其包含了三种情况：作用力与反作用力都不做功；
It contains three situations: both of force and reaction do not work;
还有一个例子证明作用力,与负反作用力相等。
That's another example where you see action equals minus reaction.
每一种作用力都有一种与之等量的反作用力。
Every kind of input strength has a reverse strength which is as big as it.
每一种作用力都有一种与之等量的反作用力。
Every kind of input strength has a reverse strength which is as big as it.
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