Newton’s second law of motion explains that a net force on an object causes an acceleration of the object and that the acceleration is proportional to the net force. In this section we see the rotational analog of Newton’s second law—the angular acceleration of a rigid object rotating about a fixed axis is proportional to the net torque acting about that axis. Before discussing the more complex case of rigid-object rotation, however, it is instructive first to discuss the case of a particle moving in a circular path about some fixed point under the influence of an external
force, Figure 22.

Consider a particle of mass m rotating in a circle of radius r under the influence of a tangential force F_t and a radial force F_r, as shown in Figure 22. A force F_r in the radial direction must be present to maintain the circular motion.