Why the bottom of a dropped Slinky hangs in mid-air: a wave has to bring the news
30seconds
Hold a Slinky by its top, let it stretch, and let go. The top rushes down while the bottom coil hovers, apparently ignoring gravity, until the collapsing coils finally reach it.
Release a simulated Slinky yourself, then scrub through the fall frame by frame. Watch the bottom coil stay put while the centre of mass drops exactly like a ball, and change how far the spring hangs to see how long the news takes to arrive.
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