Consider the undamped free vibration represented by the block and spring model in the figure below. The time-dependent path of motion of the block can be given by kx = m(d2x/dt2). As can be seen from this equation, the acceleration (d2x/dt2) is directly proportional to the block's displacement. A motion that is described in this manner is called ________________. Group of answer choices dual harmonic motion variable harmonic motion periodic motion simple harmonic motion