Have you ever watched a Slinky fall? Like close enough to see what is really going on? If you drop a suspended Slinky from one end, only the top will fall. Until the top of the toy impacts the bottom, the bottom hangs motionless in the air as though it is still suspended. What the heck is going on? (If you need to see this mind-blow for yourself, you can watch the high-speed video from Adam Shomsky above.)
Until recently, floating Slinkys were a little mystery, just one of those things that no one has taken the time to look in to. Then videos of the phenomenon spread to the point that physics papers were published on the subject. It turns out that a levitating Slinky has more to do with the speed of sound than gravity.
When you hold a Slinky off the ground, the pre-tensioned spring will come to an equilibrium. Just hanging there, the pull of gravity is cancelled out by the tension in the toy. Gravity determines how low the spring will hang. Then you let it go. While the top of the Slinky “knows” that it is no longer under tension (hanging from your hand), the bottom does not. For all it is concerned, it is still under tension from the rest of the Slinky above it. Only when the information that the toy is in free-fall reaches it does the bottom start to move.
Information is conducted through the Slinky as the molecules of the metal bump into each other, and the speed at which they do so happens to be the speed of sound (through the metal, it would be different through air). Multiply this speed by the length of the Slinky and you’ll get the time it will take for the bottom to know to fall.
So that’s the secret of the levitating Slinky. But though we know why it happens, it’s no less awesome to look at.
IMAGES: Adam Shomsky