A proton has the acceleration a=CvxB, where a is the cross product of v and B (a,v,B are vectors). C is a constant and the magnetic field is constant. Prove that the magnitude of the v and a are constant. I know I have to somehow prove the derivative of a or v to be zero, but i have no idea how to start. i know if v is constant, then a must be 0. a=CvxB (da/dt)=C(dv/dt)xB i also wrote down |a|=|Cv||B|sintheta, but that doesn't help at all.