I read this interesting thread trying to find an answer to my questions (and I got even more confused). I study radiometry units in context of computer graphics. I have a few questions, starting from the basic ones: 1. Following AlexS's note, why is speed=distance/time and not distance*time? 2. Following the definition of speed, as in speed = how much distance passed for some time, I still get confused sometimes when power operators come in, for example acceleration = distance / seconds^2, or even worse when force=kg*m/s^2. Is it possible to straight on understand the logic of the conjunction of all these units? 3. In my book, they say that differential irradiance=radiance*differential solid angle*cos(theta). Say I did get the meaning of division in physics (how much x for y, kinda..), but what does multiplication mean? Aside from the fact in order to get the irradiance unit correct, the radiance must be multiplied by a solid angle, but what does that multiplication mean? [Mentor's note: One unrelated question moved to its own thread] I know some of these questions might sound silly to you, but they have been in my head for a long time, so straightforward answers will sure help.