Why does a dropped Slinky's bottom hang motionless in mid-air?
If you hold a stretched Slinky vertically and let go of the top, why does the bottom of the Slinky stay completely still, floating in place, until the collapsing coils above reach it?
History and references
Settled in 1993 by M.G. Calkin (1993, initial analysis); confirmed and extended by W.G. Unruh (2011) and R.C. Cross & M.S. Wheatland (2012).
Calkin's 1993 analysis of a falling spring showed that the net downward force on the center of mass (gravity) is exactly balanced, coil by coil, by the release of internal tension, so the bottom of a Slinky remains in free-fall-zero net force (effectively motionless in the lab frame) until the collapse wave from the top reaches it. The result went viral after 2011 YouTube slow-motion videos, prompting Unruh's arXiv note and Cross & Wheatland's fuller 2012 AJP model including internal damping. Walker's flyingcircusofphysics.com added the Slinky drop as a companion-site update after the phenomenon became widely popularized.
A toy seems to defy gravity for a fraction of a second, and the resolution is an elegant exercise in center-of-mass reasoning.
Explanations that circulated
- The bottom is held up by the table/floor's normal force before impact
- Air resistance holds the bottom coil in place
References
- Motion of a falling spring, M. G. Calkin, American Journal of Physics, 1993
- The falling slinky, W. G. Unruh, arXiv preprint, 2011
- Modeling a falling slinky, R. C. Cross, M. S. Wheatland, American Journal of Physics, 2012
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