Question

How are the state of rest and motion relative?

Answer 1

To begin with let us start with the definition of rest and motion. And understand when a motion is said to be in rest or motion. Does the two depend on any external quantity? If yes, can that dependent quantity move?

Complete step-by-step answer:
We know that a body is said to be in motion to the observer, if the position of the body moves with respect to time. Similarly, a body is said to be in rest to the observer, if the position of the remains is fixed with respect to time.
Clearly, from the above two definitions, the observer plays a key role in defining the position of the body. Also the observer is free to move or stay at the same position if needed.
Let us take an example to understand this better phenomen better:
Assume that you and your dog are sitting in a moving bus. Let us say that you are the observer. You see through the window your friend standing on the road.
According to you the dog which is in the moving bus with you is at rest, and the friend on the road is moving.
Now if your friend is the observer, you and your friend the dog is moving with the bus while he is at rest with respect to the bus.
Do you see that the same situation has different states of rest or motion? Thus we can conclude that the state of rest and motion are relative to the observer.

Note: The state of the body depends on the position, time and the position of the observer. Hence when we describe the nature of motion of any body, we must mention the position of the observer also. For different observers, the same body might be in different states of motion or rest.