Periodic and Oscillatory Motions

In our lives, we come across various kinds of motions.

Some motions are rectilinear such as motion of a vehicle on straight road.

While some follow projectile motion, such as water falling from a certain height.

Both these motions don’t repeat themselves. But there are also some motions which repeat themselves after a certain period of time.

Such kind of motion is known as periodic motion.

Let’s try to further elaborate periodic motion.

First, let us have a look at some examples of periodic motion.

Our planet Earth takes 365 days to complete one revolution around the sun.

So, it repeats its motion after every 365 days. Hence, this is periodic motion.

Let us try to understand it further by taking one more example of a ball bouncing between our hand and the ground

Here, we can see that the ball performs periodic motion.

We know from Newton's equation of motion that the motion of the ball while falling down follows the above equation

Similarly, motion of ball returning back after hitting the ground can again is given by the above equation

If we keep on hitting the ball, it will go on repeating the motion. This motion can be graphically shown as above.

Now, let's see Oscillatory motion

Swinging on a swing.

The motion of spring.

Pendulum. All these perform a periodic to and fro motion. Hence they are considered as Oscillatory motion.

Oscillatory motion is a motion in which to and fro movement is done about its mean in a fixed interval of time or periodically

Oscillatory is also a Periodic motion.

Both Oscillatory motion and Periodic Motion are looking familiar , let's find out its differences

Difference between Oscillatory motion and Periodic Motion.

The main difference is that oscillatory motion is always periodic but a periodic motion may or may not be oscillatory motion.

The motion of the wheels of a car is only periodic, Because the wheels do not move to and fro about a mean position.

The Pendulum is both oscillatory motion and periodic motion

It is not necessary to have to and fro movement in periodic motion.

Let's take an example of the pendulum and elaborate oscillatory motion

When the pendulum is at this equilibrium point, no net external force acts on it.

So, when the pendulum is left at this point, it remains there forever.

If it is given a small displacement from the equilibrium position, there acts a force.

This force tries to bring back the pendulum again to the equilibrium position. The force is known as restoring force.

The displacement given to the pendulum and restoring force acting on it gives rise to oscillation.

Now, every oscillatory motion is periodic as it does repeat its to and fro motion after a fixed interval of time.

But every periodic motion does not necessarily mean that it will be oscillatory also.

Circular motion is periodic as there is repetition of motion but it is not oscillatory as there is no to and fro motion like pendulum motion.

Now, we shall try to know the difference between two similar things which are oscillation and vibration.

When a body does the to and fro motion slowly such as oscillation of a branch of tree, we call it oscillation.

But when the motion is very fast such as vibration of a string of a guitar, we call it vibration.

Vibration is quickly moving back and forth (or up and down) about a point of equilibrium.

Revision

The motion which repeats itself at regular intervals of time is called periodic motion.

The periodic motion also undergoing to and fro motion is known as oscillatory motion.

The motion of a ball hitting the ground and returning back can be shown as,

Every periodic motion is oscillatory but the reverse is not always true.

When the time period of motion of a body is large, we call it oscillation but when the time taken to execute the motion is small, we call it vibration.

The End