Why does slinky defy gravity?

in StemSocial3 months ago

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What if Newton sat under a slinky tree instead of an apple tree?
Would he still come up with the same inverse square law?

My toddler's Nana Ji (maternal grandpa) got her this slinky. This reminded me of this video by Veritasium. Veritasium showed that the bottom of the slinky would not start falling until the top of the slinky had collapsed upon it. If you look at the reiterative slow-motion videos I captured from my phone, you will notice that it is true. The bottom of the slinky stood still in midair as if gravity was not acting upon it. It was only after it completely collapsed it started to move down. I scratched my head for a while. It made no sense to me.

The link has the details of the physics behind it. Briefly, the answer is that Newton would have found the same laws of gravity even under the slinky tree. The slinky is like a spring. The tension in the stretched spring is holding it all together. The force of gravity is pulling it all down. But, the tension in the spring is pulling the bottom upside. When I release the slinky, the information about the loss of tension has to reach the bottom. And this information takes time to travel. Hence the bottom hangs in midair until the whole slinky is collapsed.

Discalimer: The first iteration is the slow-mo video clip directly shot via phone. The second and third iteration are slow-mo recordings of the slow-mo video themselves made for better visulisations. The music in the background is just me playing Kal Ho Na Ho tarck on guitar. You can also watch this video on my instagram

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Video by @gtg


The falling slinky
Modeling a falling slinky
How a slinky falls in slow motion

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Nice experiment! I remember my physics class when there was this type of experiment using springs. Thanks for the class

Thanks. Glad you enjoyed the class!


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