One of the things he was bothered by was the long-held belief of Aristotle thatobjects, including the Earth, naturally come to rest unlessthey are forced to move byan external force(the motion of the other planets was considered special– they had theproperty ofquintessenceso that their natural state was to move in circles). One thingthat bothered Copernicus is that nobody could imagine a force large enough to keep theEarth moving around the Sun. Galileo and Newton understood that objects had inertia–that they tended to keep moving in a straight line at constantspeed (or sit still) unlessacted on by an external force– in the cases of objects ‘naturally’ coming to rest, frictionwas identified as the responsible force. So for the Earth, there doesn’t need to be anexternal force to keep it moving, just so long as it isn’t being affected by friction. As wewill learn in subsequent chapters, gravity is what keeps theEarth moving in an ellipsearound the Sun rather than just moving through space in a straight line.
