Slap bracelets were great when I was a kid. The first one I got was pretty much just reflective tape, but I loved it. The feeling of it curling up at your wrist never seized to amaze me.
I always wondered how they work, but as time passed, the enthusiasm of a child left me and I forgot about slap bracelets. This is until I stumbled upon a video by the great Hank Green about them.
One of his kids went to a birthday party and got a slap bracelet (minecraft themed!), but it, of course, fell apart pretty soon.
The discovery
What Hank discovered amazed him and filled me with joy: It’s just a rounded piece of measuring tape. Not only the metal that’s used, but with all the markings, the yellow color - everything. This lead me into a rabbit hole to figure out why measuring tape.
Why though?
First of all: measuring tape is not the only material used for slap bracelets, and apparently it has been banned in some countries.
But what are the properties of measuring tape that make them a good interior of the bracelet? The slight rounding of the metal gives the material a little bit more stability - leading to increased stiffness.
If the material is then bent, after some point it has another stable shape: the coil shape. This property is also called bistability, meaning that a system has two different states of equilibrium.
And all of that can be learned from a slap bracelet!