Sometimes they give you the speed in miles per hour, kilometers per year, pounds per square foot.

有时候速度的单位是英里/小时,千米/年,英磅/平方米

耶鲁公开课 - 基础物理课程节选

So, the number of nuclei, 119 if we were to sit and count these as well, is 119. So, we'll multiply that by just pi, r squared, to get that cross-section, and divide all of that by 1 . 39 meters squared.

如果你们数的话，原子核的数是，我们用它乘以πr的平方，得到横截面积，除以1。39平方米。

麻省理工公开课 - 化学原理课程节选