I was at my car, loading for a trip to town, when I set my half-full water bottle on the hood. I reached into my pocket to grab my keys, my arms full, when I heard the bottle vibrating. My car was not running, rather sitting perfectly still. When I saw what was happening, I set everything down and grabbed my phone to record.

It appears the inherent motion of the water coupled with the rounded bottom edges of the bottle, concave surface of that portion of the Subaru hood, and half full water enabled that trapped kenetic energy to translate into motion. What’s astounding is how the dual-cycle standing waves in the water (meaning, the waves repeatedly return to the same position and height, amplifying the motion) “walk” the bottle down the car’s hood. The motion of the water in the bottle is essentially in equilibrium, the waves casting their energy against the inside of the bottle balanced by the forward “walking” motion invoked by the pull of gravity over an ever-so-slightly sloped surface.

But when that surface changes, the system is imbalanced, the bottle is no longer stable, and topples over the side.

I reset the bottle at the top and without any effort on my behalf, it did it all over again. So much fun!

My former high school physics professor Dan Heim found this worthy of investigation and included a full explanation as the topic for his weekly science essays at Sky Lights.