Mass

Mass is a measure of how much matter is contained within an object. It's SI unit is the kilogram.

The mass of an object can be found by pushing on it with a known force <m> F</m> and dividing the size of that force by the resulting acceleration (<m>a</m>).

<m> m = \frac {F}{a} </m>

Mass is different from weight (force) in that its value does not change if gravity changes. For example, on Earth, where the acceleration due to gravity is 9.8 m/s², a person who has 60 kg of mass weighs 588 Newtons (132 lbs). On the moon, where the acceleration due to gravity is about 1/6 of what it is on Earth, that same 60 kg person weighs just 98 Newtons (~22 lbs).

To read more about the SI unit of mass, the kilogram, click here.