Geology Reference
In-Depth Information
The density of an object is described as the mass per unit of volume.
Mass
is a measure
of the amount of matter in an object (usually measured on a scale similar to weight).
Volume
is how much space the object fills. A formula describing this relationship is writ-
ten:
D = m/v
In this formula,
m = mass (measured as grams, or g)
v = volume (measured as cubic centimeters, or cm
3
)
D = density (expressed as grams per cubic centimeter, or g/cm
3
)
To put it another way, density is the amount of stuff (matter) in a set volume of a certain
material. While density is
related to
the size (the volume), it is not
determined by
the size
(or volume). For example, two objects of the same size (or volume) may have different
densities. The difference is a result of one of the objects having more mass filling that
same space.
Consider two identical kitchen sponges. One is dry, and the other is saturated (filled)
with water. They continue to take up the same space, having the same volume, but the
dry sponge feels lighter in weight — it is less dense. In the saturated sponge, water fills
the holes of the sponge, adding more matter, increasing the mass, and therefore increas-
ing the density.
Mass is often confused with weight because you measure mass and weight in
similar terms, using a scale. But there is an important difference: weight measures
the pull of earth's gravity on an object, while mass measures the amount of atom-
ic matter in the object (see Chapter 5 for details on atoms). For example, if you go
to the moon, you will weigh less because the moon's gravity does not pull as
strongly as earth's gravity. But even on the moon, you will have the same mass as
you have on earth — the same amount of atomic matter composing your body.
Two of a Kind: Continental and Oceanic
Crust
