted by Ranger Jeff Mythbusters tried it with college track and field athletes. It didn't work. This was fairly fresh still water. No corn starch or whatever it is you can add to water to make it, um, that weird stuff.
A non-Newtonian fluid. Though I gather the name normally given to cornstarch and water suspension specifically is oobleck.
#22
02-23-2016, 07:00 AM
 Meurglys Charter Member Join Date: Jul 2003 Location: Edinburgh Posts: 2,234
Quote:
 Originally Posted by LSLGuy It also works good in very shallow water, say 1/4" or so.
Or, to quote Bootsy Collins, 'The secret to walkinŽ on water is knowing where the rocks are!' (around 35 seconds in)
#23
02-23-2016, 06:55 PM
 Greg Charles Charter Member Join Date: Apr 1999 Location: San Diego Posts: 3,916
Shaolin monk running on water. I'm torn whether this is impressive or ridiculous, but I'm leaning towards impressive.
#24
02-23-2016, 09:08 PM
 HoneyBadgerDC Member Join Date: Jul 2012 Location: Torrance Ca Posts: 7,210
Quote:
 Originally Posted by Greg Charles Shaolin monk running on water. I'm torn whether this is impressive or ridiculous, but I'm leaning towards impressive.
Not hard to believe, just like having large webbed feet. A faster runner would have done even better.

I am surprised we got no educated guesses as to the speed needed to maintain running on top of water. It would seem the flatter and faster your foot hit the water the longer the shock wave would be involving more water mass.

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