MadSci Network: Physics

Re: Can you walk on the surface of water if you walk with a certain speed?

Date: Sat Apr 29 22:48:59 2000
Posted By: John Balbach, Post-doc/Fellow, Physics, National Institutes of Health
Area of science: Physics
ID: 950891548.Ph

If you have ever rowed a boat before you are aware that if you push a wide flat object against the water, the water will push back. This force that you feel is actually related to the viscosity and mass of water and not the surface tension. It is this force that water skiers make use of to skim across the surface of water. I think I remember seeing a film clip of some sort of lizard running across the water. That would also be an example of this sort of force. The surface tension of water is measured in tenths of millinewtons per centimeter. While small insects and the like can get away with standing on top of water, most animals of any size will not.

The issue of speed comes in two ways. With the waterskier, the force on the bottom of the ski that arises from flow resistance will be dependent on the velocity of the skier. Below a certain amount, it will not be enough to support the skier. The lizard must churn its feet at a certain rate both to keep the size of the force appropriately large and also to keep the force more or less constant. The lizard must eventually pick its feet up out of the water if it wants to hang on to them, and then put its feet down again to hold itself up.

You can look up values for the surface tension of water in the CRC Handbook of Chemistry & Physics. The physics of waterskiing and the like are harder to look up easily, but you can try books about fluid dynamics. There is also a good webpage about airfoils which, while not addressing this topic directly, still outlines some concepts that are useful to understanding this question.

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