The water bottle flip

We explain the water bottle flipping craze.

In case you missed it, the bottle flip craze was an internet-driven fad that swept the country earlier this year. The challenge is to flip a partly filled water bottle and try to make it land upright. It is quite a striking phenomenon in which the bottle appears to stop rotating before performing a vertical descent.

Recording the motion and playing it back in slow-motion helps reveal what is going on. As the bottle moves through the air the liquid inside it starts to climb the sides. This increases the bottle’s moment of inertia and so decreases its rotational speed, much like a diver who extends his arms and legs to slow his spin.

Trying different amounts of water shows how important it is to get the amount of liquid right. The trick just doesn't work with a full bottle. The  liquid can’t move, and its higher centre of mass means that a full bottle topples over when it lands. Filling it to just a bit below a third seems to be optimal.