The Physics of Fidget Spinners

May 18, 2017

Credit CC0 Public Domain

Anyone who's spent any time with a kid between the ages of five and fifteen recently has probably encountered a fidget spinner. These flat, three-pronged, spinning toys are marketed as ways to help autistic students or those with ADHD focus by giving the hands something to do. However, some schools have banned fidget spinners because, well, they're too interesting. They actually grab kids' attention.

And it's no wonder. It turns out there’s a lot of physics in each small fidget spinner – rolling versus sliding friction, angular momentum, gyroscopic action, even relativity. But, what most want to know is simply this: How fast can it go? And how long can it spin?

Greg Kestin, a physics researcher and educator at Harvard University, says the theoretical limit is close to the speed of light. Slightly more attainable may be the speed of sound.

"I actually did a calculation to see when these could break the sound barrier - when would the edges of this thing be moving faster than the speed of sound," said Kestin. "That’s a few thousand rotations per second."

That's not something you want to attempt while holding one between your thumb and forefinger, especially not when you take into account the next part:

"As you have the fidget spinner moving faster, air resistance pushes harder and harder," Kestin explained. "So if you’re moving at the sound barrier - where the edges are moving at the speed of sound - then you actually have about the weight of a person pushing back and resisting this thing spinning.”

Dare ya.