A raindrop falling to the ground strikes with a certain force. But what if the drop isn't round but elongated like a cigar or flattened like a pancake? It turns out that shape is everything: the impact force can vary by more than a factor of ten just because of how the drop looks at the moment of impact.
The researchers ran experiments and found that despite the diversity of shapes, all drops follow a simple rule. If you mentally replace the drop with a cylinder of the same height and volume, you can accurately predict the force and timing of its impact on a surface. This rule works for drops of any shape — from nearly spherical to highly elongated.
This understanding matters not only for physics. Drops of different shapes are everywhere: rain erodes soil, water jets cut materials, paint is sprayed onto surfaces. Knowing how a drop's shape affects impact can help better protect soil from erosion, design wind turbines that withstand raindrop impacts, and even improve coating technologies.
A raindrop in flight is not round but resembles a flattened bun due to air resistance. Upon hitting a solid surface, it momentarily takes the shape of a pancake.