来自《耶鲁大学开放课程：基础物理 第5集 第5集》

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  and the difference between this one accelerating,
  和这个加速度不同的是
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  let us say, I go to the loop-the-loop and I drop an apple,
  比如说我坐在过山车上 掉了一个苹果
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  forget the track it's got m · g acting,
  不考虑轨道 它受到 m · g 的作用
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  it's accelerating too.
  苹果也在加速
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  What's the difference between these two?
  这两种情况有什么区别呢
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  Here, we start with zero velocity; you accelerate downwards.
  在这里的初速度为零 向下加速运动
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  That means you pick up a downward velocity.
  也就是说你获得了向下的速度
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  As it accelerates you keep
  随着速度增加 你会持续下落
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  falling down and picking up more and more speed.
  速度也越来越大
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  The same acceleration's experienced by the cart also,
  这里的车也有同样的加速度
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  and a little more because T is also pushing down.
  而且加速度还大点 因为 T 也是向下的
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  But there the change in velocity over a tiny amount of time
  但这个在短时间内变化的速度