Bacteria move back and forth by using their flagella (structures that look like little tails). Speeds of up to 50 µm/s ⎛
observed. The total distance traveled by a bacterium is large for its size, while its displacement is small. Why is this?
When the flagella move first forward and then backward, the displacement will be small because the flagella is returning back to its original position. It is similar to going to school from home and back. Home is the starting position, then you go to school, and then you go back to the starting position. In this case, the displacement would be zero, depending if you did not stop for a snack at the ice cream shop on the way home. This is why the displacement will be small also for the flagella that returns constantly back to its starting position.