Unchaining Your Trick

Unchaining Your Trick

By Guy G. Marcus


Because being the smartest kid on the long bus has certain benefits.

By Guy G. Marcus

Physics buff or not, you are undoubtedly used to things falling down rather than up. The average school child knows the deal with gravity.

However, a simple demo of a chain “falling” out of a cup might make you stop and check your intuition: The chain rises out of the cup as water does from a fountain.

See for yourself in the video that has everyone talking:

If it’s not obvious to you what’s going on here, you are not alone. The puzzle of this deceptively simple phenomenon was only just solved by the authors of a recent paper.

But beyond its cool factor, this “trick” has real-life applications. Like in space, where a project by LiftPort is looking to build a space elevator on the moon, where lower gravity and lack of atmosphere make the job a bit easier than it would be on Earth (that’s the next step!). If you’ve seen the movie Gravity, you know how dramatic forces can be in space. Elevator deployment could give Sandra Bullock’s hurtling into the abyss a run for its money.

Here’s what’s going on. (SPOILER ALERT.) For all its grandeur, the chain fountain can be understood from basic and intuitive principles of physics appearing in everyday experiences. When you throw a ball into the air, it reaches an apex (highest point) and falls down due to the force of gravity. This illustrates the principle that a force changes an object’s velocity in the direction that the force is applied. This simple concept must, at some level, be at play in the chain fountain.

The apparent defiance of gravity implies that there must be a force acting up on the chain — in the direction opposite to gravity. To understand where this comes from, think of a chain as a simple model: a number of balls linked together by thin rods.

Still with me? In the video, we see that pulling the end link of the chain is enough to start the whole chain moving. Just like you learned by playing with a toy train as a kid, each link in the chain pulls on its neighbor.

When the chain is falling over the edge of the beaker, there will always be one section at the very top of the pile that hasn’t started to move yet. The neighbor above this “surface link” will pull it up from one end, causing it to tilt — like a seesaw. One side of the link is lifted up while the other pushes against the chain pile beneath it.

Think of it like a skateboard: Just as pushing the tail of the board against the ground causes it to kick up into an Ollie, the surface link gets a push from the chain pile, called a reaction force, that lifts it into the air along the length of the chain. The repeated action of the reaction force on subsequent links is responsible for the emergence of the chain fountain. Cool, right?

And it’s useful, too. Real-world applications of this principle range from industrial design in textile manufacturing (unspooling fabric or yarn as quickly as possible) to the deployment of those space elevators. 

All the way from a chain in a cup to a railway to the sky: It’s the same physics. 

Guy G. Marcus is a doctoral student in physics at Johns Hopkins University, where he conducts research under the purview of the Institute for Quantum Matter. He received his B.A. from Wesleyan University with a dual focus in physics and science in society. Webpage: www.guygmarcus.com Twitter: @guygmarcus