1. The problem statement, all variables and given/known data I am following along my classical mechanics book by John Taylor on page 27 in anyone has it. He is saying: del r^hat ~ del phi * phi^hat then.. del r^hat ~ D[phi] * del t * phi^hat Then... dr^hat/dt = D[phi] * phi^hat Sorry for the horrible formating but I don't know how to make this better. 3. The attempt at a solution I have a few problems with this first why in the first equation is the phi needed? It would seem to me that del r^hat would just be equal to del phi^hat and the magnitude of phi would not be needed. Then how does the phi just turn into a derivative of phi in the second equation and now there is a del t? I understand the last line.