Physics Holds the Key To Performing the Flipping Water Bottle Trick (arstechnica.com)

An anonymous reader shares a report: Think of how ice skaters extend their arms and legs to slow down rotation coming out of jumps or spins. It's the same principle: conservation of angular momentum. Once the bottle is set in motion, its angular momentum remains constant. But shifting how the mass inside (the water) is distributed increases the bottle's rotational inertia (how much force is required to start or keep it moving). This slows down the bottle's rotational speed. Physicists from the University of Twente in the Netherlands decided to analyze the underlying physics [of flipping a half-full bottle of water so it lands upright] more thoroughly in a series of rigorous experiments and develop a theoretical model. For the first version of the experiment, they used a partially filled water bottle. For the second version, they reduced the variables from the large number of water molecules in the bottle to just two tennis balls in a cylindrical container. In both cases, their measurements clearly showed the dramatic decrease of the container's rotational speed, resulting in a nearly vertical descent, so the bottle landed neatly and upright. Tracking the sloshing of the liquid and the changing positions of the tennis balls demonstrated the redistribution of mass, shifting the moment of inertia.

Comments

Contrary opinion

[SuperKendall]

Well all of the Slashdot posters today seem to be gunning for most snide comment.

Maybe they didn't read the whole summary? Because I for one thought it was pretty cool they reduced the problem down to a canister and two tennis balls to demonstrate the same effect in an easier to understand way.

People may find this pointless. but you never know when understanding something others consider to be obvious or not worth knowing may be helpful. You never know what kid may get into science because of being fascinated by this reduction.

I say, good work!