because when you buy a rope,

因为你去买绳子
they will tell you

他们会告诉你
how much tension it can take before it'll snap.

绳子能承受的最大拉力是多少
If your plan is to accelerate a 3 kg mass

如果你想用 2 米每秒平方的加速度
with an acceleration of 2 meters per second,

去拉一个 3 千克的物体
you better have a rope that can furnish that force

那么你就需要一根能提供拉力的绳子
and it can take the tension of 6 Newtons.

并且它能够承受 6 牛的张力
I'm going to give the last class a problem which is pretty interesting,

我将讲解最后一个例题非常有趣
which is what happens to you when you have an elevator.

也就是当你在电梯里的情形
Here is a weighing machine

这是个称重器
and that's you standing on the elevator.

然后你在电梯里
We're going to ask,

我们想要知道