# Something amazing happens when you drop a stretched-out Slinky

When you drop a Slinky, what falls to the ground first: the top, the bottom, or everything all at once?

Derek Muller, who created the popular YouTube channel Veritasium, first took on this puzzling physics question in a 2011 video.

At the Google Science Fair on Monday, which Muller hosted, he repeated this demonstration on stage — and also with a giant, rainbow-coloured Slinky he dropped from the top of the Googleplex.

So which is it? The result may surprise you.

First, make your bets on what you think will happen. Then watch the physics unfold:

Rod Cross, a physics professor at the University of Sydney, starred in a scientific paper on modelling a falling Slinky in 2012 (PDF).

A Slinky is a loose tension spring. If you let the whole thing uncoil and hang down, the tension is enough to hold up the bottom against the pull of gravity.

Because the Slinky is dropped from the top, Cross explained in his study, it takes time for the motion wave to travel down the spiral and “communicate” to the bottom part that the top part fell — basically, that the tension is no longer there.

The time required for normal Slinkies to collapse, he calculated, is about 0.3 seconds. During that time the bottom basically floats in air.

Muller further explained what’s happening in the video:

“What’s interesting to note about this phenomenon is it’s not just a property of Slinkies. It’s a property of all objects. You can have a really long steel rod and when you let go of the top, the top really starts accelerating down first, and the bottom second. It takes time for that relaxation to travel through any material. You need that compression wave to basically pass through the entire object. A slinky just makes that nice and visible for us.”

Here it is again, from the top of the Googleplex:

See full coverage of the Fair in the video below. Muller’s segment starts at about 11:30.

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