来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  to the force pointing towards the center,
  指向圆心的力
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  and Newton has told us what the force is: G·M·m / r^2.
  牛顿已经告诉我们这个力是 G·M·m / r^2
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  Everybody understand now?
  大家都明白吗
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  The left-hand side is the effect,
  等式左边是效果
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  the right-hand side is the cause.
  等式右边是原因
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  If you're spinning a rock, spinning it in a circle,
  如果你甩动一块石头 甩成一个圆
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  that also has an acceleration toward the center
  石头就会有一个指向圆心的加速度
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  but the string is providing the force
  但提供力的是这根绳子
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  and you can work out the force of a spring, of the string.
  你可以求出绳子施的力
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  Here, it's the unseen force of gravity
  这里就是一个看不到的
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  from the Sun reaching out to a planet
  来自太阳对这个行星
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  and pulling it in.
  施加的引力