A slinky is a pre-compressed helical spring. It was invented by Richard James in the early 1940s. It can perform a number of tricks, such as traveling down a flight of stairs, while moving end over end. It can also appear to levitate for a period of time after it has been dropped.
Check out a video of a slinky:
When a slinky goes down a flight of stairs, it transfers its energy along its length in a longitudinal wave. The whole spring goes down the stairs, end over end, in a periodic fashion.
When the top end of a slinky is dropped, the information of the tension change must propagate (in the form of a wave) to the bottom end before both ends begin to fall. What happens is that the top of an extended slinky will drop while the bottom end initially remains in its original position. This results in the slinky appearing to be suspended, or levitating, for approximately half a second. But there is the potential for it to remain suspended for much longer than half as second.
Slinkys can be used to demonstrate the properties of waves. Waves travel along a slinky slow enough to be observed, which is quite useful. NASA has used slinkys in zero-gravity physics experiments inside the Space Shuttle.
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