The problem with classical mechanics is that it cannot fully explain and predict the behavior of objects at the atomic and subatomic level. It also cannot account for certain phenomena, such as the wave-particle duality of light.
Classical mechanics is based on Newton's laws of motion and works well for objects at macroscopic scales. Quantum mechanics, on the other hand, describes the behavior of particles at the atomic and subatomic level, taking into account principles such as uncertainty and superposition.
Yes, classical mechanics is still widely used in many areas of science and engineering. It is particularly useful for describing the behavior of large objects and systems, such as planets and machinery.
The study of electromagnetism and the discovery of the photoelectric effect in the late 19th and early 20th centuries led to the realization that classical mechanics was not sufficient to explain certain phenomena. This prompted the development of quantum mechanics to better understand the behavior of particles at the atomic level.
While classical mechanics is a highly accurate and useful theory, there are still some unresolved issues, such as the inability to fully explain the behavior of very small particles and the lack of a unified theory that combines classical mechanics with other fundamental forces, such as gravity.