Mass

Mass is a way of quantifying the total amount of stuff in an object. Fundamentally, it is the total amount of energy that was required to create the object, as given by E=mc². In more common use it is a measure of an objects propensity (or lack there of) to accelerate when a force is applied to it as given by Newton's Second Law F/a=m. In physics mass is extensive physical property of a system and is most frequently measured in the SI unit of kilograms. Finally, mass serves as the "charge" for the gravitational force.

Mass versus weight

Mass is frequently confused with weight which is actually a measure of the gravitational force applied to a mass. This is why you can be "lighter weight" on the moon without have a lower mass. This is a particularly annoying problem when converting between the English and metric systems because the English system uses pounds which is a unit of force where the metric uses kilograms a unit of mass.

Other units of mass

1 [kg] (SI) = 0.0685 [slug] (English)

1 [kg] (SI) = $5.60958921*10^{35}\lbrack {\tfrac {ev}{c^{2}}}\rbrack$ (SI) The "c²" is often dropped when using this unit.

1 [kg] (SI) = $1.67386428*10^{-25}$ [emu] Earth mass units

1 [kg] (SI) = $5.02785431*10^{-31}$ [solar masses]

Gravitational versus inertial mass

The fact that the same quantity serves as both the "charge" for the gravitational force and the inertial term of Newton's Second Law is neither necessary nor predicted by other laws of physics. This observed fact has led to some open problems in gravity, Albert Einstein assumed it to be true in his formulation of General Relativity

Relativistic Mass

Some interpretations of special relativity see the mass of an object increasing as the velocity increases. This view has fallen out of favor, though is still used by some physicists.