ABSTRACT
Unlike solids, the molecules in a ¬uid are not žxed to a lattice-they are free to move relative to each other. ese molecules are more widely spaced out in a gas than in a liquid, the distance between them being much larger as compared to their molecular diameter. Brownian motion, or the random motions of the molecules due to their individual thermal energies, makes all these molecules move all over the place-unlike a solid. erefore, it is not easy to track every single molecule in ¬uids: a mere picogram of water contains 1010 H2O molecules. As a practical alternative, scientists have come up with the concept of a continuum. What this means is that we assume that the ¬uid properties are the same throughout a certain volume of ¬uid. e continuum concept also assumes that the ¬uid is inžnitely divisible, in other words, its variation in properties is so smooth that we can use di•erential calculus to analyze the ¬ow. To understand this further, let us think about a specižc ¬uid property: density.
