Speed is a scalar, whereas velocity is a vector. Speed has only magnitude, whereas velocity has magnitude as well as direction. Speed depends on the magnitude of the path length described by a body and the time taken to describe this path. Velocity, on the other hand, depends on the displacement of a body and the time taken for this displacement.
A body moving in a straight line is said to be in rectilinear motion. This motion can be represented by an x-t graph. A body starts at the origin and travels through a certain distance x in time t. Let us mark two points A and B on the graph obtained. x1 and x2 are the positions of the body at times t1 and t2, respectively. The displacement Δx = x2 − x1. The time interval Δt = t2 – t1. Velocity = Δx/ Δt.
If you observe the graph carefully, you will find that the displacement is different in different intervals of time. For calculating the average velocity, we take the total displacement and total time elapsed. In other words, it is the average of different velocities obtained at different instants of time.
Average velocity is the ratio of the change in position or displacement and the time interval in which the displacement occurs. Average speed is the ratio of the total path length and the total time taken for the motion.
The SI unit of both velocity and speed is m/s. The velocity of vehicles is generally expressed in km/h
When a body moves in a straight line in the same direction throughout its motion the magnitude of the displacement is equal to the total path length. In such a case, the magnitude of the average velocity is equal to the average speed.