No matter how long the slinky is, the bottom of the slinky will stay still (hover) until the top reaches it. Even if the slinky is over 1000 feet long.
This was a fantastic episode of Veritasium. Maybe the best. The recoil of a slinky, stretched under its own weight, is a pretty fantastic physics problem, especially when you capture it in super-slo-mo.
A Cardiod, a lovely mathsy heart shape, can be constructed as shown in the animation as the combination of many circles generated from a single underlying circle. This shape describes the sensitivity regions of many directional microphones. [more] [code]