Suppose that you are standing on the moon and looking at the earth.

Why do you notice the** earth appears to be moving** while looking from the moon?

It’s because of the frame of reference.

*(i.e., While standing on the moon, your **frame of reference is the moon**)*

Similarly,

Suppose that you are standing on the earth and looking at the moon.

Why do you notice** **the** moon appears to be moving** while looking from the earth?

Again, it’s because of the frame of reference.

*(i.e., While standing on the earth, your **frame of reference is the earth**)*

From the above cases, one thing is clear…

Because of the different frame of reference, you are getting different results.

In a similar way,

Suppose that you throw up a ball in a moving train.

If you look at the ball from inside a moving train, you’ll see a **ball simply going up and down**.

Because

When you’re inside a moving train, your frame of reference is the train.

But,

If you look at the ball from outside a moving train, you’ll see a **ball following a parabolic path**.

Because

When you’re outside a moving train, your frame of reference is the ground.

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Again,

Because of the different frame of reference, you are getting different results.

So the question is,

**What is a frame of reference?**

Don’t worry.

Here, I’ll make you understand the whole concept of the frame of reference. (Step by Step)

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

## Frame of reference definition

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

*“A **coordinate system** and a **set of** **physical reference points** that can be used to determine the position and velocity of an object within that frame is known as the **frame of reference**“*

In simple words,

A frame of reference is just a** viewpoint** from which the position and velocity of an object are measured.

**Not understood properly?**

Let’s take some practical examples to understand the above definition of frame of reference.

*(You’ll surely get an exact idea)*

## Frame of reference examples

Below are some examples of frame of reference, from which you’ll clearly understand the exact meaning of a frame of reference.

- While riding a skateboard,
**skateboard**is the frame of reference

- While driving a car, a
**car**is the frame of reference

- While sitting inside a moving train, a
**train**is the frame of reference

- While flying in an aeroplane, an
**aeroplane**is the frame of reference

All these are examples of frame of reference.

*(You might have experienced any of the above situations in your life)*

Let’s understand the above examples in detail, so you’ll get an exact idea.

### #1 Skateboard

**Imagine**, you are riding a skateboard.

Suppose that while riding a skateboard, you see one boy standing on the ground.

Now, if you have noticed that…

While watching from a skateboard, a boy standing on the ground appears to be going backwards.

Also, the **ground** (along with a boy) appears to be going backwards.

Do you know the reason why this happens?

I think** you know the answer **if you have properly understood the definition of frame of reference.

Let’s find out.

The reason why a boy standing on the ground appears to be going backwards is because of the frame of reference.

*(i.e., While watching from a skateboard, your frame of reference is a **skateboard**)*

That’s why, you’ll notice both (boy and ground) going backwards.

Now,

According to the viewpoint of a boy standing on the ground, a skateboard is going forward, right.

**The reason is simple**.

See this,

When a boy is watching from the ground, a **ground is the frame of reference**.

That’s why,

A boy standing on the ground notices a skateboard going forward.

**Conclusion is**,

While watching from a skateboard, the frame of reference is a **skateboard**.

*(So a boy standing on the ground appears to be going backwards)*

Whereas,

While watching from a ground, the frame of reference is a **ground**.

*(So a boy riding a skateboard appears to be going forward)*

In short,

A frame of reference is just a **viewpoint** from which the position and velocity of an object are measured.

In other words,

*“A **coordinate system** and a **set of** **physical reference points** that can be used to determine the position and velocity of an object within that frame is known as the **frame of reference**“*

### #2 Car

**Imagine**, you are travelling in a car.

Suppose that while travelling in a car, you see one man standing on a road.

If you have noticed that…

While watching from a car, a man standing on a road appears to be going backwards.

In fact,

As the car moves forward, also the **road** (along with a man) appears to be going backwards.

Do you know the reason why this happens?

I think the reason is simple.

See this,

Here,

As you are sitting in a car, your frame of reference is a **car**.

That’s why,

While watching from a car, you’ll notice both (man and road) going backwards.

But,

According to the viewpoint of that man, your car is going forward right?

**What’s the reason behind this?**

Let’s find out.

In this case,

A man is watching from a road, so the** **frame of reference is a **road**.

That’s why,

While watching from a road, a man notices a car going forward.

**Conclusion**,

From the above two cases, one thing is concluded that…

While sitting inside a car, a car is the frame of reference.

*(So a man appears to be going backwards)*

On the other hand,

While standing on a road, a road is the frame of reference.

*(So a car appears to be going forward)*

Therefore,

A frame of reference is just a** viewpoint** from which the position and velocity of an object are measured.

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

*“A **coordinate system** and a **set of** **physical reference points** that can be used to determine the position and velocity of an object within that frame is known as the **frame of reference**“*

### #3 Train

This is a real life example of a frame of reference. (You might have experienced this situation)

**Imagine**, you are sitting inside a train which is about to start from the platform.

Suppose that after some time, a train starts moving forward and you notice a man standing on the platform.

Here’s the interesting thing,

When a train starts moving forward, you’ll see that a man standing on the platform appears to be going backwards.

Also, the **platform** (along with the man) appears to be going backwards.

Have you experienced this?

Obviously, yes.

But, do you know the reason why this happens?

It’s simple.

See this,

The reason why the platform appears to be going backwards is because of the frame of reference.

Look,

When you are sitting inside a train, your frame of reference is a **train**.

So, while watching from a train both (platform and man standing on the platform) appears to be going backwards.

Now,

Let’s talk about the viewpoint of a man standing on the platform.

According to his viewpoint, a train is going forward right?

Again, **the reason is simple**…

See this,

Here, a man is standing on the platform so the frame of reference is a **platform**.

That’s why,

While watching from the platform, a man notices a train going forward.

