134 20.7Newton Puts Them Together Newton’s major breakthrough was to show that these two different kinds of motion can be thought of as different aspects of the same thing. He did this by introducing the idea of motion being affected by a force, then expressing this idea in a quantitative way. Galileo, of course, had been well aware that motion is affected by external forces. Indeed, his definition of natural horizontal motion explicitly states that it applies to the situation where such forces can be neglected. He knew that friction would ultimately slow the ball down, and—very important—a force pushing it from behind would cause it to accelerate. What he didn’t say, though, and Newton did, was that just as a force would cause acceleration in horizontal motion, the natural acceleration actually observed in vertical motion mustbe the result of a vertical force on the body, without which the natural vertical motion would alsobe at a constant speed, just like natural horizontal motion. This vertical force is of course just the force of gravity.
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