The piston has a cross-sectional area, a, and the force pressure is force per volume area.

我们以活塞系统为例，来说明膨胀功。

麻省理工公开课 - 热力学与动力学课程节选

It's a more powerful method than something called a cross-sectional study, and I'll come back in a later lecture and talk about these in more detail, but a cross-sectional study means you're taking a snapshot of a population of people at one time.

这种方法比所谓的横断面研究更加有效,我将在以后的课中,再仔细讨论这个问题,但横断面研究是指,你一次记录下一群人的状况

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

Well, a cross-sectional study would mean that you'd get a random group of the population and measure them in 1970 and then take another random group five years later and measure them, but it wouldn't be the same people necessarily, it would be different groups of people.

那么,横断面研究就是,1970年你在人群中随意选择一组人,并记录下相关数据,然后,五年后再随意选择一组人记录数据,但这不需要是同一组人,通常是不同的人

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选