Physics
Physics builds models of the physical world. Begin with the regime that matches the system: familiar macroscopic conditions, or the high-speed, strong-gravity, atomic, and subatomic regimes where classical assumptions stop working.
Classical Physics
Best fit: ordinary speeds, macroscopic scales, and weak gravity.
Classical models describe most everyday systems extremely well. They treat motion, fields, waves, and bulk matter with continuous quantities and deterministic laws.
Modern Physics
Required for near-light speed, strong gravity, and atomic or subatomic scales.
Modern physics extends the classical picture when its assumptions fail. Relativity changes spacetime; quantum theory changes how physical states and measurements behave.
The conditions tell you which theory to use.
A newer theory does not make an older one useless. It explains where the older model works, and what must change outside that range.
Are speeds far below light speed and gravity relatively weak?
Mechanics, thermodynamics, electromagnetism, and waves cover most human-scale phenomena.
Are speeds close to light speed, or is gravity strongly curving spacetime?
Special relativity handles inertial high-speed motion; general relativity handles gravity as geometry.
Does the system depend on atoms, nuclei, photons, or quantized states?
Atomic and nuclear physics apply quantum rules to particular kinds of matter and interaction.