mass

Mass is the number that tells you how stubborn a body is. It is what you feel when you try to accelerate something — the resistance that a bowling ball offers and a tennis ball does not. In Newton's second law, F = ma, mass is the constant of proportionality between applied force and resulting acceleration.

Mass is not the same thing as weight. Weight is the force of gravity on an object, and it depends on where you are. On the Moon, a person has the same mass but one-sixth the weight. Out in deep space, weight goes to zero, but mass does not — push a floating astronaut and she still resists accelerating.

There are, at a deeper level, two distinct concepts called mass. Inertial mass is the resistance to acceleration. Gravitational mass is how strongly gravity pulls on the object. Newton noticed that the two appear to be identical for every object he could test, and Einstein elevated the equivalence to a postulate and built general relativity on top of it.

The modern concept of mass as distinct from weight emerged in the seventeenth century, most clearly in Newton's Principia (1687). In 2019 the kilogram was redefined in terms of Planck's constant, severing SI from any specific chunk of metal.