would be negatively charged, so they would all repel each other. This would keep
them from passing through each other, and would also keep the surface in a sta-
ble, fixed state. The spheres would be mostly empty space inside, but there
wouldn’t be much space in between them. They would be tightly packed together.
The foregoing is an oversimplification, but it should give you an idea of the
reason why solids don’t normally pass through each other, and why many solids
resist penetration even by liquids such as water, or by gases such as air.
DENSITY OF SOLIDS
The density of a solid is measured in terms of the number of kilograms (kg) per
cubic meter (m
3
). That is, density is equal to mass divided by volume. The kilo-
gram per meter cubed (kg/m
3
or kg × m
−3
) is the measure of density in the
International System of units (SI), also known as the meter-kilogram-second
(mks) system. This is a rather awkward unit in most real-life situations. Imagine
trying to determine the density of sandstone by taking a cubical chunk of the
stuff measuring one meter (1 m) on an edge, and placing it on a laboratory scale!
You’d need a construction crane to lift the boulder, and it would smash the scale.
Because of the impracticality of measuring density directly in standard inter-
national units, the centimeter-gram-second (cgs) unit is sometimes used instead.
This is the number of grams of mass (g) per cubic centimeter (cm
3
) of the mate-
rial in question. Technically it is called the gram per centimeter cubed (g/cm
3
or
g × cm
−3
). To convert the density of a given sample from grams per centimeter
cubed to kilograms per meter cubed, multiply by 1000 (10
3
). To convert the den-
sity of a sample from kilograms per meter cubed to grams per centimeter cubed,
multiply by 0.001 (10
−3
).
You can think of solids that are dense, such as lead. Iron is dense, too.
Aluminum is not as dense. Rocks are less dense than most metals. Glass has
about the same density as silicate rock, from which it is made. Wood, and most
plastics, are not very dense.
MEASURING SOLID VOLUME
Samples of solids rarely come in perfect blocks, cubes, or spheres, which are
shapes that lend themselves to calculation of volume by mathematical formulas.
Most samples are irregular, and defy direct dimensional measurement.
Scientists have an indirect way of measuring the volumes of irregular solid
samples: immerse them in a liquid. First, we measure the amount of liquid in a
CHAPTER 1 Background Physics
3