# Frame of Reference : Inertial Frame and Non-inertial Frame

We have learned about velocity, acceleration, and displacement. But all these quantities need a frame of reference from which they are measured. In this article, we will be learning about the frame of reference in detail.

## What Is a Frame of Reference?

We have learned about velocity, acceleration, and displacement. But all these quantities need a frame of reference from which they are measured.

In physics, a frame of reference consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements within that frame.

Let’s consider the following figure:

If we ask A what velocity of B is, he will say it is at rest. But if we ask the same question to C, he will say B is moving with a velocity V in the positive X direction. So we can see before specifying the velocity we have to specify in which frame we are or in simple terms, we need to define a frame of reference.

## Types of Frame of Reference

Once we have chosen our reference they can be of two types:
• Inertial Frame of Reference
• Non-inertial Frame of Reference

### Inertial Frame of Reference

An inertial frame of reference is a frame where Newton’s law holds true. That means if no external force is acting on a body it will stay at rest or remain in uniform motion. Suppose a body is kept on the surface of the earth, for a person on earth it is at rest while for a person on the moon it is in motion so which is my inertial frame here?

Actually, the term inertial frame is relative i.e. first we assume a reference frame to be the inertial frame of reference. So a more general definition of an inertial frame would be: Inertial frame is at rest or moves with constant velocity with respect to my assumed inertial reference frame.

### Non-inertial Frame of Reference

Now we can define a non-inertial frame as a frame that is accelerated with respect to the assumed inertial frame of reference. Newton’s law will not hold true in these frames. So in the above example if I assume earth to be an inertial reference frame the moon becomes a non-inertial reference frame as it is in accelerated motion with respect to earth. But if we want to make Newton’s law hold here we need to take some mysterious forces also known as pseudo forces.

## Frequently Asked Questions – FAQs

Q1

### What is meant by a frame of reference?

In physical science, a frame of reference
comprises a group of physical reference points and an abstract coordinate system that helps to standardise calculations within the given frame.
Q2

### What are the two types of frames of reference?

The frame of reference is mainly of two types:
inertial frame of reference and non-inertial frame of reference.
Q3

### What is meant by an inertial frame of reference?

An inertial frame of reference is the type of frame where Newtonian laws are true. This implies that if no outside force is exerted on an object, it will continue to stay in uniform motion or state of rest.
Q4

### What is meant by a non-inertial frame of reference?

A non-inertial frame of reference can be defined as a frame that is in the state of acceleration relative to the considered inertial reference frame. Newtonian laws are irrelevant in these reference frames.
Q5

### Which transformation is applied in the special theory of relativity?

In relativity, the transformation between multiple inertial frames is the Lorentz transformation. In Newtonian mechanics, it is replaced by a Galilean transformation.
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