Mass (physics)
Mass is a fundamental property of matter that quantifies the amount of substance in an object and determines its resistance to Acceleration. It appears in two contexts that are experimentally equivalent but conceptually distinct.
Inertial mass describes how much an object resists changes in motion—the "m" in F = ma. A more massive object requires greater force to accelerate.
Gravitational mass describes how strongly an object experiences gravitational attraction and how strongly it attracts other objects. It determines the Weight you feel on Earth or the force between celestial bodies.
The equivalence of these two forms is so precise that Albert Einstein elevated it to a fundamental principle, founding General Relativity. This insight revealed that Gravity isn't truly a force but a curvature of Spacetime caused by massive objects.
Mass differs from Weight, which is the gravitational force exerted on an object and varies by location. An astronaut has the same mass on Earth and the Moon, but weighs less on the Moon.
In Quantum Field Theory, mass emerges from interactions with quantum fields rather than being intrinsic. This remains one of physics' deepest mysteries.