And that's really neat to think about, because photons, of course, are massless particles, they have no mass, so it's neat to think about something that has no mass, but that actually does have a momentum.

而且那真的不容易想明白，因为光子，当然是无质量的粒子，它们没有质量，所以这个真的不容易想明白,一些物体没有质量，但是它们事实上确实有动量。

麻省理工公开课 - 化学原理课程节选

So, massless bodies will always have, like a massless rope, equal and opposite forces on the two ends.

所以无质量的物体,比如这个绳子,两边所受的力永远都相等

耶鲁公开课 - 基础物理课程节选

We know there is no thing called a massless rope, but most ropes have a mass, but maybe negligible compared to the two blocks you are pulling, so we'll take the idealized limit where the mass of the rope is 0.

我们都知道无质量的绳子是不存在的,是绳子就有质量,但是相比于这两个物体就可以忽略不计了,所以我们有了这个理想的条件,即绳子的质量都是 0

耶鲁公开课 - 基础物理课程节选