Now I Know - Now I Know: Slinky Magic

A couple of days ago, one of my kids told me how Slinkys do this really cool thing if you drop them, if you set up the right conditions beforehand. I knew that already, because I had written about it -- in 2012! This is a re-run of that story. It flows a bit differently than other, more recent Now I Knows because I guess my writing style has changed over the years. Oh, and the best part? This is a science experiment that you CAN try at home! (And should. It's fun!) Enjoy! -- Dan
 

Slinky Magic

Take a Slinky or a generic version thereof — the bigger, the better. Find a balcony or window a few stories above ground, making sure that outside, there aren’t any passersby coming. Dangle the Slinky out the window until it is mostly still. You’ll want the other end of the Slinky to be about halfway to the ground — if it isn’t, go up another few stories. If you have the required distance, count to five and let the Slinky go.

The top will fall. The bottom will wait until the top gets there.

Don’t have a Slinky handy? Or can’t get to a third-story window? Watch the video below (or check out this animated gif):

Watch the Video

See? The top falls. The bottom waits.

What’s going on here? The Slinky comes with a small, barely visible jet pack which allows–  no, wait. It’s just physics, even if counterintuitively so.

Let’s start with gravity. Drop something — a ball, your cell phone (which certainly happens all too often), a Slinky, or anything, and gravity will start to pull it down. That’s pretty straightforward. It’s why the top of the Slinky immediately falls once released, and it’s why we expect the rest of the Slinky to fall as well. But that’s not the only force acting on the Slinky. There’s also the tension in the spring.

From the perspective of the Slinky’s bottom, the tension is an upward force. Literally, the tension is pulling the bottom of the Slinky back up toward the top. When you are holding the top end of the Slinky, tension is what keeps it from unraveling entirely and falling to the ground as it stretches and dangles. When you drop it, the spring’s tension doesn’t just disappear, It’s still there and, in this case, pulling up at the same rate that gravity is pulling it downward. So the bottom stays in place as the Slinky compresses.

But in the end, gravity wins. When the top and bottom meet, the tension goes to zero, and the bottom of the Slinky joins the top in its descent back to the ground.



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Bonus fact: The Slinky was created by accident. A naval engineer named Richard James was working with springs, hoping to find a way to counter the rough seas ships often encountered, which in turn caused their instruments to bounce around. A spring fell off a shelf and started tumbling around, “walking” from place to place, until it recoiled and came to rest. James tinkered with the tension until he created one which could walk down stairs.

From the Archives: Flying Signs: How physics makes it hard for airplanes to fly banners behind them (and how we get around that). Also, Just Say No to Gravity: The tale of a quixotic attempt to remove gravity from our lives.
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