来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  In the y direction
  在 y 方向上
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  I write N - m · g · cos(θ) = m · ay.
  方程是 N - m·g·cos(θ) = m·ay
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  That's simply writing down Newton's Laws as two laws,
  这样就把牛顿定律写成两个公式
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  one along x and one along y,
  一个沿着 x 轴 一个沿着 y 轴
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  and putting in whatever we know.
  把已知的条件都加进去了
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  What happens next is up to what else we know.
  接下来要考察另外一些已知条件
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  Well, we know that this block is sliding down the hill,
  我们知道这个物体会从上面滑下来
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  it's not going into the block,
  不会到斜面里面去
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  nor is it flying out of the block; it's moving along the block.
  也不会飞离斜面 它会沿着斜面运动
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  That's the reason we chose a coordinate to be perpendicular to the block,
  所以我们才选择了一个垂直于斜面的坐标系
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  because that coordinate,
  因为在这个坐标系中
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  the y coordinate, it's not changing.
  y 坐标是不变的