If your plan is to accelerate a 3 kg mass with an acceleration of 2 meters per second, you better have a rope that can furnish that force and it can take the tension of 6 Newtons.

如果你想用 2 米每秒平方的加速度,去拉一个 3 千克的物体,那么你就需要一根能提供拉力的绳子,并且它能够承受 6 牛的张力

耶鲁公开课 - 基础物理课程节选

kg is being pulled by the rope on the right with a force that I'm going to call T, which stands for tension.

千克的物体受到绳子向右的一个拉力,我称它为 T,也就是拉力的意思

耶鲁公开课 - 基础物理课程节选

We'll give it a name, but by linking that T to that T, we can also figure out the same T must be exerted on this one by Newton's third law, because if this block is the only one that could be pulling this rope with T.

我们先给它命个名,把这个 T 和那个 T 联系起来,利用牛顿第三定律,我们可以知道这个 T 也一定作用在这个物体上,因为如果这个物体是唯一,受到绳子拉力 T 的物体

耶鲁公开课 - 基础物理课程节选