Thursday, May 26, 2016

Oh, patents! The Slinky®

Copyright Françoise Herrmann


If you are interested in a patented toy based on enough Newtonian mechanics to make you marvel without ever having to master any of it, then just try the Slinky® !

The Slinky® is a spiral or helical spring toy that unravels and bounces back and forth from one hand to another, from one step to another down the stairs, or just across a flat surface! Slink (!), slink (!), slink (!)…. It will probably also drive your pooch wild!...

Richard T. James, an American naval engineer and the inventor of the Slinky® recited this invention in 1946 in the patent US2415012 titled Toy and process of use. The toy is purported to have been discovered accidentally when the engineer dropped a tension spring to the ground during a naval research project (Wikipedia). However, beyond the accidental spark of inspiration, there is enough embodied physics theory to build an altar.

US2415012 explicitly recites the theoretical mechanical properties of the spring invoked in the design of the toy, and also incorporates a description of how to manipulate the spring for entertainment purposes.

In particular, the invention invokes the transfer of turns from one end of the spring to the other, when the spring is bent into a semi-circular form, and the ends are moved up and down (see the patent figure drawing below). The invention also invokes a starting force and the force of gravity for the spring to “walk” down steps or across a plane surface. In particular, the inventor provides further specification of the equations of the oscillating system to explain how the spring actually “walks” on a plane surface after it has gained momentum from the initial bending step of the spring through a 180-degree arc.  

In other words…. the Slinky® gracefully tumbles down stairs as its potential energy is converted into kinetic energy (i.e.; energy-in-motion) under the force of gravity, depending on a series of physical properties such as “the radial cross-section relative to the axial cross-section of the spring” and “the compression or tension between turns”, in turn expressed as the Mass (m) Constant (C) and Stiffness (k) of the spring    

Otherwise, the actual generation of energy-in-motion that explains how the Slinky® “walks” is in fact calculated using sound wave physics, where the energy transferred from one coil to another is seen as transferred along a longitudinal or compressional wave,  which “resembles a sound wave that travels through a substance by transferring a pulse of energy to the next molecule.”

Below, figure drawing no, 4 extracted from US2415012 titled Toy and Process of use and above an image of the marketed product.


The Slinky® is still marketed today, 70 years after it was patented, and sold as “Slinky®, the walking spring toy” by Slinky®, which is part of the Alex Brands family of toys.

To date more than 3 million Slinkys® have been sold, and many more people continue to marvel at the Slinky® phsyics embodied in this toy!

References
Wikipedia - Richard T. James
Slinky physics - Michigan Reach Out

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