Meaning of Inertial Frame of Reference [Explained with Examples]

Imagine, you are standing on a road, waiting for a car.

Suppose that you notice a car running on a road.

While standing on a road, why do you notice that car going forward?

It’s because of your frame of reference.

(i.e., While standing on a road, you are in inertial frame of reference)

Similarly, 

Imagine, you are standing on a terrace, looking at the sky.

Suppose that you notice an aeroplane flying in the sky.

While standing on a terrace, why do you notice that aeroplane moving forward?

It’s because of your frame of reference.

(i.e., While standing on a terrace, you are in inertial frame of reference)

In a similar way,

Suppose that you are sitting inside a truck which is moving with constant velocity.

While sitting inside a truck, why do you notice other vehicles moving on a road?

Again, 

It’s because of your frame of reference.

(i.e., While sitting inside a truck, you are in inertial frame of reference)

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In above cases, 

Because you’re in an inertial frame of reference, you’ll notice a car, aeroplane and other vehicles moving.

The question is,

What is an inertial frame of reference?

Here,

I’ll explain to you the exact meaning of an inertial frame of reference. (In simple language)

Let’s quickly start off with the definition of inertial frame of reference.

Inertial frame of reference definition

In physics, the definition of inertial frame of reference can be stated as:

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference“

In simple terms,

A frame of reference which is not accelerated is known as the inertial frame of reference.

You might have not understood the above statement properly.

Don’t worry !!

Let’s consider some real life examples to understand the above definition of inertial frame of reference.

So you’ll get a clear idea.

Inertial frame of reference examples

Here are the Real Life examples of inertial frame of reference:

• A frame of reference of a bus

• A frame of reference of a ship

• A frame of reference of a bicycle

• A frame of reference of a cable car

• A frame of reference of a missile

Any object whose frame of reference is either at rest or moving with constant velocity is in an inertial frame of reference.

You might be thinking, HOW?

Let’s understand the above examples in detail. (One by One)

#1 A bus at rest

Imagine, one bus standing at the bus stop.

Note that a bus is initially at rest.

Suppose that you are sitting inside that bus.

While sitting inside a bus, can you guess your frame of reference?

It’s simple.

You are in an inertial frame of reference.

(Because, while sitting inside a bus, your frame of reference is not accelerated)

In short, 

When a frame of reference is not accelerated, it is known as the inertial frame of reference.

#2 A bus moving with constant velocity

Imagine, you are sitting inside a bus.

Suppose that a bus is moving with constant velocity on a road.

Now, let me ask you one question.

While sitting inside a bus, can you guess your frame of reference?

The answer is simple.

You are in an inertial frame of reference.

As you know, a bus moving with constant velocity doesn’t mean that it is accelerated.

In short,

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference”

That’s how you can remember the definition of inertial frame of reference.

#3 A ship at rest

Consider one ship standing at the port.

Note that a ship is initially at rest.

Suppose that you are standing on that ship.

While standing on a ship, what do you think of your frame of reference?

I think you have guessed the answer, right?

It’s simple. Look, 

When you are standing on a ship, your frame of reference is not accelerated.

So, you are in an inertial frame of reference.

#4 A ship moving with constant velocity

Consider a ship moving in a sea with constant velocity.

Suppose that you are standing on that ship.

While standing on a ship, can you guess your frame of reference?

The answer is simple.

Again, you are in an inertial frame of reference.

Because, while standing on a ship, your frame of reference is not accelerated.

(As you know, a ship moving with constant velocity doesn’t mean that it is accelerated)

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference”

#5 A bicycle at rest

Consider that you are standing on a ground along with your bicycle.

Note that a bicycle is initially at rest.

Now, let me ask you one question…

While standing on a ground, what is your frame of reference?

I think you have guessed the answer, right?

While standing on the ground, your frame of reference is not accelerated.

So, you are in an inertial frame of reference.

In the same way…

#6 A bicycle moving with constant velocity

Consider a bicycle moving on the ground.

Note that a bicycle is moving with constant velocity.

In this case, 

While moving on a bicycle, what do you think of your frame of reference?

Again, you are in an inertial frame of reference.

(Because, a bicycle moving with constant velocity doesn’t mean that it is accelerated)

In short, 

A frame of reference which is not accelerated is known as the inertial frame of reference.

Or you can remember it as:

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference”

#7 A cable car at rest

Imagine, you are sitting inside a cable car.

Note that a cable car is initially at rest.

While sitting inside a cable car, what do you think of your frame of reference?

I think you can now answer this question.

You are in an inertial frame of reference.

Because, 

While sitting inside a cable car, your frame of reference is simply at rest. (i.e., Not accelerated)

Similarly,

#8 A cable car moving with constant velocity

Suppose that you are sitting inside a cable car which is moving with constant velocity.

While sitting inside a cable car, can you guess your frame of reference?

It’s simple. Let me explain it to you.

As you know, a cable car moving with constant velocity doesn’t mean that it is accelerated. 

Also, your frame of reference is not accelerated, when a cable car is moving with constant velocity.

That’s why, you are in an inertial frame of reference.

In short, 

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference”

#9 A missile at rest

Let’s consider a real life situation.

Imagine, a missile is about to launch from the earth.

(Note that a missile is initially at rest)

When a missile is on earth, what is the frame of reference of a missile?

You know the answer, right !!

As a missile is simply standing on earth, the frame of reference of a missile is not accelerated.

That’s why, a missile is said to be in an inertial frame of reference.

In a similar way,

#10 A missile moving with constant velocity

Here, a missile is launched from the earth and it is moving upward in the sky. 

Suppose that a missile is moving with constant velocity.

At this moment, what do you think of its frame of reference?

Here also, 

When a missile is moving with constant velocity, the frame of reference of a missile is not accelerated.

And that’s why,

A missile Is considered to be in an inertial frame of reference.

So remember this way,

“A frame of reference which is at rest or moving with constant velocity is known as inertial frame of reference”

Is earth an inertial or non-inertial frame of reference?

From the definition of inertial frame of reference, it is clear that…

When an object’s frame of reference is not accelerated, an object is said to be in an inertial frame of reference.

Now, 

If we consider the object as earth, then what do you think about the frame of reference of earth?

Is earth an inertial frame of reference?

You might be thinking that earth is an inertial frame of reference, right?

(As the earth’s frame of reference appears to be steady, while standing on the earth)

If you are thinking so, you are wrong !!

Okay, let’s make it simple.

Imagine, you are in space and looking at the earth.

Now what do you think, is earth an inertial or non-inertial frame of reference?

The answer is simple.

While watching from space, the frame of reference of earth appears to be accelerated.

(As earth is rotating on its own axis as well as revolving around the sun)

So, earth is a non inertial frame of reference.

Conclusion,

In both cases, the earth’s frame of reference is accelerated.

(Either you are standing on earth or watching an earth from space)

That’s why, 

Earth is considered as a non inertial frame of reference.

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What do you think, is it easy for you to understand this concept of inertial frame of reference?

(Let me know in the comments below)