As a runner reaches the finish line, she pushes harder with her leg muscles and speeds up for the last few yards

Newton's Second Law

Explanation:

The scenario explained above could be best described by Newton's Second Law.

Newton's Second Law state that when a net force is acted on an object, the force will lead to the acceleration of that object, such that force is equal to momentum change per change in time.

Hence, as the runner pushes harder with her leg muscles and speeds up for the last few yards, the speed is directly proportional to the force with which it pushes her leg muscles.

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