Strictly speaking, mass is related to the inertia of a moving object and the effort needed to stop it. Even though two astronauts weigh only one sixth as much on the moon as on Earth, if they begin fighting the blows land just as hard and hurt just as much because the mass behind the blows is the same as on Earth. In contrast, weight is the force with which a planet pulls on a given mass.

If m is the mass in grams and g is the gravitational acceleration in centimeters per second per second, the weight of the object, w, in grams cm sec , or dynes, is given by w = m x g.

We ordinarily do not make the distinction between weight and mass. When we say that an object "weighs one gram," what we really mean is that it "weighs what a one-gram mass would weigh on Earth." The gravitational constant, g, is one sixth as large on the moon, so the same mass will have only one sixth the weight there that it has on Earth. When we say that a one-gram object "weighs only one sixth of a gram on the moon," we mean that it is pulled toward the moon with the same force that the Earth would exert on a one-sixth-gram mass. As long as we are making only Earth-bound comparisons, no confusion need arise between weight and mass. We will use the terms "atomic weights" and "atomic masses" interchangeably.