来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  I apply F = m · a,
  应用公式 F = m·a
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  that's it;
  就是这样
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  and I'm going to get everything by applying F = ma,
  利用 F = m·a 解决所有问题
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  to this guy.
  对这个物体
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  So, here is that mass.
  这是小球的质量
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  The tension on the rope can only be along the rope.
  拉力只沿着绳子方向作用
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  The rope is not a rigid object.
  绳子不是刚体
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  It cannot exert a force perpendicular to itself.
  不可能产生垂直于自己的力
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  It can only pull it along each direction.
  力的方向只能沿着绳子
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  That's the tension on the rope.
  那是绳子上的拉力
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  There is m · g, and that's it.
  这是 m·g 就是这样
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  Under these things it's going to obey F = m · a.
  根据这些条件必须满足 F = m·a