A particle is moving with constant speed if the magnitude of the velocity is independent of time. Thus v∙v = constant and this gives 2v∙(dv/dt)=0. Hence there are three possibilities: v = 0, or dv/dt = 0, or v is perpendicular to dv/dt. The third possibility shows that the velocity is not necessarily constant. One such example is when a particle is moving in a circle with constant speed, and then the acceleration is directed to the circle’s centre.