Imagine someone firing a gun horizontally, and at the same instant someone standing next to the shooter and dropping a bullet from the same height as the gun. When the two bullets reach the ground, they will be far apart. The one shot from the gun will have traveled hundreds of yards; the other will rest in the grass directly below the spot where it was dropped. Which bullet will hit the ground first? Surprisingly, both reach the ground at exactly the same moment. That’s what it means for the bullet’s vertical motion—its fall—to be independent of its horizontal motion. For Newton, that was
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