OMG second time I'm opening a thread in the wrong forum FFS!!!! Damn bookmarks!! MODS move it please. 1. The problem statement, all variables and given/known data Not a home work question, just something i cam across and need a clarification. Could One show me how to derive to the equation that X(cm)=int(X dm)/M int=the deformed S of the integral(2 lazy to write in Latex XD). 2. Relevant equations X(cm)=sigma(Xi Dmi)/M 3. The attempt at a solution I know It's simple, But I can't imagine how sigma(Xi Dmi) becomes int(X dm), I don't understand what it means , trying to convert it to words just doesn't work for me,so could some 1 explain that for me? AFAIK sigma(Xi Dmi) means the mass distribution,but how does the integral takes place here? I really need to understand the math part in physics. Thanks a lot in advanced !