Final answer:
The runner is propelled forward by a reaction force from the ground as a result of them pushing backward on the ground, implementing Newton's third law of motion.
Step-by-step explanation:
The force propelling a runner forward when they are moving is called the reaction force. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When a runner's foot pushes backward against the ground, the ground pushes back with an equal and opposite force, which propels the runner forward. This action-reaction pair of forces is why the runner accelerates.
Moreover, during a sprint, when a runner exerts the maximum force in a short time while their foot is in contact with the ground, they generate a significant amount of thrust. This thrust is the reaction force that acts in the opposite direction of the push against the ground, and it adds to the runner's kinetic energy, helping them to maintain or increase speed during the race.
Using Newton's second law (Fnet = ma), we can also calculate the average force exerted by the ground on the runner to produce the required acceleration, assuming we have their mass and the acceleration information. Factors such as air resistance are typically neglected in these simplistic models.