Bill M. answered • 09/12/15

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Physics PhD: 25 years experience: courses from HS to University level.

To run 100 m from rest an athlete must first accelerate from rest to her top speed. The average speed during this time is less than her top speed. In fact, if her acceleration is constant for this period is half of her top speed. This makes her average speed for the entire 100 m less than her top speed.

If she runs in a relay she can accelerate to her top speed before she is passed the baton. Assuming, as before, that she can keep running at her top speed for 100 m, then her average speed will be equal to her top speed.

In brief: it takes some time to accelerate to the final speed: this lowers the average speed a bit for the entire 100 m.