Introduction

Circular motion is one of the two related-but-different kinds of motion this chapter is built around. When every particle of an object moves in a circle about some point or axis that lies OUTSIDE the object itself, we call it revolution -- the Earth going around the Sun, or a stone whirled at the end of a string. When an object turns about an axis that passes THROUGH the object itself, we call it rotation -- a fan's blades, a spinning top, or a wheel on its axle. Both share the defining geometric feature of circular motion: every particle traces a circular path. The first part of this chapter (sections 1.2 to 1.4) revisits and extends the circular-motion ideas built in Class XI -- kinematics, centripetal/centrifugal force, and a set of important real-life applications (vehicles on flat and banked roads, the well of death, the conical pendulum, and vertical circular motion). The second part (sections 1.5 onward) then builds the full machinery of ROTATIONAL DYNAMICS proper -- moment of inertia, radius of gyration, the parallel and perpendicular axes theorems, angular momentum, torque, and rolling motion -- which describes not just a single revolving particle but an entire rigid, extended object turning about an axis, with every one of its particles moving in its own circle simultaneously.