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
You cannot have a spring pulled only at one side,

你不可能对弹簧只施加一边的拉力
because then it will then accelerate with infinite acceleration

因为那样的话弹簧就会以无穷大的加速度加速
in that direction.

就在这个方向上
This spring is getting squashed on either side,

这个弹簧两边都会受到压缩
and it'll squash by certain amount x,

压缩量为 x
that depends on your mass,

它由你的质量决定
and that x will be turned into a motion of a needle

x 会转换成指针的运动
and that'll read your mass.

你从而可以读数