There's the hard sphere. It wants to collide against the wall to create a force on the wall, and I have a couple of the hard spheres that are nearby, right, and in the absence of any interactions, I get a certain pressure.

给墙一个作用力,现在它的附近，还有许多小球,没有相互作用时,它将直接冲向这面墙。

麻省理工公开课 - 热力学与动力学课程节选

There are no other molecules on that side of the wall. So there's an attractive force that makes the velocity within not quite as fast.

墙的另一边没有分子,因此吸引力,会降低球的速度。

麻省理工公开课 - 热力学与动力学课程节选