Why are belly flops painful? Scientists actually conducted a study to find out

PROVIDENCE, R.I. — It might be funny to watch people doing belly flops into a pool, but it’s no fun on the body. Belly flopping can be painful, and Brown University scientists are now diving into why that is. The stinging pain that follows such a dive is a result of the abrupt change from air to still water, causing the water’s surface to resist the body’s motion fiercely.

“All of a sudden, the water has to accelerate to catch up to the speed of what’s falling through the air,” says Daniel Harris, an assistant professor in Brown University’s School of Engineering who specializes in fluid mechanics, in a university release. “When this happens, that large reaction force is sent back to whatever’s doing the impacting, leading to that signature slam.”

While understanding the physics behind a belly flop may seem like fun trivia, it has significant implications for naval and marine engineering. Many structures in these fields need to withstand high-impact air-to-water slamming forces. Therefore, researchers have been studying this phenomenon for decades.

Traditionally, research in this area has focused on rigid bodies slamming into water, with their overall shape remaining unchanged during impact. However, researchers explored what happens when the impacting object is flexible, allowing it to change shape or deform upon impact. To investigate this, they attached a soft “nose” to a cylinder and used a system of flexible springs, similar to a car’s suspension, to soften the impact.

Their experiments yielded unexpected results. While the flexible system sometimes effectively reduced impact forces, it occasionally increased the maximum impact force compared to a rigid structure — a counterintuitive finding. To understand this paradox, the researchers conducted extensive experiments and developed a theoretical model.

They discovered that the key factor was the stiffness of the springs. Depending on the drop height and spring stiffness, the body experienced not only the slam’s impact but also the vibrations of the structure as it entered the water, intensifying the slamming force.

“The structure is vibrating back and forth due to the violent impact, so we were getting readings from both the impact of hitting the fluid and an oscillation because the structure is shaking itself,” explains Harris. “If you don’t time those right, you can basically make the situation worse.”

Their research has led to new insights and the realization that soft springs are crucial for effectively absorbing the impact without causing additional rapid vibrations that add to the overall force.

Scientists are now inspired by diving birds, which perform specific maneuvers to reduce the forces they experience when entering the water. They are exploring the development of a robotic impactor that can actively maneuver during water entry, aiming to apply these principles to blunt objects.

“Biological studies of these birds have shown that they perform certain maneuvers as they enter the water to improve the conditions so they don’t experience such high forces,” says John Antolik, a Brown University graduate student. “What we’re moving towards is trying to design what is essentially a robotic impactor that can perform some active maneuver during water entry to do the same for blunt objects.”

This study received support from the Office of Naval Research and the Naval Undersea Warfare Center.

The study is published in the Journal of Fluid Mechanics.

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