Physics divide in different fields because lot of innovations is in the field therefore there comes a need of division. Classic mechanics deals much with motion and Equilibrium of bodies, e.g. Trusses, levers, pulleys and gears etc. And electrodynamics deals with force exist in neighboring current carrying conductor. Statistical mechanics deals with the statistical prediction properties of matter in bulk from a knowledge of the properties of atoms or molecules and forces between them. The special theory of relativity refers to the non accelerated frame of reference. Quantum mechanics is the is a mathematical theory developed from quantum theory and concerned with the mechanics of atomic and related systems in terms of measurable quantities.
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