Classical mechanics describes so basically accurate most of phenomena mechanics observable directly in our daily life. The fundamental branch is constituted by Continuum Mechanics based on the hypothesis of ’ continuous body. The validity of this hypothesis is defined by Knudsen number. If such an assumption cannot be applied are addressed by the laws of statistical mechanics, It is a part of the thermodynamics.
We observe a considerable discrepancy between the predictions of classical mechanics and experiments both systems where the speed at stake are comparable with the speed of light and both systems of spatial dimensions comparable to those nukes or molecular studies (for which the fundamental constant with which face is the Planck's constant). In these cases the classical mechanics are replaced respectively by relativistic mechanics and from the quantum mechanics.