The Science of Slinkies

The Slinky's strange behaviour

Most of us have come across a “Slinky” sometime when we were growing up (who can forget the adorable Slinky Dog that was first introduced to the world in Toy Story?) The Slinky was invented by Richard T. James in 1943, an engineer who accidentally discovered it after experimenting with springs as a way to stabilise naval equipment (check out this article here to find out more).

More than just a toy to keep children amused, this unassuming rainbow-colored coil actually has a few interesting tricks up its sleeves.

(Taken from https://unsplash.com/photos/tvuFYFSyv2A)

In particular, a Slinky dropped vertically downwards exhibits pretty unique behaviour. Think about this for a second: What happens to a Slinky when it is dropped? Does the whole coil drop and collapse together once it reaches the ground? Or does it compress mid-air and then fall to the ground?

While too fast to catch for the naked eye, a slow-motion camera can easily capture the falling motion of a Slinky. And the results are pretty startling; check the video out below!

When the Slinky is released, the bottom coil remains suspended in mid-air, and only falls after the top of the coil reaches it. This seemingly counterintuitive falling motion can be explained by science.

Before the Slinky is released, the bottom coil is in a state of equilibrium. Gravity is pulling it downwards, while the tension in the coil is pulling it upwards, and these forces are equally balanced against each other.

Due to the length of the extended Slinky, when it is released, it takes time for the top coil to travel downwards towards the bottom coil, and for the bottom coil to “realise'' that there is no longer an upward force holding it up! Once the Slinky compresses on itself, there is no longer any force that is resisting the pull of gravity, and so the whole thing falls to the ground.

Very impressive indeed! A simple toy with a lot to teach us about forces. Another interesting motion of the Slinky that you might have come across is a Slinky “walking” down a flight of stairs, its own momentum continuously driving it forward. Can you figure out what is happening here?

To find other great science toys to play with, check out our class plans (we offer free trial classes on mechanical puzzles and science toys) and rental kits. Do contact us if you have any other queries!

References

Wolchover, Natalie. “The Cool Physics of 7 Classic Toys.” Published 2 December, 2011.

https://www.livescience.com/33614-the-cool-physics-of-7-toys.html