来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  The rope is not a rigid object.
  绳子不是刚体
• 
  
  It cannot exert a force perpendicular to itself.
  不可能产生垂直于自己的力
• 
  
  It can only pull it along each direction.
  力的方向只能沿着绳子
• 
  
  That's the tension on the rope.
  那是绳子上的拉力
• 
  
  There is m · g, and that's it.
  这是 m·g 就是这样
• 
  
  Under these things it's going to obey F = m · a.
  根据这些条件必须满足 F = m·a
• 
  
  So, what I do with this tension is
  我对拉力的处理就是
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  I trade this guy for two forces.
  把它分解为两个力
• 
  
  This is T.
  这是 T
• 
  
  If that angle is θ; let me see,
  如果那个角度是 θ 我想想
• 
  
  that angle is θ; so this is T · sin(θ),
  那个角是 θ 所以这是 T·sin(θ)
• 
  
  and this is T · cos(θ).
  这是 T·cos(θ)