I just met with a very basic question. (Might even sound silly!) My textbook kinda says(not exactly), 'Whatever flows is a fluid'. That got me wondering because we are creating a whole category of matter just because they flow! So there must be some significance to 'flowing'. That further led me to ask why in the first place should we say liquids and gases "flow" and not "move"?! It seems to tell me that there should be a major difference between the physics of flow and movement. What is it?
PS:- I don't want the difference in meaning from a dictionary but a scientific difference. Please don't get too mathematical. I haven't acquired good mathematical skills YET.
Edit:- Okay. Since a comment below says "Movement is actually seldom defined very rigorously", I suppose I must refine my question here. Consider someone is saying that a box moves on a table as you applied a force on it. Now why is that person saying it 'moved' rather than it 'flowed', here? What is the difference between flow and movement in this case and how can we generalize the idea?
Best Answer
In very basic terms, flow is movement with continuous deformation while regular old movement is without continuous deformation, but there could be discrete deformation I suppose.
Water will flow when spilled, it tumbles and rolls and breaks up and rejoins. But when you push a ball across the table, the ball stays together, moving rigidly. It could be a rubber ball, which will deform locally and may even stay deformed if it's inelastic, but as a whole, the object moves as one unit.
How an object moves and it's relation to shear stresses is what defines states of matter which I describe very simply in the linked answer. Flowing really implies movement with no fixed shape (apart from shape imposed by the boundaries, such as containers or channels) which distinguishes it from the movement of solids.