The equivalence principle is a key concept of general relativity, linking two fundamental notions: gravitational mass and inertial mass. Although these two types of mass describe different phenomena, they are equivalent.
Albert Einstein (1879-1955) envisioned a hypothetical situation where a person is in a completely closed elevator cabin, unable to see outside. This cabin can either be in free fall in a gravitational field or in an accelerating reference frame in space. He formulated this idea to show that the person could not distinguish whether they feel a gravitational force or an inertial force due to acceleration.
According to the equivalence principle, these two masses are equal: $m_g = m_i$. This equivalence has been experimentally verified with high precision.
The equivalence principle states that the effects of a gravitational field are indistinguishable from those of an acceleration in a non-inertial reference frame (amusement park ride, car turning, Earth rotating around its axis, etc.). This leads to a remarkable consequence: all objects, regardless of their mass or composition, fall with the same acceleration in a gravitational field in the absence of air resistance.
In general relativity, gravitation is no longer considered a classical force, but rather a manifestation of the curvature of space-time. The equivalence principle helps explain why an object in free fall does not feel a gravitational force, as it follows a geodesic in curved space-time.
The Speed of Light: The Ultimate Limit That Nothing Can Exceed 
Reality Escapes Us: Truths We Can Never Prove 
The Physics of the Universe in 50 Equations: User Guide 
The Kaya Identity: The Equation Complicating Our Decarbonization 
The Unsurpassable Speed in the Universe: When Energy Becomes Infinite 
Electromagnetic Runaway: The Secret of the Speed of Light 
Understanding the Photoelectric Effect: Light and Electrons 
How far is the horizon? 
How Do Solar Panels Inject Electricity into the Grid? 
Dynamics of Momentum to explain the propulsion of rockets or jellyfish 
How Electron Energy Dictates Chemical Properties 
The Key Role of Quantum Uncertainty: No Particle Can Be at Rest 
Energy and Power: Don't Confuse Them, Time Makes All the Difference 
Why is there a limit to cold, but not to heat? 
Galileo's Law of Falling Bodies 
The Ideal Gas Law: One Equation, Thousands of Applications 
Schrödinger's Equation Revolutionized Our View of Matter 
The Magic of Noether's Theorem: From the Principle of Least Action to Conservation Laws 
Relationship between gravitational mass and inertial mass and the equivalence principle 
Third Equation of Physics: Momentum to Understand Collisions 
The second essential equation in physics: The intuition of a conserved quantity 
The First Equation of Physics: How to Mathematize Force 
The electromagnetic force or Lorentz force 
The solar energy received depends on
the angle of incidence 
Why is marble colder
than wood? 
Why does a photon, which has no mass, have energy?
Bayes Formula and Artificial
Intelligence 
The seven fundamental constants of physics 
What temperature does it feel like in interstellar space? 
Black body radiation curves: Planck's law 
The equivalence principle, gravitational effects are indistinguishable from acceleration 
E=mc2: The four fundamental concepts of the universe revisited 
How to weigh the sun? 
Equation of the free fall of bodies (1604) 
Coulomb vs Newton: The Mysterious Similarity of the Universe's Forces 
Boltzmann's equationon entropy (1877) 
Special relativity equations (1905) 
The equation of general relativity (1915) 
Planetary Rotation Equations: Between Angular Momentum and Gravitational Balance 
Equation of the orbital velocity of a planet 
Planck's equation 
Understanding Schrödinger's Equation Without Math 
Newton's Three Laws: From the Falling Apple to the Orbiting Planets 
Maxwell's equations 
Paul Dirac's equation 
Conservation of energy 
Equation of electromagnetic induction 
Why do elementary particles have no mass? 
Difference between heat and temperature