来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  this would become an integral of F, from x1 to x2.
  就是 F 从 x1 到 x2 的积分
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  And if F really is a function of x
  如果 F 只是 x 的函数
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  and not of also velocity,
  而和速度无关
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  the laws of calculus tell you,
  微积分的法则会告诉你
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  you may write this integral as the difference of two numbers.
  你可以把这个积分写成两个数之差
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  Then, you rearrange the expression to get K1 + U1;
  然后 将表达式化为 K1 + U1
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  U1 is shorthand for U of x1;
  U1 是 U(x1) 的简写
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  is K2 + U2.
  等于 K2 + U2
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  And that means the energy E1 to begin with, the energy at the end.
  这表示初态能量 E1 和末态能量
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  If there is a frictional force,
  如果存在摩擦力
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  then in this integral you break it up into two parts:
  那么这个积分可以分成两部分
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  the part, say, due to a spring and the part due to friction.
  一部分关于弹簧 另一部分关于摩擦力