We've all had the misfortune of botching a dive into the pool and ending up in a painful belly-flop—or perhaps we've done it deliberately to show off and instantly regretted that decision. Hitting the water in that body position can feel like hitting concrete and lead to bruising or (if one is falling from a greater height) internal injuries. While the basic physics is well-understood, scientists are always looking for greater insight into the phenomenon in hopes of finding novel ways to ameliorate the impact.
Scientists at Brown University have found that, surprisingly, adding a bit of extra spring to a body hitting the water can actually increase the impact force instead of decrease it under certain conditions, according to a new paper published in the Journal of Fluid Mechanics. The implications go beyond protecting divers; a better understanding of the hydrodynamics will improve designs of naval ships, seaplanes, or projectiles, as well as underwater autonomous vehicles.
From a physics standpoint, we're talking about an elastic body hitting the surface of water. The stress of moving from the medium of air to the much denser medium of water exerts a huge force as that body displaces it. The cohesive forces between water molecules are stronger at the surface, making it harder to break through. (It's why diving competitions often use aerators to create bubbles in the water, breaking the surface tension to protect the divers.) A large volume of fluid must be accelerated (displaced) in a short timeframe to match the speed of the impinging body. The larger the surface area of the object hitting the water, the more resistance there will be—and with belly-flops there will be a much larger surface area than with a simple swan dive, resulting in that signature slam.
