Classical Physics

1. Overview

Classical Physics is the framework that describes the physical world at macroscopic scales—where objects are large, speeds are much smaller than the speed of light, and quantum effects are negligible. It is built on deterministic laws, meaning that given the initial conditions of a system (position, velocity, forces), its future evolution can be predicted precisely.

  • Classical physics explains most of everyday reality.
  • It is deterministic and mathematically structured.
  • It laid the foundation for modern physics.
  • Its limitations led to quantum mechanics and relativity.

Key principles

  1. Determinism: The future state of a system is fully determined by its initial conditions and governing laws.
  2. Continuity: Physical quantities vary smoothly over space and time.
  3. Objectivity: Measurements are independent of the observer.
  4. Absolute space and time: Space and time exist independently of matter.

These assumptions enabled the formulation of universal laws that apply across a wide range of physical systems.

Classical physics seeks to answer questions such as:

  • What governs the motion of objects?
  • How is energy transferred and conserved?
  • How do heat and temperature behave?
  • How do electric and magnetic fields interact?
  • How does light propagate?

Even today, classical physics remains the backbone of engineering and real-world systems—from bridges and vehicles to power grids and communication systems.

2. Knowledge map of classical physics

CLASSICAL PHYSICS
|
+--- FOUNDATIONS
|   |
|   +--- determinism
|   +--- conservation laws
|   |    +--- energy
|   |    +--- momentum
|   |    +--- angular momentum
|   +--- mathematical framework
|        +--- calculus
|        +--- vector mechanics
|        +--- differential equations
|
+--- MECHANICS
|   |
|   +--- kinematics
|   +--- dynamics
|   +--- work and energy
|   +--- rotational motion
|   +--- gravitation
|
+--- THERMODYNAMICS
|   |
|   +--- temperature and heat
|   +--- laws of thermodynamics
|   +--- entropy
|   +--- heat engines
|
+--- ELECTROMAGNETISM
|   |
|   +--- electric fields
|   +--- magnetic fields
|   +--- Maxwell equations
|   +--- electromagnetic waves
|
+--- WAVES AND OPTICS
|   |
|   +--- wave motion
|   +--- interference
|   +--- diffraction
|   +--- reflection and refraction
|
+--- LIMITATIONS
    |
    +--- fails at atomic scale → quantum mechanics
    +--- fails at high speeds → relativity

3. Foundations of classical physics

At its core, classical physics is grounded in a few powerful principles.

3.1 Newton’s laws of motion

These laws describe how objects respond to forces:

  • First law (inertia): Objects remain at rest or in uniform motion unless acted upon by a force.
  • Second law: Force is proportional to acceleration.
  • Third law: Every action has an equal and opposite reaction.
F = m * a

3.2 Conservation laws

Conservation laws are among the most powerful ideas in physics:

  • Energy is conserved — it cannot be created or destroyed.
  • Momentum is conserved — especially in isolated systems.
  • Angular momentum is conserved — critical for rotational systems.

3.3 Mathematical framework

Classical physics relies heavily on mathematics:

  • Calculus — describes continuous change
  • Vectors — represent direction and magnitude
  • Differential equations — describe system evolution

3.4 Questions to think about

  • Why do conservation laws hold universally?
  • Is motion fundamentally deterministic?
  • Can all physical systems be reduced to equations?

4. Mechanics (motion and forces)

Mechanics is the study of how objects move and interact through forces.

4.1 Kinematics (describing motion)

Kinematics focuses on motion without considering forces. It introduces quantities such as displacement, velocity, and acceleration.

  • Displacement — change in position
  • Velocity — rate of change of position
  • Acceleration — rate of change of velocity

It answers:

  • How fast is an object moving?
  • How does its motion change over time?

4.2 Dynamics (forces and motion)

Dynamics explains why motion occurs. It introduces forces and explains why objects accelerate.

Common forces include gravity, friction, tension, and the normal force.

F = m * a

4.3 Energy and work

Energy provides a universal way to analyze systems. Instead of tracking forces at every instant, one can often analyze energy transformations.

