1. The problem statement, all variables and given/known data The problem says to show that the dot product u dot a = u*u-dot where u-dot is the differentiation with respect to time. How is u-dot different from just the acceleration? My teacher said that "a is the magnitude of the 3-acceleration. u-dot is the time derivative of the speed. In the first case, you first differentiate the velocity with time and then take the magnitude. In the second case, you first take the magnitude and then differentiate with time. These are not necessarily the same. For example, consider uniform circular motion. In that case, u-dot is always zero, but a is never zero." but I am not sure I understand this... 2. Relevant equations N/A 3. The attempt at a solution No idea where to begin. I know that if u and a are perpendicular them the dot product is 0, but that is about it.