来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  that was given by integral of F times dx
  那么它等于 F 乘以 dx 的积分
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  and the integral of F times dx from x1 to x2.
  积分的下限是 x1 上限是 x2
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  If F is a function only of x,
  如果 F 是只关于 x 的函数
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  from the rules of calculus,
  那么根据微积分的运算法则
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  the integral can be written as
  积分可以写成
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  a difference of a function at this limit minus that limit,
  某函数在这一点的极限减去在另一点的极限
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  and that was U(x1) - U(x2),
  即 U(x1) - U(x2)
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  where U is that function whose derivative with a minus sign is F.
  函数 U 的导数的相反数为 F
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  That's what we did.
  这就是我们上一课的内容
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  Then, it's very simple now to take the U1 to the left-hand side.
  接下来就很简单了 把 U1 移项到左边
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  Let me see, K1 to the right-hand side and U2 to the left-hand side,
  不对 K1 移到右边 U2 移到左边
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  to get K2 + U2 = K1 + U1 and that's the conservation of energy.
  可以得到能量守恒定律 K2 + U2 = K1 + U1