NEWTON’S 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.

Topical Notes, Problems, Presentations, Quiz, Test, Investigations and Videos
6.1. Circular motion
6.2.Newton’s law of gravitation                                                       

Circular motion a motion in a circular path in which the speed of the body remains constant in magnitude, but where the direction in which the body is moving changes constantly. We shall show that this type of motion is produced by a force which is always constant in magnitude but continually changing in the direction in such a way that it is always at right angles to the velocity of the body.

Circular motion