On our spherical Earth, gravity pulls "down" us toward the planet's center of mass. So on a flat surface, we naturally stand up straight.

在我们地球上，重力会把我们拉向星球的质量中心，因此在平坦的地表，我们自然站得很直。

youdao

The common aim of all such systems is to represent an ellipsoidal shape on a flat surface — distortion in some places is almost inevitable.

这些系统的共同目标是在平面上表示椭圆形状 — 有些地方失真也是不可避免的。

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The app works best when your phone is resting on a flat surface, like a table.

该应用程序在手机搁在桌子之类的平面上效果最佳。

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It is a small device that you move around on a flat surface in front of a computer.

VOA: special.2010.08.01

"Well,one of the things that's interesting about these flexible sheets is that they can be curved, so you could imagine putting them in unconventional forms, like on the surface of a vehicle or something like that, where you don't have a flat surface."

VOA: standard.2010.02.19

Before, 50-year-old Trevor Prideaux had to balance the phone on his arm or put it on a flat surface.

FORBES: Man Installs Nokia Phone In His Prosthetic Arm

When placed on a flat surface, the stack redirects the waves such that reflected waves are exactly as they would be if the stack were not there at all.

BBC: Acoustic 'cloaking device' shields objects from sound

Because peregrine falcons don't build nests but lay their eggs on a flat surface, officials with the Port Authority of New York and New Jersey started protecting the eggs several years ago by building nesting boxes on the George Washington and the other bridges the authority operates.

WSJ: 4 falcon chicks living on NY-NJ bridge are banded