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  • Have you ever noticed that some rounded stones are almost spherical, while others are kind

  • of flat like a bar of soap?

  • The spherical stones form that way because they never have the chance to settle into

  • a preferred orientation: they're either sloshing around in pot holes, or rolling along the

  • bottom of a river, or if they're small enough, getting blown around by the wind.

  • Since they have no preferred orientation, these stones get weathered the same amount

  • on all sides, and they'll end up looking the same from all sides as well.

  • That's spherical symmetry!

  • But for flat stones, even if they're a smooth disk perfect for skipping, there's obviously

  • a preferred, or special, orientation - the direction perpendicular to, or 'away from'

  • the flat sides.

  • Where did this preference come from?

  • And how do flat stones get flat?

  • Gravity.

  • It's kind of like the difference between dumplings and biscuits.

  • A dumpling is a lump of dough dropped in a pot of water, and gravity doesn't really affect

  • its orientation very much when it's rolling and bobbing about.

  • A biscuit, on the other hand, is a lump of dough pulled down by gravity onto a baking

  • sheet, so it tends to be flat and has a clear top and bottom.

  • And while the details are ultimately more complicated, when sedimentary rock forms,

  • it forms layers, or when stones too heavy to roll are worn down by abrasion at the bottom

  • of a river or lake, they tend to be worn a bit flat.

  • In both cases the preferred direction is dictated by gravity.

  • And gravity is also why we build buildings in layers, with a basement, floors and a roof

  • if we were in outer space without a preferred direction from gravity, we'd probably just

  • live in blobs.

Have you ever noticed that some rounded stones are almost spherical, while others are kind


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B1 中級

SMBC物理学。石はなぜ丸いのか?ザック・ヴァイナー (SMBC Physics: Why are stones round? feat. Zach Weiner)

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    林宜悉 に公開 2021 年 01 月 14 日