Laboratory Manual for Introductory Geology 4e

than a change of 1°F (180°/100°). To convert Fahrenheit to Celsius or vice versa,
see Appendix 1.1.
Mass refers to the amount of matter in an object, and weight refers to the force
with which one object is attracted to another. The weight of an object on the Earth
therefore depends not only on its mass, but also on the strength of the Earth’s gravitational
field. Objects that have more mass than others also weigh more than others
on the Earth because of the force of the Earth’s gravity. But whereas the mass of an
object remains the same whether it is on the Earth or on the Moon, the object weighs
less on the Moon because of the Moon’s weaker gravity.
Grams and kilograms (1 kg 5 1,000 g) are the units of mass in the metric system;
the U.S. customary system uses pounds and ounces (1 lb 5 16 oz). For those
who don’t read the metric equivalents on food packages, 1 kg 5 2.2046 lb and
1 g 5 0.0353 oz.
We saw earlier that the density (d) of a material is a measure of how much mass
is packed into each unit of volume. Density, a property useful in studying minerals
and rocks, is generally expressed in units of grams per cubic centimeter (g/cm 3 ).
We instinctively distinguish unusually low-density materials such as Styrofoam and
feathers from low-density materials such as water (d 5 1 g/cm 3 ) and high-density
materials such as steel (d 5 ∼7 g/cm 3 ) because the very low-density materials feel
very light for their sizes and the high-density materials feel unusually heavy for their
sizes (FIG. 1.4).
FIGURE 1.4 Weights of materials with different densities.
High density
(Steel)
Low density
(Water)
Very low density
(Feathers)
To measure the density of a material, we need to know its mass and volume.
Mass is measured with a balance or scale, and the volumes of regular geometric
shapes such as cubes, bricks, spheres, or cylinders can be calculated from
simple formulas. For example, to calculate the volume of a bar of gold, you
would multiply its length times its width times its height (FIG. 1.5a). But rocks
rarely have regular geometric shapes; more typically, they are irregular chunks.
To measure the volume of a rock (or another irregular object), submerge it in
a graduated cylinder partly filled with water (FIG. 1.5b). Measure the volume of
the water before the rock is added and then with the rock in the cylinder. The
rock displaces a volume of water equivalent to its own volume, so simply subtract
the initial volume of the water from that of the water plus the rock to obtain the
volume of the rock. The density of a rock can then be calculated simply from the
definition of density: density 5 mass 4 volume. Exercise 1.6 provides practice
in determining density.
1.3 UNITS FOR GEOLOGIC MEASUREMENT
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