Water strider

Aquarius

Method of propulsion of a water strider

Animals such as water striders that live on the surface of water need to push something backwards to generate a reaction force (that is, Newton's third law of motion).

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It was originally thought that water striders transferred momentum to the water by the creation of capillary waves on the surface. However, biophysicist Mark Denny showed that to do this, some object must move faster than about 0.25 m/s—far faster than a water strider can move its legs. This apparent contradiction is known as Denny's paradox.

Related Topics:
Momentum - Capillary waves - Mark Denny - Denny's paradox

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Water striders beat Denny's paradox by generating not capillary waves but hemispherical vortices in the water. These vortices carry sufficient backwards momentum to propel the animal forwards.

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In a series of experiments, mathematician David L. Hu and coworkers showed that during the rowing stroke, water striders drive their middle legs backwards without penetrating the surface, and can attain speeds of up to 1.5 m/s.

Related Topics:
Mathematician - David L. Hu

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