Friday, September 14, 2012

The gravity-defying slinky

Here's a weird bit of informative entertainment for your Friday. If you drop a slinky, the bottom of the slinky appears to defy gravity as it hangs in the air while waiting for the rest of the slinky to meet it before it decides to join the fall.

Take a look at the below video and re-calibrate your intuitions about gravity.

The video doesn't properly explain what is happening, so I've done a little research (emphasis on the little) to determine what is going on. The centre of mass of the slinky behaves exactly as a falling object should. The bottom of the slinky appears to defy gravity for a while because the forces of the spring pulling the bottom up neatly balance the force of gravity pulling the spring down - that's why the slinky initially stretched to the length it did instead of reached all the way to the ground. If you stretched the slinky from the bottom as well as holding it at the top, and then released both ends simultaneously, the bottom would actually travel up while the centre of mass travelled down - we've all seen that sort of behaviour in a slinky/spring, but that just doesn't seem as weird as the bottom of the slinky simply hanging in the air.

Via What Would JT Do

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