i am using textbook in classical mechanics by douglas gregory. It is known that rate of change of displacement with time is velocity , but it is given that the velocity at any instant is the equal to the product of speed and its unit tangent vector at that instant in vector form i.e, V=vT (this expression is reasonable), but the method he proves this expression questions me, he says that rate of change of displacement with time is velocity i.e, dR/dt=V then he said distance is changing by time and by chain rule (dR/ds)(ds/dt) was that mean 's' is also the function of displacement? if yes how? another doubts: why it is displacement so important( i know vector concepts are abstract concepts)?, why the velocity is measured in meter per second and speed in kilometer per hour? we can use the distance as the measure of time but we can't use the displacement as the measure of time why?