Question

Which of the following are examples for periodic motion?
A) Oscillation of a pendulum
B) The motion of a swing
C) Motion of the earth around the sun
D) All of the above

Answer 1

Hint: Every object in the universe is in the state of motion or in a state of rest. The term motion and rest are relative to each other. In some periodic motions, a body moves to and fro along a given path between extreme positions. It means after some time, we can see the particle(body) in the same position again.

Complete step by step answer:
Most of the time we get confused with periodic motion and oscillatory motion. Both are different.
The motion which repeats at regular intervals of time is called ‘periodic motion’.
The periodic motion in which a body moves to and fro about a fixed point is called ‘oscillatory motion’.
All periodic motions need not be oscillatory but all oscillatory motions are periodic.
In a given question we can differentiate periodic motion as well as oscillatory motion.
All options represent the periodic motion. It repeats at a regular interval of time. That is the oscillation of a pendulum, motion of a swing both are to and fro motion. That is, particles move from mean position to extreme position again from an extreme position to mean position. This process takes place for regular intervals of time. Similarly, the motion of the earth around the sun is purely in periodic motion. It is not oscillatory motion.
Thus, we can say all the three options are periodic motion. Oscillatory motion is only the oscillation of a pendulum and the motion of a swing.
There are a few examples for better understanding.
Examples for oscillatory motion which are periodic:
(i) Swinging a pendulum in a clock.
(ii) To and fro motion of atoms in a substance.
(iii) The vibration of strings in a guitar.
Examples of periodic motion which are not oscillatory:
(i) Uniform circular motion
(ii) The motion of planets in the solar system is orbital.

$\therefore $ We can say all the options are correct. So, option (D) is correct.

Note:
The "period" is the smallest interval of time after which the motion is repeated. It is denoted by the $t$ and the $SI$ unit is $\text{second}$. The reciprocal of the time ($t$) gives the number of repetitions that occur per unit time. This quantity is called the frequency ($f$) of periodic motion. Its unit is $Hz$ or $\text{ per second.}$
We always need to remember that “All periodic motions need not be oscillatory but all oscillatory motions are periodic.”