# The concept of Relativity and frames of reference

The concept of Relativity and frames of reference:

The term relativity comes from the word “relative that is in relation”. In this universe, everything is relative, there is nothing like absolute rest. If you think that you are sitting somewhere and you are in the state of rest, then my dear you are wrong. You are also in the state of motion, because if someone is able to see you from outside the earth (suppose from moon), then you are also moving. Therefore, everything is relative.

Here question arises with respect to which we can study the motion of another object? This problem is solved with the concept of frame of reference.

Definition: The frame of reference is defined as any coordinate system with respect to which one can study the motion of the another object. For example, a bus can be a frame of reference with respect to which one can study the motion of another object.

Types of the frame of reference:

1. Inertial frame of reference:

[This you can understand only if you know the Newton’s first law or law of inertia. So what is that? Remember of forgot! Let us revise it. Newton’s first law is that a body at rest will remain at rest; a body in uniform motion will remain in uniform motion, until and unless some external force is applied on it.

Here a body at rest will remain at rest, until and unless some external force is applied on it is also called law of inertia of rest.

A body in uniform motion will remain in uniform motion until and unless some external force is applied on it is also called law of inertia of uniform motion.]

Thus inertial frame of reference is that frame in which the law of inertia is obeyed. In other words, the non-accelerated frame of reference is called inertial frame of reference.

2. Non – inertial frame of reference:

[It should be simple for you know if you understood the inertial frame of reference].

Non-inertial frame of reference is that frame of reference in which the law of inertia is not obeyed. In other words, the accelerated frame of reference is called inertial frame of reference.

Brain Teaser: Can you give an example of inertial frame of reference that is non-accelerated one?

Answer:  The answer is nothing can be perfect inertial frame. But we consider earth as an inertial frame because acceleration of the earth is negligible.

Types of theory of relativity:

On the basis of this, theory of relativity is divided into two types:

1. Special theory of relativity: This theory deals with inertial frames of reference.

2. General theory of relativity: This theory deals with non-inertial frames of reference.