<P> Typically, the mass of objects is measured in relation to that of the kilogram, which is defined as the mass of the international prototype kilogram (IPK), a platinum alloy cylinder stored in an environmentally - monitored safe secured in a vault at the International Bureau of Weights and Measures in France . However, the IPK is not convenient for measuring the masses of atoms and particles of similar scale, as it contains trillions of trillions of atoms, and has most certainly lost or gained a little mass over time despite the best efforts to prevent this . It is much easier to precisely compare an atom's mass to that of another atom, thus scientists developed the atomic mass unit (or Dalton). By definition, 1 u is exactly one twelfth of the mass of a carbon - 12 atom, and by extension a carbon - 12 atom has a mass of exactly 12 u . This definition, however, might be changed by the proposed redefinition of SI base units, which will leave the Dalton very close to one, but no longer exactly equal to it . </P> <P> In special relativity, there are two kinds of mass: rest mass (invariant mass), and relativistic mass (which increases with velocity). Rest mass is the Newtonian mass as measured by an observer moving along with the object . Relativistic mass is the total quantity of energy in a body or system divided by c . The two are related by the following equation: </P> <Dl> <Dd> m r e l a t i v e = γ (m r e s t) (\ displaystyle m_ (\ mathrm (relative)) = \ gamma (m_ (\ mathrm (rest))) \!) </Dd> </Dl> <Dd> m r e l a t i v e = γ (m r e s t) (\ displaystyle m_ (\ mathrm (relative)) = \ gamma (m_ (\ mathrm (rest))) \!) </Dd>

The pull of gravity on the mass of a substance is known as