来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  Here's a simple example;
  这是个简单的例子
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  you can show that if this mass and that mass are equal,
  你可以发现如果这个质量和那个质量是一样的
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  and say this one is at rest, that one comes and hits this.
  这个是静止的 那个过来撞击
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  You can show under certain conditions this one will come to rest
  在特定条件下 这个会停下来
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  and this will start moving with the speed of the--
  这个会以一个速度开始运行
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  the target will move at the speed of the projectile.
  目标以发射物的速度运行
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  So, momenta of individual objects have changed.
  个体的动量改变了
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  One was moving before, it is not moving;
  一个之前运动 现在不运动了
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  one was at rest, it's moving,
  一个之前静止 现在运动了
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  but when you add up the total, it doesn't change.
  但你加起来时 它没有改变
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  This is called the Law of Conservation of Momentum.
  这是动量守恒定律
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  So, that's so important
  这个定律非常重要