New explanation proposed for viral chain-of-beads video

Nature : A year ago a video was posted to YouTube by British science presenter Steve Mould that became a huge sensation. It showed the behavior of a long string of beads in a jar when one end of the string is pulled out of the container and allowed to drop toward the floor. Mould’s explanation for the fountain-like effect was based on the inertia of the beads in the jar versus the inertia of the falling strand. In a new paper, however, John Biggins and Mark Warner of the University of Cambridge argue that Mould’s explanation is wrong. They modeled the chain of beads as a series of short rods, each made of three beads and two connectors. When one end of any such rod is picked up, the other end pushes downward. When the downward-moving end is in contact with the jar, it receives a push back, which provides the extra kick that causes the chain to climb into the air, like water from a fountain, before beginning its descent. A similar but reversed effect can be observed in chains that are falling onto a surface, which fall faster than chains falling into open space. Although Biggins and Warner note that other factors are involved, such as the arrangement of the chain in the jar, they believe that in ideal conditions the fountain-like peak would remain steady.