To learn the basic terminology and relationships among the main characteristics of simple harmonic motion. motion that repeats itself over and over is called periodic motion. there are many examples of periodic motion: the earth revolving around the sun, an elastic ball bouncing up and down, or a block attached to a spring oscillating back and forth. the last example differs from the first two, in that it represents a special kind of periodic motion called simple harmonic motion. the conditions that lead to simple harmonic motion are as follows: there must be a position of stable equilibrium. there must be a restoring force acting on the oscillating object. the direction of this force must always point toward the equilibrium, and its magnitude must be directly proportional to the magnitude of the object's displacement from its equilibrium position. mathematically, the restoring force f_vec is given by \vec f=-k\vec x, where x_vec is the displacement from equilibrium and k is a constant that depends on the properties of the oscillating system. the resistive forces in the system must be reasonably small. in this problem, we will introduce some of the basic quantities that describe oscillations and the relationships among them. consider a block of mass m attached to a spring with force constant k, as shown in the figure (intro 1 figure) . the spring can be either stretched or compressed. the block slides on a frictionless horizontal surface, as shown. when the spring is relaxed, the block is located at x=0. if the block is pulled to the right a distance a and then released, a will be the amplitude of the resulting oscillations. assume that the mechanical energy of the block-spring system remains unchanged in the subsequent motion of the block. part aafter the block is released from x=a, it willremain at rest. move to the left until it reaches equilibrium and stop there. move to the left until it reaches x=-a and stop there. move to the left until it reaches x=-a and then begin to move to the right.