Does physics allow us to run on water? Despite what we may have seen on various YouTube videos, running on water is impossible for people. This myth was tested in the 2011 episode of MythBusters, where U.S. sprinter Wallace Spearmon attempts – and fails miserably – to run across a pond.
Running on Water: Animals Can Do It – Why Not Us?
Although impossible for humans, there are animals that can run across water. The basilisk lizard, for example, has this ability. The Western Grebe, a species of water bird found in Western North America, is also known for “rushing,” a spectacular display of courtship as a pair of birds lunge forward in synchronicity and race across the water side by side.
In 1996, J. W. Glasheen and T. A. McMahon determined the various mechanisms basilisk lizards generate the forces needed to run across water. This model shows how the lizard’s mass and stride frequency allows it to run across water. It also shows why other species can’t; body size and proportions, appendages and limited muscle power prevent this. However, if gravity were reduced, running on water might require less muscle power.
To test this hypothesis, Francesco Lacquaniti and his research team strapped test subjects to a reduced gravity simulator, a specially designed harness to reduce a subject’s weight, to test this hypothesis. This work doesn’t just show that it is possible for humans and robots to run under reduced gravity conditions but may answer questions on how gait dynamics evolved and might exist under different gravity conditions. The team was awarded the 2013 Ig Nobel Prize for discovering “that some people would be physically capable of running across the surface of a pond —- if those people and that pond were on the moon.”
The Physics of Running on Water
The basilisk lizard is able to support its weight and run across water by slapping its feet across the water’s surface. As they do this, they create a tiny air pocket that prevents them from sinking. To avoid having to pull its foot back up against water drag, the lizard must withdraw its foot before water can fill the cavity.
This effect of slapping and stroking their feet into the water and the stride frequency, described by Glasheen and McMahon, affects a momentum transfer that provides both lift and forward thrust. By knowing how all these variables connect, Steven Floyd and other Carnegie Mellon scientists successfully built small basilisk-type robots that run across water. This model also indicates that humans are far too big and weak to splash their feet hard enough to support their weight. We would need to stamp the water’s surface at speeds greater than 30 m/s; about fifteen times greater than our available muscle output.
Could Humans Run on Water in Alien Worlds?
Although our body size and proportions, appendages and limited muscle power preclude water running, the model predicts that if gravity is reduced, it may require less muscle power and thus be possible. To test this hypothesis, Lacquaniti and his research team outfitted volunteers with small fins and strapped them into a harness attached to a constant force unloading system to reduce their weight.
Study author, Yuri P. Ivanenko told Decoded Science, “The robots designed by Floyd and colleagues are based on similar principles that were developed for a Basilisk lizard. Our experiments confirmed that a hydrodynamic model of lizard running on water holds also for humans, despite the enormous difference in body size and morphology. There was a fairly good correspondence between predicted and actual simulated gravity levels at which humans can run on water.”
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