来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  by definition it cannot exert a force along its own length.
  按照定义 它就不能产生一个沿着斜面的力
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  So it can only exert a force in this direction
  所以只能有一个这个方向的力
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  and we're going to call that N.
  我们把它记作 N
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  Then, there's only one other force;
  这样 就只剩下了另外一个力
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  that's the force of gravity which we agreed we have to remember.
  也就是之前强调要牢记的重力
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  Even though the Earth is not touching this block,
  虽然地球没有碰到这个物体
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  the Earth is somewhere down here,
  地球在下面某个地方
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  it's able to reach out and pull this block down.
  但它确实能影响到这个物体 把它往下拉
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  So, let me show you the forces.
  我来把这些力画出来
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  So here is N, and here is m · g,
  这个是 N 这个是 m·g
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  these are your forces.
  就是这些
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  And with no friction, that's it.
  没有摩擦力的话 就是这样