Average speed is useful in describing the motion of a body, but it does not tell everything
about the motion. For instance, a car traveling at an average speed of 100 kilometers per
hour during a certain hour could conceivably have traveled at 150 km/h for 40 minutes,
Energy, Ch. 3, extension 1 Calculating average speed and velocity 3
then halted for the last 20 minutes as a patrolman gave the driver a ticket (picture above).
Its average speed really does not describe that hour’s journey very well.
The way to deal with such a situation is to take the average over a shorter interval, and do
this more times. In the example given above, if the speed were sampled every 15 minutes
instead of over the entire hour, we would list average speeds of 150 km/h at the start (0 to
15 minutes), 150 km/h (15 to 30 minutes), 100 km/h (30 to 45 minutes), and 0 km/h (45
to 60 minutes).