Final answer:
Periodic motion occurs when motion or sound repeats at regular intervals, exemplified by a vibrating guitar string with a consistent period for each oscillation.
Step-by-step explanation:
The recurrence of motion or sound at regular intervals occurs in periodic motion. An example of this is when a guitar string vibrates after being plucked, producing a steady tone with successive vibrations taking the same amount of time. This constant time to complete one cycle is called the period (T). In physics, periodic motion is fundamental to understanding oscillatory systems such as springs, pendulums, and sound waves. The frequency (f) is related to the period and is defined as the number of oscillations per unit time.