Friday is here, and with it the weekly "A Taste of Physics" column — number 42.
This week: the fascinating physics of suspension bridges, and their connection to bicycles, spiderwebs, and birds.
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Suspension bridges are remarkably striking structures.
From a distance, the cables appear to hang as naturally as a clothesline, as though they aren't supporting the enormous weight of thousands of tons.
Yet behind this elegant design lie some genuinely interesting physics.
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The spacing between the towers and the shape in which the cables hang serve a single purpose — to minimize the tensile forces acting on the cables.
If the cables were stretched horizontally, the towers would have to pull them with tremendous force to counteract gravity.
If the towers were too close together, more weight would be concentrated at the center of the cable rather than being transferred to the supporting towers.
The precise spacing allows the cables to hang in a way that balances their own weight against gravity, giving the bridge maximum stability.
Spiderwebs, high-voltage power lines, and playground swings all operate on the same principle.
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Designing a suspension bridge requires special attention to preventing resonance.
Every material has a "natural frequency" at which it responds.
This means the material resonates at certain frequencies and amplifies them — much like the strings of a musical instrument.
A suspension bridge that responds to the frequency of the wind, for example, can amplify its own oscillations more and more until it collapses — something that actually happened in the United States in 1940.
By the same principle, loudspeaker enclosures are designed so they don't resonate at the speaker's own frequency, and buildings are designed so they don't resonate at the ground frequency during an earthquake.
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Another interesting principle in bridge construction is triangulation.
The towers, cables, and bridge deck together form a triangular shape that transfers the maximum load to the bridge's support columns, making the structure significantly stronger.
A bicycle frame is built as a triangle for exactly the same reason, and in nature this structure can be found in the wing bones of birds.
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In the video: the world's highest suspension bridge, expected to open this year in China.
Shabbat Shalom 😊
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