来自《耶鲁大学开放课程：基础物理 第11集 第11集》

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  I come to the floor now.
  现在轮到地面了
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  The floor is exerting a force N,
  地面对梯子施加了一个力 N
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  standard name for normal or perpendicular.
  这是法向作用力的常用名
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  Then, it's exerting a frictional force f, which will be inward.
  还对梯子施加一个向里的摩擦力 f
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  Because the ladder is trying to slide out,
  因为梯子趋于向外滑
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  you will apply a static frictional force inward
  你必须施加一个向里的静摩擦力
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  to keep it from moving.
  才能让它保持不动
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  That's it. These are the forces,
  就这么多 就是这些力
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  and subject to these forces.
  和受力的物体
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  We have to write down all the conditions.
  我们还需要写下所有的条件
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  This angle is θ here.
  这是角 θ
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  For the x motion, the fact that all the--
  对于 x 轴方向运动来说 事实上