Which of the following is not an example of periodic motion?

CONCEPT:

• Periodic motion: When a body follows the same path after a fixed interval of time is called periodic motion.
• Some of the examples are the motion of the earth around the sun and on its axis both, pendulum motion, spring-mass system motion, etc.

EXPLANATION:

• Option 1: The earth moves around the sun. It is an example of periodic motion because
  • It follows the same path again and again. 
  • It follows its path after a fixed interval of time i.e. 1 year.
• Option 2: Simple pendulum is also an example of periodic motion because
  • It follows the same path again and again. 
  • It follows its path after a fixed interval of time i.e. time period of the pendulum.
• Option 3: Striking of strings of a guitar is not periodic motion because
  • Striking of a guitar is not done with a fixed interval of time.
  • Sometimes less gap, sometimes more gap after one strike. This depends on the song.
  • It is not necessary that after fixed time the same string will be struck.
• So the correct answer is option 3.