Speed: It is rate of change of distance covered by the body with respect to time
Speed = Distance travelled /time taken
Speed is a scalar quantity .Its unit is meter /sec. and dimensional formula is[M0L1 T-1]. It is positive or zero but never negative.
Velocity: The rate of change of position of an object in a particular direction with respect to time is called velocity. It is equal to the displacement covered by an object per unit time.
Velocity =Displacement /Time
Velocity is a vector quantity, its SI unit is meter per sec. Its dimensional formula is [M0L1 T-1]. It may be negative, positive or zero.
