Whereas weight measures the gravitational force that is exerted on an object, mass measures how much of something there is; thus, mass is closely related to volume. The weight of an object can change depending on its location (e.g., on the Earth or on the Moon), but the mass of the object (how much of it there is) always stays the same.
Mass and weight are often confused, because our two systems of measurement use different terms. In the metric system, kilograms and grams are measures of mass, but in the U.S. customary system, ounces and pounds are measures of weight. When using the metric system, we should really state that we are measuring mass, saying, for example "I have a mass of 60 kg" rather than "I weigh 60 kg," but this goes against convention. Throughout this course, we will use both terms (but regardless of the term we use, mass is what we'll be finding!). Note 10
The base unit of mass is the kilogram (kg). In the 1790s, a kilogram was defined as the mass of 1 L (cubic decimeter, or dm3) of water:
Though that definition has changed somewhat with time, here is a definition that is close enough for ordinary purposes: There are 1,000 g in 1 kg, and 1,000 g occupy a volume of 1,000 cm3, or 1 L. Therefore, 1 g of water weighs the same as 1 cm3 of water and occupies 1 mL of space. In other words, for water:
1,000 g = 1 kg = 1,000 cm3 = 1 dm3 = 1 L
1 g = 1 cm3 = 1 mL
Kilograms are used to weigh just about everything but very light objects (which are weighed in grams) and very heavy objects (which are weighed using metric tons). A gram is almost exactly the weight of a dollar bill. A metric ton is equivalent to 1,000 kg (so it can also be thought of as a megagram) and should not be confused with the common American ton in the U.S. customary system. In fact, the metric ton is often referred to by its French and German name, tonne, to distinguish it as a metric measure. Most cars have a mass of between 1 and 2 tonnes; a large diesel freight locomotive has a mass of approximately 165 tonnes.
As with metric lengths, it is useful to establish benchmarks for metric mass measures.