How fast must a person be going to run on water? Is this possible? I know it is possible to skid across water barefoot if you are being pulled by a fast boat.
Does gravity play a roll in this? Would someone be able to run on water more easily in lower gravity since they would sink slower?
You would not be able to run on water because when your feet touch the water they are stationary (no matter how fast you are running). For something to stay on top of the water it needs to be sliding across it.
The problem with that reasoning is that there are numerous creatures that do run across water, so quite clearly it is possible.
I suspect that what you are overlooking is inertia. Although the foot is momentarily stationary no matter how fast an animal runs the same is true of the water. For an infinitely short period of time the water won’t move. As the weight of the foot provides more and more energy the water will move faster and faster.
The trick to running on water is to lift the foot up before its weight can displace so much water that the leg gets stuck. That becomes easier the lighter an animal is and it also becomes easier the larger the animal’s footprint is. But you sure don’t need to be aquaplaning to stay on top of the water.
I can’t actually answer the OP, I’m thinking it would require some pretty complex calculations to take into account the viscosity of the water, the surface tension, how the surface tension is broken, the surface area of the foot, how much force needs to be exerted on each step to maintain momentum and so forth.
Picture of a basilisk running on water, and quite clearly not sliding.
http://www-math.mit.edu/~dhu/Press/kennis_files/99135_962_1060686240391-lizard.jpg
There are plenty of other lizards around the world that use the same trick, as well as birds.
Creatures that size can take advantage of waters surface tension which is negligble when talking about something with the weight of a human over the small area of the foot.
I agree that surface tension is very small at human weights, but it still exists meaning that the inertia effects can still come into play. Obviously the speeds involved for a human to run on water would have to be well outside the range of physical possibility, but that wasn’t the question. The question was just how fast would a person need to run.
A total WAG would be somewhere in excess of 200km/hr if we simply scale up a normal sprinting motion, which although obviously impossible is still the type of answer requested. Interestingly it would probably be lower if we simply scaled up the walking motion because there is more foot area making contact with the surface.
I don’t know about these things, but I suspect that SDR, [1920s Style Death Ray] may still be correct in saying that inertia is not really the key point. These creatures are small enough that their feet can stay on top of the water for x seconds, which enables them to maintain a certain running stride.
Because of the fact that we’re so much heavier and surface tension of water so slight, it’s harder to assign a value for how long a human foot can stay on top of the water without sinking in… my wild guess would be one 50,000th of a second. If the average human running stride is .9 meters, then that would give a speed of 45km/s, or about 162 million km/h.
YMMV 
The basilisk lizard doesn’t actually run on top of water. The lizards are big enough that their feet actually plunge beneath the surface and skim backwards-- kind of swimming the crawl. When they face plant and go under, they swim and porpoise up until they can run again.
Not very fast at all, especially with snow shoes.
The physics involved in the bouncing 9000 lb dam buster bombs might give us some idea if the proposition is possible. I suspect footwear to be of utmost importance. I see discs.
aka the “jesus christ lizard”
I had a water dragon that ran like that, never got a chance to see if it could run on water though.
I believe it was in Discover magazine, several years back, where they analyzed the forces necessary for the basalisk lizard to run on water… and extrapolated it to humans.
I think they said that an average human would have to be able to legpress 1000 pounds, and be able to run 60 mph.
Wish I could find the cite for ya.
BasIlisk. BasIlisk. Jeez.
Anyway, I managed to find a semi-cite here
I’m pretty sure that people and to some extent lizards would have to rely on relative motion of the foot downward with respect to the water to generate enough force to stay up while running. Surface tension is almost certainly neglible for the human and perhaps also for the lizard. Note, though, the foot wouldn’t be stationary at any important point in this process.
People can waterski without skis (barefoot, that is). No doubt their feet are angled like ramps trying to climb out of the water. The downward component of the foot speed is probably what it takes to stand on the water - running with support less than half the time would take more than double this force, so the speed would have to be more than sqrt(2) greater. Maybe triple the speed that barefoot skiing works?
Not really, no. The feet do disturb the water of course, the fact that there are ripples present is all the proof we need of that. I don’t think anyone suggested that a person or lizard could run on water without disturbing the water at all.
However high speed photographs have shown that the lizard’s foot never goes under the water, or conversely there is never water over the foot. That’s very different to swiming where there is always water over the limb/the limb always goes under water.
http://jeb.biologists.org/content/vol206/issue23/cover.shtml
The foot is only beneath the surface of the water in the trivial sense that a pocket of water forms that is lower than the general level. The limb is not under water in the more important and true sense that there is never water above the foot. The water never closes in.
Minor nitpick here. 45 km/s converted to km/hr, gives 162 thousand km/hr, not million. Still plenty fast though.
Kinda related, I’ve seen motorcycles and atvs skim across water.
link about half way down
Also kinda related, I saw something on WTTW a while back that featured snowmobiles racing across water in the summer.
Apparently, it isn’t that unusual
If you’ve got iTunes, you can grab a recent “Quirks & Quarks” podcast about the basilisk lizard.
The site’s available at www.cbc.ca, but since the Mother Corp locked out employees you can’t grab the 'cast from the main page, and have to go through iTunes instead.
What about my second question? Would gravity have any effect on this? Is it humanly possible to run on water where the pull of gravity is less than it is on earth.
>The foot is only beneath the surface of the water in the trivial sense that a pocket of water forms that is lower than the general level. The limb is not under water in the more important and true sense that there is never water above the foot. The water never closes in.
Why would it matter that the water does not get the chance to close in over the lizard’s foot?
What matters is whether the water pushes upward against the foot. And, of course, it does. Imagine throwing a dinner plate down hard on the water, while maintaining it horizontal. It will slow down suddenly (accelerate upwards, that is) when it hits the water, which it is trying to displace. You can sense this also by smacking the water with your hand. There’s quite a lot of upward force. Water closing again above the object isn’t very important.