Classical Physics isn’t applicable to very high-speed situations. But, there is another problem with classical Physics, it is anot applicable to the ‘small scale of the universe’ also . It isn’t applicable to atoms and objects and particles smaller than atoms.
This problem is not only with classical Physics but also with the general theory of relativity. (But, the special theory of relativity works fine on the small-scale.)
This article mainly focuses on the basic concepts of Quantum Physics including, Planck’s Quantum Theory, Wave-Particle Duality and the Uncertainty Principle.