来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  One says, aim at your enemy and fire like this.
  一种是 直接瞄准敌人 像这样射击
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  Then it lands on your foot,
  然后它就会落在你的脚边
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  because assuming the cannon is at zero height,
  因为假设大炮的高度为 0
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  the cannonball certainly comes out towards the enemy,
  炮弹笔直地出膛打向敌人
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  but has no time of flight.
  却没有时间飞行
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  Other one says, maximize the time of flight
  另一种是 让飞行时间最长
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  and you point a cannon like this.
  像这样进行瞄准
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  It goes up, stays in the air for a very long time,
  它先上升 在空中停留很长时间
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  but it falls on your head.
  但是它会直接落在头上
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  We know the truth is somewhere between 0 and 90.
  最适宜的角度应该介于 0° 到 90° 之间
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  Now, the naive guess may be 45,
  现在 简单的猜测是 45°
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  but it turns out the naive guess is actually correct.
  但事实上这样的猜测是正确的