1. The problem statement, all variables and given/known data In the attachment. 2. Relevant equations 3. The attempt at a solution First of all, sorry for the English, I'm from Israel, speak Hebrew here. Ok, I found v = (-w*R*sin(wt), w*R*cos(wt)), a = (-w^2 * R*cos(wt), -w^2*R*sin(wt)). I did tg to ay / ax and found the angle of a. The problem is when I do tg to v, I come out with tg = -ctg, how can I solve that? Hope u got it, 10x in advance.