Everyone loves a Slinky, especially scientists! Most people know them as cool springy things that can go down the stairs or end up in a giant tangled mess. We want to focus on something else: what happens when you dangle a slinky from the top until it is fully extended, then drop it? You might expect the whole slinky to fall to the ground. As you can see in the video, it’s a little more interesting than that. The bottom of the slinky stays completely motionless until the top of the slinky catches up. But why?
The slinky isn’t breaking physics or ruining science. It’s really just a loose spring. The more a spring is stretched, the more it pulls back towards the center. When the slinky is extended vertically like this, gravity pulls down on every part of it, including the bottom. Before the drop, we allow the slinky to hang until it stops moving, which means that the gravity pulling down on it is exactly countered by the spring force pulling back up.
The whole slinky is pulled down by gravity with the same acceleration. Both ends are also pulled towards the middle due to its springy characteristics. The top is pulled down by gravity and the spring, so it falls extra fast. The bottom, even as the slinky is dropped, is still being pulled up by the spring. With the bottom going nowhere, and the top going faster, it all averages out. The whole slinky together is falling at the exact rate that gravity dictates.
There is a bit more to this explanation. The slinky actually shows a wave and the transmission of information as well. The information from the top (“we’re falling!”) needs to get to the bottom before it can take effect there. You can see this wave– called a compression wave– if you watch the top of the slinky in the gif below. The spring clumps up as the fast-falling top part catches up with the parts below it.
There is nothing special about our Slinky. It doesn’t need to be rainbow colored or giant, any old Slinky will do the trick. Don’t believe us? Try this one at home! And when you’re done, take it to the stairs!