来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  friction is doing the maximum thing it can do;
  摩擦力也是最大
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  then, we can certainly say
  然后我们能够肯定地说
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  m · g · sin(θ) is then equal to the static friction times N,
  m·g·sin(θ) 就等于静摩擦系数乘上 N
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  which is now μs times m · g · cos(θ).
  现在即 μs·m·g·cos(θ)
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  Again, the mass cancels.
  同样地 质量被消掉了
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  So, it doesn't matter what kind of car you parked on the slope,
  所以不管你把什么车停在坡上 效果都一样
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  it only depends on the following.
  只和以下这些量有关
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  But g cancels too.
  这里 g 也消掉了
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  So, whether you park your car on the Earth or park it on another planet,
  也就是说 无论把车停在地球上还是别的星球上
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  this is going to be the same restriction,
  所受到的限制条件是相同的
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  which is that tan(θ) has to be less than or equal to μs;
  即 tan(θ) 必须小于等于 μs
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  equal to μs is the critical value,
  其中等于 μs 已是临界状态