Friday is here, and the 'A Taste of Science for the Weekend' corner is back — number 78.
This time: the complex scientific engineering behind bungee jumping cords, and why jumping in summer is actually better.
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Bungee jumping is a sport that began on the islands of Vanuatu near Australia, before making its way to Britain and the rest of the world.
Originally, the bungee cord was made from vine branches, and it carried a very real risk of snapping or stretching too far — ending in a crash into the ground.
Modern bungee cords are far more sophisticated, enabling a relatively safe jump even from particularly nerve-racking heights.
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Bungee jumping is a genuine physics challenge, because it requires a precise balance: the jumper must be slowed to a stop before hitting the ground and then pulled back up — without being decelerated too abruptly.
As the jumper bounces up and down repeatedly until coming to rest, it's important that the number of oscillation cycles be as low as possible.
Surprisingly, synthetic polymers are not up to the task.
Synthetic materials tend to be slightly too stiff, braking the fall too quickly.
Additional problems are that stretched synthetic rubber doesn't fully return to its original shape — it stays slightly elongated — and in cold conditions it becomes rigid and brittle.
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To meet this challenge, modern bungee cords are built around a core made of thousands of thin strands of natural rubber, derived from the Brazilian rubber tree (Hevea brasiliensis).
Natural rubber is a viscous liquid, and to transform it into the familiar elastic material it undergoes vulcanization — a process of heating combined with the addition of sulfur.
The molecules of natural rubber are structured as coils that can be stretched. The sulfur forms cross-links between these coils, producing a more stable network structure that always tends to return to its original shape.
The drawback of natural rubber is its high sensitivity to UV radiation.
When exposed to sunlight its molecular structure degrades, so the core is encased in a sheath of synthetic nylon fibers.
The sheath protects the core from rapid deterioration while simultaneously preventing the rubber from overstretching.
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Two fascinating physical principles are involved in braking the fall.
The first is hysteresis — a process by which motion is converted into heat and energy is lost. The second is the lesser-known Gough–Joule effect, which describes the tendency of certain materials to contract when heated, rather than expand.
As the cord stretches, internal friction generates a great deal of heat. This heat is a consequence of hysteresis, which causes the elastic strain energy to dissipate rapidly — thereby reducing the number of bouncing cycles before the jumper comes to rest.
The Gough–Joule effect means that as the cord stretches and heats up, its tendency to contract increases sharply, so it brakes the fall more effectively.
Natural rubber also becomes stiffer at lower temperatures, which is why a winter jump may feel less elastic and comfortable compared to a summer one.
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In the mesmerizing video: the world's highest bungee jumping site according to the Guinness World Records.
It is located beneath the deck of a suspension bridge in Guizhou Province, China, at a height of 370 meters.
Shabbat Shalom 😊
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