1. The problem statement, all variables and given/known data Show that for a particle moving with velocity v(t), if v . v'=0 then the speed v is constant. 2. Relevant equations 3. The attempt at a solution Let v = (v1,v2,...,vn) and let v'= (v'1,v'2,...,v'n) So, v . v'= v1v'1 + v2v'2 + ... + vnv'n = 0 This is the only way i can think of to go about it but i'm stuck here. What do i do to show v is constant when v.v'=0?