内容 向心力定律 · 18
Motte 1729 · reading base
Newton can begin to infer a force law once the curve, focus and areal speed are fixed.
一旦轨道、焦点和面积速度固定，牛顿就能开始反推力的规律。
命题
Proposition 6 · inverse-square entry shows how motion can be read by areas, tangents and departures from tangents.
命题 6 · 平方反比入口 说明运动可以由面积、切线和切线偏离量来读。
读法检查 worked example
题
Ask what changes if the same curve is read from a focus rather than from its center.
问同一条曲线若从焦点而不是中心来读，会改变什么。
解
The chosen center of areas becomes part of the force law, not a decorative point.
用来量面积的中心会进入力律，不是装饰性的点。
互链 context
与阿基米德穷竭法 archimedes-selections/overview、极限思想史 math-meta/topic-limit-history、微积分发明史 math-meta/topic-calculus-invention、柯西严格化 cauchy-cours/overview 和牛顿代数面 newton-arithmetica/overview 对读。
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