I realize that this may be an old question, but Google didn't help me last time I tried it.
My physics teacher told me about an old proffesor of his who asked new students this question, though he didn't remember the answer, since then I'ts been niggling in the back of my head, but we have'nt learnt anything in physics that pertains to answering it, and I though that I'd get the answer before my physics final next Thursday to ease my curiosity before re-reading EM and electronics for the test .
So, what is the speed/velocity necessary for an adult human weighing at ~80 kgs to run across a clear body of water without sinking into it?
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DEATH wrote:I realize that this may be an old question, but Google didn't help me last time I tried it.
My physics teacher told me about an old proffesor of his who asked new students this question, though he didn't remember the answer, since then I'ts been niggling in the back of my head, but we have'nt learnt anything in physics that pertains to answering it, and I though that I'd get the answer before my physics final next Thursday to ease my curiosity before re-reading EM and electronics for the test .
So, what is the speed/velocity necessary for an adult human weighing at ~80 kgs to run across a clear body of water without sinking into it?
Glasheen & McMahon (1996a) estimate that a human would be able to run on water only if he were able to achieve speeds of order 30 m/s,
From "WALKING ON WATER: Biolocomotion at the Interface" out of the "Annual Review of Fluid Mechanics 38(1):339".
Further, from Comparative Biomechanics: Life’s Physical World:
basilisk lizards are able to run on water (and why a human would have to weigh just 4.6 g to accomplish the same feat
I have no idea how to calculate it though.
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I don't really see how you could possibly calculate such a figure from first principles without knowing a lot more about the details, such as the manner in which the feet are contacting the water, the shape of the feet and/or flippers/shoes/coverings, etc. There would have to be a lot of assumptions.
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To further Mike's example. Some creatures can "run" on water at speeds under 5 miles an hour. It has to do with surface tension and the means of which you touch the water. If your light enough and spread out on the water, you can't break its surface tension and so you can easily walk on it.
I believe the weight of the human trying to walk on the water and the size of their shoes are very important factors which can greatly affect the speed required to run on water.
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Side note to the calculation, the surface area would not be based on the sole of he foot, because as a runners velociry increased the point of impact moves forward. During a sprint the runner shifts to balls of the feet. So the impact surface to the water would only be the toes & the balls of the feet.
Surface tension would not be much of a factor since you can't make a human light enough to make surface tension significant relative to his weight. So the mechanism through which a human would be able to run on the water would be the viscosity of the water. To put it simply, water resists movement, and if you pound on the water fast and hard enough, you'll be able to stay on top of it. That's how water-skiing works; the water resists downward movement with enough force to keep your body above the water once you reach sufficient velocity.
Therefore, the size of your feet and the rapidity of your strides is the most important thing. It's not exactly like water-skiing because your feet aren't constantly in the water, so what you have to do is make sure that your feet strike the water so often that it's equivalent to water-skiing.
"It's not evil for God to do it. Or for someone to do it at God's command."- Jonathan Boyd on baby-killing
"you guys are fascinated with the use of those "rules of logic" to the extent that you don't really want to discussus anything."- GC
"I do not believe Russian Roulette is a stupid act" - Embracer of Darkness
"Viagra commercials appear to save lives" - tharkûn on US health care.
If you just increase the surface tension of the water (freeze the pond) you don't even need to move to stay on top of it (at which point, naturally, the water is too cold for swimming).
Assuming the water is liquid of course, I'm not sure how fast waterskiiers go, but I think it is well faster than a human can run spread over much greater area than human feet normally occupy.
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wilfulton wrote:Assuming the water is liquid of course, I'm not sure how fast waterskiiers go, but I think it is well faster than a human can run spread over much greater area than human feet normally occupy.
It is possible to waterski barefoot. I think that a typical speed for a waterskiier is between 15 and 35 mph, depending on skill. Of course: it may be that the aforementioned barefoot skiiers need to travel at the top end of that spectrum, or indeed beyond it.
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