Water bottle flipping has become quite the craze, and in a recent video The Backyard Scientist presented his own take on the subject, testing whether you could flip a bottle with mercury rather than water. As it turns out, fluid dynamicists have studied this topic, too, by dropping partially-filled elastic spheres containing water, isopropyl alcohol, and glycerin. The key physics here comes from the sloshing of liquid inside the container. When the elastic ball bounces, energy that would otherwise go into the sphere’s rebound instead gets distributed into sloshing the fluid inside. The result is that the sphere bounces less on its subsequent impacts.
Interestingly, the researchers found that the properties of the fluid inside the ball made very little difference to its rebound height. Instead, the most important feature was the volume of fluid in the container. Balls filled to approximately 30% of their volume had the most damping – that’s totally consistent with the best water bottle flips, which use bottles about 1/3rd full.
The main difference between flipping a bottle and dropping a ball is what goes on in the first bounce. When a bottle hits a surface, the liquid inside has already been disturbed by the bottle’s rotation. For a ball being dropped, that first impact is what disturbs the fluid. So while a water-filled ball’s first rebound will reach nearly the same height as an empty ball, the spinning water bottle is, in effect, already on its second bounce. The motion of the fluid inside the bottle acts as a damper, allowing the bottle to stick the landing. (Image credit: Mercury Bottle Flip – The Backyard Scientist, source; Water Ball Bounce – The Splash Lab, source; research credit: T. Killian et al.)