mass Mass is a measure of inertia of a body. The Newton’s second law of motion states that the acceleration, a produced on a body is proportional to the impressed force, F. The mass, m is the constant of proportionality,

m = F/a

Mass defined in this way is called inertial mass.

Mass can also be defined from the law of Gravitation.

F = G [ m₁ m₂ /r² ]

Einstein showed that the two masses are equivalent. See also kilogram, atomic mass unit.