Humans cannot run on water under normal circumstances. The scientific principles governing mass, force, and liquid properties explain why such a feat is beyond human capability. Understanding these principles clarifies what is required for any organism to traverse a water surface.
Why Humans Don’t Run on Water
Humans are too large and slow to generate the necessary forces to stay atop water. Our significant body weight and relatively limited speed mean we cannot produce enough upward force or distribute our mass effectively to counteract gravity and water properties. When a human attempts to run, their feet quickly break through the surface.
Water’s density is insufficient to support a human’s weight without rapid force application. A human foot applies too much concentrated pressure for too long, causing it to sink. Even the fastest human sprinter cannot generate the required speed or exert force rapidly enough. For a human to run on water like a basilisk lizard, a speed of about 65 miles per hour would be necessary.
The Physics of Water Surface Movement
Moving across a water surface without sinking relies on surface tension and hydrodynamic forces. Water molecules exhibit strong cohesive forces at the surface, creating surface tension. This property allows the water’s outermost layer to behave like a thin, elastic skin, capable of supporting light objects.
Hydrodynamic forces also play a role when an object moves rapidly through water. When an object applies a force to the water, the water exerts an equal and opposite reaction force. Generating enough upward force through rapid, forceful movements, often by pushing water downwards, can create temporary support. Distributing weight over a large area or applying force very quickly before the water surface yields are critical for remaining afloat.
Animals That Traverse Water
Several animals traverse water surfaces by exploiting these physical principles, each with unique adaptations.
Water striders are small insects that utilize water’s high surface tension to “walk” across ponds and streams. Their long, slender legs are covered in microscopic, hydrophobic hairs that trap air, preventing them from breaking the surface tension. By distributing their minimal weight across six legs, they create small depressions on the water’s surface. They propel themselves by pushing against the water with their middle legs without penetrating the surface.
The basilisk lizard, also known as the “Jesus Christ lizard,” employs a different strategy, relying more on speed and hydrodynamic forces. These lizards have specialized hind feet with long toes and skin fringes that splay out when they strike the water, significantly increasing their foot’s surface area. As they run, they rapidly slap their feet, creating temporary air pockets. This rapid “slap and stroke” motion generates upward force and resistance to support their weight for short distances, allowing them to sprint across the surface to escape predators.