Mass

Mass measures an object's resistance to acceleration and contributes to its gravitational interaction. Its meaning and measurement differ across classical mechanics, relativity, and quantum physics.

Inertial mass: A measure of how strongly an object's acceleration resists an applied force. It captures the resistance to acceleration that defines mass in Newton's second law.
Rest mass: The invariant mass of a body, measured in a frame where its total momentum is zero. It is the modern relativistic measure of an object's mass.
Aristotle: An ancient Greek philosopher whose natural philosophy shaped European thought for centuries. His account linked bodies' motion to their composition and heaviness, unlike modern mass.
Relativistic mass: A frame-dependent quantity sometimes defined as an object's total energy divided by the speed of light squared. Modern treatments usually reserve mass for invariant rest mass instead.
Gravitational mass: A measure of an object's coupling to gravitational fields and sources. It describes mass's second role: participating in gravitational attraction.
Energy: A conserved physical quantity associated with the capacity to produce change or perform work. Relativity relates an object's rest mass to its rest energy.
Galileo Galilei: An Italian scientist who advanced mathematical and experimental studies of motion. His investigations of falling bodies challenged accounts based on heaviness alone.
Mass–energy equivalence: The relation between mass and energy expressed by Einstein's equation E = mc² for an object's rest energy. It shows mass is not a separate, immutable store apart from energy.
Equivalence principle: The principle that inertial and gravitational mass are equivalent, foundational to general relativity. Their experimentally observed equality connects free fall with inertial motion.
Momentum: A vector quantity equal to an object's mass times velocity in classical mechanics. Mass helps determine momentum and its relativistic generalization.

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