So the spring is being pushed by mg here and mg here.

所以弹簧受到一个向下的 mg 还有一个向上的 mg
Therefore, it'll compress by an unknown x,

因此它的压缩量为 x
which is equal to mg divided by the force constant of the spring.

也就等于 mg 除以弹簧的弹性系数
By the way, that is a subtle thing people may not have realized.

顺便说一下有一个人们不曾发觉的微妙细节
Even in the case of this spring,

甚至是在这样的弹簧问题中
when you pull it, if you pull it to the right by some force.

当你拉它的时候如果你用力向右拉
Remember, the wall is pulling to the left with the same force.

记住这个墙也要向左用相同的力拉它
So springs are always pushed or pulled

所以弹簧两边永远都是受到
on either side with the same force.

相同的压力或拉力
We focus on one because we are paying for it,

我们只关注某一边因为我们只对这一边施力
but the wall is doing the opposite.

但是墙对另一边有反向的作用力
We don't pay any attention to that.

我们不用考虑墙的问题