Quantum mechanics describes nature at the smallest scale. As we approach the atomic scale, classical mechanics starts to fall apart, and we have to develop new physics to explain what is happening. 

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I use the following textbooks for reference and provide solutions for (most) of the problems in these books:

• Principles of Quantum Mechanics (2nd ed.)-Shankar

• Modern Quantum Mechanics-Sakurai

• Quantum Mechanics-Cohen-Tannoudji, Diu, and Laloe