Hello, - I am a CS student and I am trying to simulate movements of people using a theory described in Helbing's paper. I have put the paragraph in below address: www.host111.com/question/helbing.jpg - The problem is that I have passed college physics and math about 10 years ago ! Please Register or Log in to view the hidden image! and now need to understand above paragraph to be able to find a numerical solutions for it. (unfortunately I have been a software developer since then and shame that I have forgotten most of the things) 1- Would someone please help me understand this paragraph of the article? 2- What that Gradian mathematically and physically mean? 3- Would you please introduce me some quick references for a) physical side of it i.e potential functions b) mathematical side ie gradian etc? Thank you very much in advance. Mac