Speed velocity and acceleration
Unless he is travelling on a motorway, a motorist cannot usually maintain a constant speed for any length of time. In the ordinary way, traffic conditions frequently cause him to change speed or stop. When deciding the time to allow for a particular journey, the motorist must therefore have some idea of the average speed at which he will be able to travel.
Speed is defined as the rate of change of distance moved with time. Thus, if a journey from London to Birmingham, 176 krn, takes four hours, distance 176
Average speed = 4 = 44 km time
During such a journey, however, the actual speed of the car at any instant will vary considerably from this figure.