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A spinning coin explained

By Carol Saab, 4 July 2013 Activity

What happens when you spin a coin on a table? It spins and rolls (or ‘spolls’), for starters. How it stops is much more fascinating.

Spinning a coin is simple enough to do, but the physics of its motion and how the coin comes to rest has been studied since the 18th century. When you spin a coin on its side, it slowly loses energy and starts wobbling around. Then a curious thing happens – as the flat surface of the coin comes closer to the table, the spinning increases in frequency. In other words, it spins faster as it’s about to stop. Take a look:

Video transcript available here.

The whole spinning-faster-as-it-slows-down thing is called a finite-time singularity. It’s like bouncing a really elastic ball. If no energy was lost, the ball would bounce at a constant frequency forever. But since energy is being lost, the ball’s maximum height decreases while its bouncing frequency increases.

Without  friction or vibration, the coin (or disc) would spin forever. Euler’s Discs, coins and even Conan O’Brien’s wedding ring have been spun in a vacuum to see if it’s either air resistance or friction slowing it down. In the case of O’Brien’s wedding ring, spinning in a vacuum had no identifiable effect while spinning it on a Teflon surface gave a record time of 51 seconds. The general consensus is that rolling friction is the main force slowing these objects down, and ultimately makes them stop.

If you’re a parent or teacher, download our teacher’s notes with curriculum links.

Keep spolling around with last week’s video on Spinning, rolling and hula hooping.

2 comments

  1. Reblogged this on News @ CSIRO and commented:
    When you spin a coin on its side, it spins faster as it’s about to stop. In other words, as the flat surface of the coin comes closer to the table, the spinning increases in frequency. Check out this new video from our Helix blog for more on this physics pro-tip (suitable to teach in the home, classroom, or win a bet at the pub).

      Reply
  2. The momentum of a spinning coin at its best 🙂 Its often simple physics like this that is the most shocking. Where does all that energy come from? Not much, just gravity and velocity held along for a lengthy period of time. I post about intriguing physics topics as well on my blog 🙂

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