Everyone loves a falling slinky

Dec 18, 2012
Everyone loves a falling slinky
Associate Professor Rod Cross experimenting with a falling slinky.

(Phys.org)—If a slinky is not at the top of your Christmas list you might want to think again. Making a slinky walk down steps or draping it over your arm to imitate a robot is fun but the coiled spring toy hides a more intriguing capability.

"Hold a slinky by its top and let it hang under gravity, and then release it. What happens next is not visible to your naked eye but, remarkably, the bottom section of the slinky remains suspended in midair until the entire top section collapses onto it," says Associate Professor Mike Wheatland, from the University of Sydney's School of , whose interview on falling slinkies has become a Youtube hit.

This video is not supported by your browser at this time.

He and Associate Professor Rod Cross, also from the School of Physics, have co-authored a paper published in the this month, which reveals exactly what takes place when a slinky falls.

"It might seem like a frivolous exercise, but in fact it provides us with an insight into the fascinating physics of everyday objects. The physics of a falling slinky also applies to a falling steel bar - with a steel bar it just happens over a much shorter time," said Associate Professsor Wheatland. "The bottom of the bar does not start to move until a fraction of a second after the top starts to move."

Wildly popular after its invention in 1943 by a naval engineer, and making a comeback through the character 'Slinky Dog' in the 1995 film Toy Story, the slinky still had secrets to be uncovered.

"The slinkies that star in our movies show that when dropped from a height they collapse from the top down. The bottom of the slinky remains hanging in space for an estimated 0.3 seconds," said Professor Rod Cross.

By filming real falling slinkies the researchers improved on a previous theoretical model.

"We took metal and plastic slinkies and used high speed cameras to capture their falling motion at 300 frames a second," said Associate Professor Cross.

Analysing the film frame by frame the researchers found that the tension in the slinky spring collapses from the top downwards, behind a wave front which runs down the coil. During the collapse the top of the slinky turns collide, one by one, as the tension is released at the wave front.

"It takes a finite amount of time for the information or 'signal' about the changed conditions to be transmitted from the top to the bottom of the slinky," said Associate Professor Wheatland.

"The falling slinky's behaviour might seem counterintuitive, but physics can be like that. Dynamics - the interplay of forces around us - often need to be examined to become clear."

Explore further: IHEP in China has ambitions for Higgs factory

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baudrunner
not rated yet Dec 18, 2012
Experimentation with spiraling EM fields around a rail gun assembly comes to mind, as a means to counter expectations of the behavior of gravity.
Ensa
not rated yet Dec 18, 2012
I didn't watch the video so maybe I missed the full explanation, but I don't understand how this is counter-intuitive. If you release them from the top then the top falls first. If you release them from the bottom then the bottom falls first. I am not seeing the counter-intuitive bit.
baudrunner
not rated yet Dec 18, 2012
You can't release them from the bottom if they are stretched to their limit, so far as gravity is concerned. If you do release them from the bottom then that means that the spring is fully compressed, and the object acts like the steel beam.

I think that you might be impressed by a full understanding of deBroglie, or matter waves. They get pretty freaky on the macro scale, and their freakyness is analogous to that of the above demonstration.
Ensa
not rated yet Jan 15, 2013
You can release objects from the bottom first. What happens then is that the bottom begins to fall first and then what is above begins to fall as it becomes unsupported (by pressure say).
So it is when you release from the top first. That falls and as the lower parts are unsupported (in this case by tension) then they fall.
Seems pretty intuitive to me.
VendicarD
not rated yet Jan 15, 2013
"If you release them from the bottom then the bottom falls first." - Ensa

And if you release them from the inside then the inside falls first.

Oddly though, if you release them from the before than the after falls first.