In the example of an athlete running back and forth on a straight track, when will the displacement of the athlete be zero? What will be the total distance travelled in that case?                                          [Pg. No. 51]

An athlete starts from O (0 m) at $t = 0 \text{ s}$. Reaches A (100 m) at $t = 10 \text{ s}$. Then runs back and reaches B (40 m) at $t = 16 \text{ s}$. Displacement becomes zero when the athlete returns to the starting point O. Total distance travelled = $100 + 100 = 200 \text{ m}$. Hence, when the athlete returns to the starting point, the displacement becomes zero and the total distance travelled is 200 m.