Final answer:
The action-reaction pair when a sprinter uses a set of starting blocks for the start of a sprint race is the sprinter exerting a force on the blocks, and the blocks exerting an equal and opposite force on the sprinter.
Step-by-step explanation:
The action-reaction pair when a sprinter uses a set of starting blocks for the start of a sprint race is:
Action: The sprinter exerts a force on the starting blocks backward.
Reaction: The starting blocks exert an equal and opposite force on the sprinter forward.
This action-reaction pair allows the sprinter to push off the blocks and propel themselves forward with more force, helping them achieve a faster start in the race.