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Classical Physics
Branches include mechanics (Newtonian physics), sound, light, heat (thermodynamics) and electricity and magnetism. Classical physics is a flawed theory, but it is only dramatically flawed when dealing with the very small (atomic size, where quantum mechanics is used) or the very fast (near the speed of light, where relativity takes over). For everyday things, which are much larger than atoms and much slower than the speed of light, classical physics does an excellent job. Plus, it is much easier to use than either quantum mechanics or relativity (each of which require an extensive amount of math).
Modern Physics Superstring theorists believe they are on their way to developing a framework that incorporates both general relativity and quantum mechanics. If they succeed, they will have discovered a theory that solves the greatest problem in physicsthat of unifying the laws of nature. If superstring theory turns out to be true, every one of the "fundamental" particles below are made of identical strings, with the only difference between the strings being their vibrational patterns. Modern physics studies things on the extremes. Very small things (components of neutrons and protons), the building blocks of matter, and extremely large things/forces (black holes). It also tries to come up with a unifying theory that will explain the four major forces.
Disciplines now include:
particle physics, quantum theory (quantum field theory, quantum gravity, quantum electrodynamics, quantum mechanics, ...), string theory, gravitation, astrophysics, nanotechnology, geophysics, etc. The quantum theory and the theory of relativity together form the theoretical basis of modern physics. Just as the theory of relativity assumes importance in the special situation where very large speeds are involved, so the quantum theory is necessary for the special situation where very small quantities are involved, i.e., on the scale of molecules, atoms, and elementary particles. Superstring theorists believe they are on their way to developing a framework that incorporates both general relativity and quantum mechanics. If they succeed, they will have discovered a theory that solves the greatest problem in physics  that of unifying the laws of nature. Quantum theory is the modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles;
Fay Dowker, Lecturer in Theoretical Physics at Queen Mary, University of London, states "The development of quantum mechanics was a major advance in our understanding of the physical world. However, quantum mechanics has not yet come fully to fruition because it has not replaced classical mechanics in the way that general relativity has replaced Newtonian gravity. In the latter case, we can start from general relativity and derive the laws of Newtonian gravity as an approximation; we can also predict when  and quantitatively to what extent  that approximation is valid.
See Also:
Physics at answers.com
