A chain of beads can defy gravity, acting like water spouting from a fountain, and now physicists reveal the secret behind this odd phenomenon.
The findings could have surprising implications for everything from tethered satellites to elevators reaching from space to Earth.
Chains are among the simplest, oldest and most widespread of technologies. As such, one might imagine scientists comprehensively understood their behavior.
However, a recent online video of a chain from the BBC seen by more than 2.5 million viewers astonished many, including many physicists. The video shows a strange effect one can easily recreate at home — if one lays a long chain of beads in a neat pile within a pot, beaker or similar vessel, pulls an end of the chain over the rim of the vessel and then releases it, the chain will not only flow to the floor due to gravity but also spontaneously arc upward as it moves. [See Videos of Weird Phenomenon of Beads Forming a Fountain and Macaroni Fountain]
Chain physics
Scientists were launching a project to teach physics to high school students, the Rutherford School Physics Partnership, when they discovered this video.
"We thought it was cool, and thought we should figure out what was going on and set up a question of it for the high-school students," said study lead author John Biggins, a physicist at the University of Cambridge in England."It then quickly transpired that we couldn't explain the leaping of the beads above the pot using the traditional ways of thinking about chains being picked up and put down, and that to explain it we were going to have to revisit apparently set-in-stone ideas from textbook classical mechanics.
"This was the point we realized that we had an interesting research problem on our hands."
Although the weight of the chain clearly pulled it downward, scientists didn't know why the beads leapt upward before falling. Viewers of this event sometimes mistakenly believe "that the beads are magnetic in some way," Biggins said. But "magnetism has nothing to do with this phenomenon."
However, both of these ideas "are fundamentally wrong," Biggins said.
Since their calculations showed the driving force behind this effect did not come from the part of the chain flowing away from the vessel, the scientists deduced that the force causing the beads to leap upward ultimately came from the pile of chain within the vessel somehow pushing upward.
"The push from the pot is the main result and the big surprise," Biggins told LiveScience.
Connected rods
The key to understanding where this push comes from is the fact that chains are essentially series of connected links or rods. Imagine that a rod in the pot is lying horizontally, waiting to move. It then gets pulled upward by a force acting on one of its ends. This force comes from the part of the chain flowing away from the vessel. [The 9 Biggest Unsolved Mysteries in Physics]
If this rod were alone, the force it experiences on one end would make it lift and rotate, causing the other end to move downward. However, since the rod is connected to other rods, "the far end of the rod bounces off the pot or other links in the chain, and this bounce provides the anomalous push," Biggins said.
"It is rare in physics for schoolchildren to be able to understand real research results, but in this case we think they will be able to," Biggins added.
Although the scientists conducted this research solely due to curiosity, the results "might have engineering implications," Biggins said. "People deploy chains and strings from piles all the time in a wide range of industrial and technological situations."
For instance, textile manufacturing often involves strings released from spools. In addition, satellites and spacecraft often deploy items on tethers.
"In situations like space engineering where energy and mass need to be reduced as far as possible, it may be advantageous to harness this push in the deploying of chains and tethers," Biggins said. "For example, if you want to tether two satellites, you need to deploy a chain between them from a pile on one satellite.
"Our work says that when you deploy that chain, by pulling on its end, your pull is supplemented by a push from wherever the chain is stored. So the pull you provide can be smaller than you originally thought. Therefore you can deploy the chain with a smaller force, and hence with a smaller, lighter motor and with less consumption of energy."
In what may be the most far-out possible application, the researchers also noted that plans to construct space elevators— giant structures reaching from space to Earth — often involve incredibly long fibers unspooled in space to stretch down to Earth. These findings could help complete such megastructures.
Biggins and his colleague Mark Warnerdetailed their findings online Jan. 14 in the journal Proceedings of the Royal Society A.
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