A world-class sprinter accelerates to his maximum speed in 4.0 s. He then maintains this speed for the remainder of a 9.1-m race, finishing with a total time of 9.1 s.
(a) What is the runner's average acceleration during the first 4.0 s?
(b) What is his average acceleration during the last 5.1 s?
(c) What is his average acceleration for the entire race? (d) Explain why your answer to part (c) is not the average of the answers to parts (a) and (b).