Sample question

On your way to class, you leave home and walk at 3 m/s east towards campus.

After one minute, and after covering 180 m, you realize that you left your physics assignment at home, so you turn around and run, at 6 m/s, back to get it. You're running twice as fast as you walked, so it takes 30 seconds to get home again.

For the 90 second out-and-back trip, what is your average speed?

  1. 4 m/s
  2. 4.5 m/s
  3. 5 m/s

The average speed is the total distance covered divided by the total time.
That works out to 360 m / 90 s = 4 m/s

What is your average velocity?

  1. zero
  2. 1.5 m/s West
  3. 3 m/s West
  4. 4 m/s West
  5. none of the above

Average velocity is the net displacement divided by the total time.
Since the net displacement is zero, vavg = 0.