How we did it:
Head outside with a slinky and a smartphone. Hold the slinky as steady as possible. Then, drop it and carefully observe what happens!
Strange! It looks like the slinky is levitating — the bottom doesn’t start to fall until the top comes down to meet it. But it all happens so quickly. How might you investigate this at a slower speed to see it more clearly?
Hold the slinky still like in Step 1, but this time, have a friend or parent point the smartphone at the slinky. Open up the camera app, and set it to record in slow motion. Start recording, then release the slinky!
Watch your slow-motion video. Does the slowed-down footage match your normal-speed observations?
The bottom of the slinky does float in midair before the rest of the slinky catches up to it. But there’s no magic at work here - this happens because of tension.
Tension is what keeps the dangling slinky from unravelling like a noodle. It’s the force of each coil trying to get the slinky back to its normal shape, pulling up against gravity and down against your hand as the slinky stretches out.
When you let go of the top coil, it gets pulled down by the tension in the next coil. Then they get pulled down by the next coil. The coils spring together like a chain of falling dominoes.
But the lower coils don’t move until the falling coils reach them. Their tension holds them in place. Once all the of coils get to the bottom, though, the tension is totally gone, and the whole slinky falls.