Circular Motion and Gravitation
first law tells us that objects will move in a straight
line at a constant speed unless a net force is acting upon them.
That rule would suggest that objects moving in a circle—whether
they’re tetherballs or planets—are under the constant influence
of a changing force, since their trajectory is not in a straight
line. We will begin by looking at the general features of circular
motion and then move on to examine gravity, one of the principal sources
of circular motion.