来自《耶鲁大学开放课程：基础物理 第3集 第3集》
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we don't know what force it exerts.我们不知道它施加了多大的力 -
We do know it exerts a force,我们只知道它施加了一个力 -
so here's what you do.所以你应该这么做 -
You hook one end of the spring to a wall你把弹簧的一端固定在墙上 -
and you pull it from rest by some amount然后拉伸一段距离 -
and you attach the one kilogram mass to it.接着你把 1 千克物体固定在上面 -
Yes?请说 -
A wall that will not move?固定的墙吗 -
You're right.你说的没错 -
What can actually happen其实真实的情况就是 -
is that if you're in an enormous laboratory,如果你在一个巨大的实验室里 -
which is made up of an enormous object,这个实验室由巨大的物体组成