Motion

What is motion?

Motion is the change in position of a body w.r.t time.

How it can be observed? 

For observation, an observer is needed.

We attach a frame of reference with the observer.

Then we measure the change in position of the body relative to that frame of reference.

In this way, we observe the motion of the body.

If there is no change in position of the body relative to the attached frame of reference, then we say that the body is motionless or it has a constant position.

What does Newton's first law say about this? 

Newton's first law states that the motion of the body can't be changed unless it is compelled by a force.

What is absolute frame of reference? 

We can take anything as frame of reference.

So there is no such absolute frame of reference.

Why we say that 'every thing is moving' ?

Since there is no absolute frame of reference, we cannot determine absolute motion. So we say that in the universe, every thing is moving.