**Conclusion is**,

While sitting inside a train, a **train** is the frame of reference.

*(So a platform appears to be going backwards)*

On the other hand,

While standing on a platform, a **platform **is the frame of reference.

*(So a train appears to be going forward)*

In short,

** viewpoint** from which the position and velocity of an object are measured.

Or

You can remember the meaning of frame of reference as,

*“A **coordinate system** and a **set of** **physical reference points**frame of reference**“*

### #4 Aeroplane

**Imagine**, you are flying in an aeroplane.

Suppose that you see one boy standing on the ground from an aeroplane.

If you have noticed that,

While watching from an aeroplane, a boy standing on the ground appears to be going backwards.

Also, the **ground** (along with a boy) appears to be going backwards.

Now you should know the reason why this happens.

It’s simple. Look,

In this case,

As the observer is watching from an aeroplane, the frame of reference is an **aeroplane**.

So while watching from an aeroplane, both (boy and ground) appear to be going backwards.

Now,

According to the viewpoint of a boy standing on the ground, an aeroplane is moving forward.

And I think, now you know the reason why this happens.

Let’s see.

In this case,

A boy is watching an aeroplane from the ground, so the frame of reference is the **ground**.

Therefore,

While watching from the ground, an aeroplane appears to be going forward.

**Conclusion is**,

While sitting inside an aeroplane, an **aeroplane **is the frame of reference.

*(So a ground appears to be going backwards)*

Whereas,

While standing on a ground, a **ground **is the frame of reference.

*(So an aeroplane appears to be going forward)*

That’s why,

A frame of reference is nothing but a **viewpoint** from which the things are measured.

Or

*“A **coordinate system** and a **set of** **physical reference points**frame of reference**“*

## Types of frame of reference

From the definition of frame of reference, one thing is clear that…

A frame of reference is just a** reference point** or a **viewpoint** from which the object’s position and velocity can be measured.

Now, let’s discuss the different types of frame of reference.

In physics, a frame of reference (or a reference frame) can be classified into two types:

**Inertial frame of reference**

**Non inertial frame of reference**

Let’s understand these two different reference frames with practical examples. (So you’ll get an exact idea)

### Inertial frame of reference

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 words,

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

Let’s take one example to understand the definition of inertial frame of reference.

Consider a bike at **rest**.

Can you guess the frame of reference of a bike?

It’s simple.

See this,

Here, the frame of reference of a bike is at rest.

*(i.e., A bike is in the frame of reference which is **not accelerated**)*

So, a bike is said to be in the inertial frame of reference.

In a similar way,

Guess the frame of reference of a bike, when a bike is moving with **constant velocity**.

Again,

A bike is said to be in the inertial frame of reference, as the frame of reference of a bike is **not accelerated**.

Now, you might be thinking that…

**As a bike is moving**, how is the frame of reference of a bike not accelerated?

The reason is simple.

Look,

According to Newton’s first law… *“An object will accelerate further, only if an **unbalanced force** acts on it”*

It means that a bike will accelerate further, only if an unbalanced force acts on it.

*(i.e., A bike will continue moving forward with constant velocity, unless an unbalanced force acts on it)*

And that’s why **Newton’s first law is valid** in the inertial frame of reference.

In short,

In both cases, a bike is not accelerating further and also the frame of reference of a bike is **not accelerated**.

So, a bike is said to be in the inertial frame of reference.

### Non inertial frame of reference

The definition of non inertial frame of reference can be stated as:

*“A frame of reference which is not moving with constant velocity or is accelerated is known as **non inertial frame of reference**“*

In simple words,

When a frame of reference is **accelerated**, it is known as the **non inertial frame of reference**.

Let’s consider a similar example of a bike as discussed above.

But in this case, a bike is **accelerated**.

Now the question is,

Is a bike in the inertial or non inertial frame of reference?

Let’s find out.

As you can see,

When a bike is accelerated, the frame of reference of a bike also gets **accelerated**.

So, a bike is said to be in the non inertial frame of reference.

Now,

Why is Newton’s first law invalid in the non inertial frame of reference?

The reason is simple.

Look, when a bike is accelerated, a bike is not at rest or moving with constant velocity, right?

And you know that… Newton’s first law is valid only when an object is at rest or moving with constant velocity.

That’s why **Newton’s first law is invalid** in the non inertial frame of reference.

In short,

When a bike is accelerated forward, the frame of reference of a bike also gets** accelerated**.

So, a bike is said to be in the non inertial frame of reference.

### Conclusion

From this chart, you can easily remember the two different types of frame of reference.

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

On the other hand,

When a reference frame is** accelerated**, it is known as the** **non inertial reference frame.

And, always remember that…

Newton’s first law is **valid** only in the inertial frame of reference.

## Difference between Inertial and Non inertial frame of reference

Here are some points, which shows the basic difference between inertial and non inertial frame of reference.

Sr No. | Inertial frame of reference | Non inertial frame of reference |
---|---|---|

1. | A frame of reference which is at rest or moving with constant velocity is known as the inertial frame of reference. | A frame of reference which is not moving with constant velocity is known as the non inertial frame of reference. |

2. | Inertial frame of reference is a frame of reference which is not accelerated. | Non inertial frame of reference is a frame of reference which is accelerated. |

3. | Inertial frame of reference is also known as the stationary frame of reference. | Non inertial frame of reference is also known as the Non stationary frame of reference. |

4. | Newton’s first law is valid in the inertial frame of reference. | Newton’s first law is invalid in the non inertial frame of reference. |

5. | Example of inertial frame of reference: A bike which is at rest or moving with constant velocity. | Example of non inertial frame of reference: A bike which is accelerated. |

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

(Let me know in the **comments** below)

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