Final answer:
The displacement of a runner completing a 400-m track lap and returning to the starting point is zero, because displacement measures the change in position, and there is no net change in starting and ending positions.
Step-by-step explanation:
The displacement of a runner who completes one lap around a 400-m oval track and returns to her starting position is zero. Displacement is defined as the change in position of an object and is a vector quantity, which means it has both magnitude and direction. Since the runner ends up at the starting position, the initial and final positions are the same, resulting in no change in position. Thus, the magnitude of the displacement is zero. It's important to note that displacement is different from distance which is a scalar quantity that represents the total ground covered. The runner's distance is 400 meters, but the displacement is zero because the start and end points are the same.