Video: Ball Chain

In this demonstration, we will see how a chain of balls and links can seem to defy gravity, rising out of its container to unexpected heights.


Video Overview

To observe a chain fountain, a chain consisting of many linked spherical beads is used.

The chain is placed inside a glass, filling it to the brim.

The end of the chain at the brim of the glass is pulled slightly away from the rim of the glass and released.

The surprising thing that happens is that once the chain starts to flow out of the glass, the beads of the chain first travel a height upward before they fall back downward.

As the chain flows out of the glass, gravity pulls the part of the chain outside the glass downward. The tension in the chain pulls the end of the chain that is inside the glass upward.

If the only force that acts on a bead as it is leaving the glass is the tension in the chain produced by gravity, the chain should only rise to the height of the rim of the glass. For the chain to rise a significant distance above the rim of the glass, some force other than this tension must act on the beads.

The first explanation for what the additional force on the beads might be was that a group of consecutive beads would act like a thin rod. When the bead at the end of the rod was pulled upward, the rod would rotate, pushing downward on the surface below the opposite end of the rod to the end being pulled upward.

This explanation has however been shown to be incorrect. A chain made up of actual rods rather than of spheres will produce a fountain in this way, but a chain of spheres will only do so if the chain is resting on an uneven surface. The shape of an uneven surface that a chain of spheres must initially rest on for it to produce a fountain includes the kinds of shapes that would naturally exist at the top of a glass containing a chain of spheres that was placed into a glass in an uncontrolled way that allowed the chain to become tangled.

Current models and experiments involving chain fountains are not able to explain why an uneven surface enables a chain of spheres to produce a fountain.

The scientific method for understanding phenomena ideally involves trying to measure the changes in some variable and comparing them to changes in a different variable that have been given chosen values, and doing this while, as far as possible, keeping all other variables constant. For the chain fountain phenomenon, it is not obvious what variables significantly affect whether a chain does or does not produce a fountain. Are, for example, the following variables significant?

  • The size of the spheres
  • The spacing distance between the spheres
  • The material from which the spheres are made
  • The total length of the chain
  • The shape of the chain inside the glass

Each variable could be significant, and each variable could affect other variables. For example, the shape of the chain inside the glass affects the shape of the surface at the top of the glass.

Also, each variable could be insignificant alone, but a combination of variables could be significant; perhaps neither just changing the size of the spheres nor just changing the spacing distance between the spheres affects whether a fountain is produced, but changing both the size of the spheres and the spacing distance between them does affect whether a fountain is produced.

The chain fountain is a good example of how the scientific method needs to be used in subtle ways to understand some phenomenon; while simple explanations may be incorrect, they might still be able to help in finding better explanations.

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