Over 2000 years ago, Aristotle incorrectly claimed that a force, a push or pull, was needed to sustain motion. Galileo and Newton got it right: They recognized that uniform motion—straight-line motion at constant speed—is a natural state that requires no explanation. What does require explanation is any change in motion. That’s what forces do: they cause change in motion, which can be in speed, direction, or both. Newton summarized the role of force with his 3 laws of motion.