Average velocity: Displacement divided by elapsed time.

Average velocity equals displacement divided by the time it takes for the displacement to occur.

For example, if it takes you two hours to move positive 100 miles (160 kilometers), your average velocity is +50 mi/h (80 km/h). Perhaps you drive a car at a constant velocity. Perhaps you drive really fast, slow down for rush-hour traffic, drive fast again, get pulled over for a ticket, and then drive at a moderate speed. In either case, because your displacement is 100 mi and the elapsed time is two hours, your average velocity is +50 mi/h.

Since the average velocity of an object is calculated from its displacement, you need to be able to state its initial and final positions. In Example 1 on the right, you are shown the positions of three towns and asked to calculate the average velocity of a trip. You must calculate the displacement from the initial to final position to determine the average velocity.

A classic physics problem tempts you to err in calculating average velocity. The problem runs like this: “A hiker walks one mile at two miles per hour, and the next mile at four miles per hour. What is the hiker's average velocity?” If you average two and four and answer that the average velocity is three mi/h, you will have erred. To answer the problem, you must first calculate the elapsed time. You cannot simply average the two velocities. It takes the hiker 1/2 an hour to cover the first mile, but only 1/4 an hour to walk the second mile, for a total elapsed time of 3/4 of an hour. The average velocity equals two miles divided by 3/4 of an hour, which is a little less than three miles per hour.