Classical physics

Physics that does not use quantum mechanics or the theory of relativity. Newtonian mechanics, thermodynamics, and Maxwell’s theory of electromagnetism are all examples of classical physics. Many theories in classical physics break down when applied to extremely small objects such as atoms or objects moving at close to the speed of light.
Classical mechanics refers to the classical physics of bodies and forces, particularly Newton’s laws of motion and the principles of mechanics based on them.

Meaning and branches of classical physics.
Sometimes, scientific breakthroughs mean we throw out everything we thought we knew. This almost happened to physics when Albert Einstein came up with his theory of relativity and quantum mechanics. The old theories couldn’t explain everything—but they could still explain a lot. Because they were still important and useful, but because they concerned a completely different way of looking at the world, physics split into two main branches: classical physics and modern physics.

Classical physics includes classical mechanics (the study of the motion of fluids and particles), thermodynamics (the study of temperature and heat transfer), and electromagnetism (the study of electricity, magnets, and electromagnetic waves). Classical physics can be defined as the study of the physical world that is visible to the naked eye—in other words, things that are macroscopic. It is based on Isaac Newton’s laws of motion and gravitation (hence classical physics is also called Newtonian physics) and is the basis of various other branches of science such as chemistry and biology.

However, in the late 1800s, scientists began to discover things that classical physics could not adequately explain. This led to the theories of relativity and quantum mechanics, which required a new model of physics. But in the “normal world”—the visible, physical world—the old laws of physics continued to work just fine. Classical physics is what you might call “practical physics” — for most practical, everyday purposes, classical physics is still important and useful.

Modern physicists may engage in studies that go far beyond the practicalities of classical physics, but that does not mean that they no longer pay attention to classical physics. There are many questions in classical physics that are still unanswered, or about which scientists are still debating.