来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  is they put bodies in various circumstances
  将物体置于不同的环境中
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  and they deduce various forces.
  得到不同的力
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  This is the force of gravity.
  这个是重力
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  This is the force of the spring.
  这个是弹簧产生的力
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  Here's another force you might find.
  你可能会发现还有一种力
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  You put a chunk of wood on a table
  你把一大块木头放在桌子上
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  and you try to move it at constant speed.
  试着让它匀速运动
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  Then you find that you have to apply minimum force.
  你就会发现 所需外力有最小值
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  We are moving at constant velocity.
  我们现在是让物体匀速运动
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  That means the force you're applying is cancelled by another force,
  意味着你施加在物体上的力被另一个力抵消了
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  which has got to be the force of friction.
  这个力就是摩擦力
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  So force of friction is yet another force.
  摩擦力就是另外一种力