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Physics · 术语表

什么是 Gravitational field？

Definition 14.13 High School Physics · 第 14 章 — Electric and Gravitational Fields

If a test mass $m$ placed at a point feels a gravitational force (a weight) $\vect P$ , the gravitational field there is

\vect g = \frac{\vect P}{m} \qquad \text{(in $\mathrm{N}/\mathrm{kg}$)},

and conversely $\vect P = m \vect g$ . An earlier chapter introduced the number $g$ ; $\vect g$ adds the direction a dropped stone starts to move.

The Earth’s gravitational field: radial, pointing at the center, of magnitude GM/d2 — the exact portrait of the field of a negative point charge. — The Earth’s gravitational field: radial, pointing at the center, of magnitude $GM/d^2$ — the exact portrait of the field of a negative point charge.

示例

Example 14.15 (The field where the Moon lives)

At the Moon’s distance $d = 3.84 \times 10^{8}\,\mathrm{m}$ :

g = 6.67 \times 10^{-11} \times 5.97 \times 10^{24} / (3.84 \times 10^{8})^2 \approx 2.7 \times 10^{-3}\,\mathrm{N}/\mathrm{kg}.

Earth’s field never stops — it only thins out as $1/d^2$ ; this is the field that holds the Moon on its orbit.

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