Why does a spinning coin's rattle speed up right before it stops?
When a spinning coin or disk settles flat on a table, why does the frequency of its wobbling/rattling sound rise dramatically just before it comes to rest, instead of dying away smoothly?
History and references
Still disputed in the literature as of 2026.
Moffatt's 2000 Nature paper modeled the settling motion of a spun disk (popularized by the 'Euler's Disk' toy) as approaching a mathematical finite-time singularity, attributing the rapid energy dissipation mainly to viscous drag in the thin layer of air squeezed between the disk and table. Later the same year, van den Engh, Nelson, and Roach published experiments spinning a coin in a near-vacuum and found the behavior barely changed, arguing that rolling/slipping friction at the contact point, not air viscosity, dominates the dissipation. Subsequent studies (into the 2010s) have examined rolling friction, imperfect disk shape, and contact mechanics as contributing factors, but no single, fully agreed quantitative account of the dissipation mechanism has emerged.
A spinning coin encodes a real mathematical finite-time singularity in an everyday kitchen-table sound, and physicists still argue about exactly why.
Explanations that circulated
- The spin-up is caused only by simple mechanical friction with no fluid or vacuum dependence
- There is no genuine finite-time singularity, only ordinary exponential decay (Moffatt's own model predicted a true finite-time stop, which is itself debated)
References
- Euler's disk and its finite-time singularity, H. K. Moffatt, Nature, 2000
- Euler's disk and finite-time singularity (comment), G. van den Engh, P. Nelson, J. Roach, Nature, 2000
Solutions 0
Sign in or create an account to publish a solution