Newton read about Descartes view on motion (this I have explained in my previous post). Noticing the flaws in it, he went on a quest to understand true nature of motion of objects. He performed an experiment with an ordinary bucket filled with water. Here is an illustration to show what he did: He hung the bucket from ceiling with rope and twisted the rope in (say) anticlockwise direction. This caused the bucket to spin clockwise along with the water in it. Here is how he studied about motion of water and bucket.
Before bucket was spinning (bucket at rest) surface of water was flat.
When the bucket started to spin, water surface was still flat.
As the spin of bucket increased, water started to catch up with the spin and its surface became concave as in figure (roughly cone like). He explained that this was due to friction and centrifugal force which caused the particle to feel force towards the inner wall of the bucket.
When the rope loose all of it’s tension and coiled the other way, bucket retarded and came to rest. At this time water surface was still concave.
Just when the bucket from rest started spinning in anticlockwise direction, water surface became flat again.
It was a strange behavior. Relative motion could not explain the behavior of water surface. Newton worked on the problem. After rigorous thinking, he came to a conclusion.
Imagine, that we had taken the bucket (and water inside it) into an empty space. A space with nothing. If we spin the bucket in this space, the water surface would still display this phenomenon. How can this be possible? What is bucket spinning with respect to? How is it moving when its motion is relative to nothing (as the space in which it is kept is empty)?
Here, Newton gave concept of absolute space. Here is what he said:
“Absolute space, in its own nature, without regard to anything external, remains always similar and immovable. Relative space is some movable dimension or measure of the absolute spaces; which our senses determine by its position to bodies: and which is vulgarly taken for immovable space … Absolute motion is the translation of a body from one absolute place into another: and relative motion, the translation from one relative place into another”
Now he can fully explain how the motion of bucket work.
When bucket started moving, water remained at rest with respect to the absolute space and its surface remained flat.
When the bucket came to rest, water was in motion with respect to absolute space. This resulted in its surface becoming concave.
So relative motion together with absolute space gave a new definition to concept of motion. This broke down with Theory (now a law) of relativity given by Albert Einstein, but nevertheless mankind was able to describe what motion is for nearly 200 years.