Hello Karling,  Good morning,  when Object or any substance is moving repeatedly along a fixed point with fixed interval of time,  Such motion is know as periodic motion.
Same activities are performed repeatedly with fixed interval of time.
Real time exam of periodic motion is swing of pendulum,  Swing of chair.
I hope it will help you.
Thanks.

Hello,
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I will help you to understand in simple words, so that you can understand well and can write it on your own.
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Periodic motion means a motion repeating at a regular intervals of time. It means a thing which does same work at regular intervals.
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Examples,

• A clock ( minute, second,hour hands work at a regular interval of time)
• A pendulum ( same like a clock)

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If you are moving around the chair repeatedly in the same time without creating disturbance in the time, then even you are in a periodic motion

Dear Aspirant,

periodic motion is a repeated motion, which repeated at equal intervals of time. For example, motion performed by a moving chair, a bouncing ball, a vibrating spring, etc. In each case the interval of time for a repetition, or cycle, of the motion is called a period, while the number of periods per unit time is called the frequency. Simple harmonic motion is a special case of periodic motion.

Frequency = 1/time period

Hope this helps. Thank you.