Fundamental Laws of Physics: Thermodynamics, Mechanics, Waves

Posted on Jan 11, 2025 in Physics

Fundamental Laws of Physics

Zeroth Law of Thermodynamics

If two systems are in thermal equilibrium with a third, they are in thermal equilibrium with each other.

First Law of Thermodynamics

Heat supplied to a system increases its internal energy and does work.

Second Law (Kelvin-Planck Statement)

No process can solely convert heat from a reservoir into work.

Clausius Statement

No process can solely transfer heat from a colder to a hotter object.

Newton’s Law of Cooling

The rate of cooling is proportional to the temperature difference: dΘ/dt = k(T2-T1)

Postulates of Kinetic Theory of Gases

  • Gas is a collection of many molecules.
  • Molecules are perfectly elastic.
  • Molecules are in random motion.
  • Molecules collide with each other and the container walls.
  • Molecular size is negligible compared to the distance between them.
  • Kinetic energy is proportional to absolute temperature.


Newton’s First Law (Law of Inertia)

An object remains at rest or in uniform motion unless acted upon by an external force.

Newton’s Second Law

The rate of change of momentum is proportional to the applied force: F= dp/dt

Newton’s Third Law

For every action, there is an equal and opposite reaction.

Triangle Law of Vector Addition

If two vectors are represented by two sides of a triangle, their resultant is the third side in reverse order.

Parallelogram Law of Vector Addition

If two vectors are represented by adjacent sides of a parallelogram, their resultant is the diagonal.

Pascal’s Law

Pressure applied to a fluid in equilibrium is transmitted equally to all points.

Hooke’s Law

Within the elastic limit, stress is proportional to strain: stress/strain = constant (modulus of elasticity)


Principle of Homogeneity of Dimensions

For an equation to be correct, dimensions on both sides must be the same: [LHS] = [RHS]

Kepler’s First Law (Law of Orbits)

Planets revolve around the sun in elliptical orbits with the sun at one focus.

Kepler’s Second Law (Law of Areas)

The line joining a planet to the sun sweeps out equal areas in equal times.

Kepler’s Third Law (Law of Periods)

The square of a planet’s period is proportional to the cube of its semi-major axis: T2 ~ a3

Impulse = Force × Time

Law of Conservation of Linear Momentum

If the external force is zero, linear momentum is constant.

Impulse-Momentum Principle

Impulse = change in momentum

Transverse Wave

The direction of vibration is perpendicular to the direction of wave propagation. Eg: Harmonic wave on a string.

Longitudinal Wave

The direction of vibration is parallel to the direction of wave propagation. Eg: Sound waves.