  • Kinetic energy — energy of motion
  • Potential energy — energy stored due to position
  • Work — energy transferred by force
KE = (1/2) m v^2

The work-energy theorem connects force-based and energy-based approaches.

4.4 Rotational motion

Rotational systems extend linear mechanics into angular motion. Torque replaces force, angular velocity replaces linear velocity, and moment of inertia replaces mass.

This is critical in systems such as turbines, wheels, and planetary motion.

4.5 Gravitation

Newton’s law of gravitation describes attraction between masses.

F = G * m1 * m2 / r^2

This law explains planetary orbits, tides, and much of celestial mechanics (before relativity).

4.6 Questions to think about

  • Why does inertia exist?
  • Why does gravity follow an inverse-square law?
  • Is force fundamental, or derived from energy?
  • What determines stability in motion?

5. Thermodynamics (heat and energy systems)

Thermodynamics studies how heat and energy behave in systems where energy transfer and transformation are central. 

ΔU = Q - W

5.1 Temperature and heat

  • Temperature measures the average kinetic energy of particles.
  • Heat is energy transfer due to a temperature difference.

5.2 Laws of thermodynamics

  1. Zeroth law: Defines temperature equilibrium.
  2. First law: Energy conservation.
  3. Second law: Entropy increases (irreversibility).
  4. Third law: Absolute zero cannot be reached.

5.3 Entropy

Entropy measures disorder (or the number of microstates). It introduces irreversibility into physics—unlike mechanics.

It helps explain:

  • Why processes are irreversible
  • Why time has a direction
  • Why energy spreads out

5.4 Heat engines

Devices that convert heat into work:

  • engines, turbines, refrigerators
  • efficiency limited by thermodynamic laws

5.5 Questions to think about

  • Why can’t entropy decrease naturally?
  • Why is time directional?
  • What is the microscopic origin of heat?

6. Electromagnetism

Electromagnetism unifies electricity and magnetism into one framework.

6.1 Electric fields

  • Produced by charges
  • Exert forces on other charges

6.2 Magnetic fields

  • Produced by moving charges
  • Influence other moving charges

6.3 Maxwell’s equations

These four equations describe all electromagnetic phenomena, and unify electricity, magnetism, and light.

One of Maxwell’s equations can be written as:

∇·E = ρ / ε0

They predict the existence of electromagnetic waves, including light.

6.4 Electromagnetic waves

Light is an electromagnetic wave traveling at constant speed. This discovery directly led to relativity.

  • Light is an EM wave
  • It travels at the speed of light
  • It includes radio, microwave, visible, and X-rays

6.5 Questions to think about

  • Why do electric and magnetic fields exist?
  • Why does light have a constant speed?
  • Are fields fundamental or emergent?

7. Waves and optics

Waves describe oscillations that transfer energy.

7.1 Wave motion

  • Characterized by wavelength, frequency, and amplitude
  • Can be mechanical (sound) or electromagnetic

7.2 Interference

When waves overlap:

  • Constructive interference — amplification
  • Destructive interference — cancellation

7.3 Diffraction

Waves bend around obstacles and spread out.

7.4 Reflection and refraction

  • Reflection — bouncing off surfaces
  • Refraction — bending due to medium change

7.5 Geometric optics

  • Ray approximation of light
  • Used in lenses and mirrors

7.6 Questions to think about

  • Why do waves interfere?
  • What determines wave speed?
  • Is light fundamentally a wave or particle?

8. Famous milestones

  • Galileo — experimental method
  • Newton — laws of motion
  • Faraday — field concept
  • Maxwell — unified EM theory
  • Carnot — thermodynamic efficiency

9. Tools that enabled classical physics

  • Telescope — planetary motion
  • Prism — light spectrum
  • Electrical circuits — EM experiments
  • Pendulum — time measurement
  • Vacuum chambers — gas laws

10. Limitations of classical physics

Classical physics fails in extreme conditions:

  • Atomic scale — quantum mechanics required
  • High speeds — relativity required
  • Strong gravity — general relativity required

11. Further references

Books

  • Feynman Lectures on Physics
  • Classical Mechanics — Goldstein

Online

  • MIT OpenCourseWare
  • Stanford Physics Lectures