5.2Classification and Behavior of Forces
One of the most basic and important tasks of physics is to classify the
forces of nature. I have already referred informally to “types” of forces such
as friction, magnetism, gravitational forces, and so on. Classification
systems are creations of the human mind, so there is always some degree of
arbitrariness in them. For one thing, the level of detail that is appropriate
for a classification system depends on what you’re trying to find out. Some
linguists, the “lumpers,” like to emphasize the similarities among languages,
and a few extremists have even tried to find signs of similarities between
words in languages as different as English and Chinese, lumping the world’s
languages into only a few large groups. Other linguists, the “splitters,”
might be more interested in studying the differences in pronunciation
between English speakers in New York and Connecticut. The splitters call
the lumpers sloppy, but the lumpers say that science isn’t worthwhile unless
it can find broad, simple patterns within the seemingly complex universe.
Scientific classification systems are also usually compromises between
practicality and naturalness. An example is the question of how to classify
flowering plants. Most people think that biological classification is about
discovering new species, naming them, and classifying them in the class-
order-family-genus-species system according to guidelines set long ago. In
reality, the whole system is in a constant state of flux and controversy. One
very practical way of classifying flowering plants is according to whether
their petals are separate or joined into a tube or cone — the criterion is so
clear that it can be applied to a plant seen from across the street. But here
practicality conflicts with naturalness. For instance, the begonia has separate
petals and the pumpkin has joined petals, but they are so similar in so many
other ways that they are usually placed within the same order. Some taxono-
mists have come up with classification criteria that they claim correspond
more naturally to the apparent relationships among plants, without having
to make special exceptions, but these may be far less practical, requiring for
instance the examination of pollen grains under an electron microscope.
In physics, there are two main systems of classification for forces. At this
point in the course, you are going to learn one that is very practical and easy
to use, and that splits the forces up into a relatively large number of types:
seven very common ones that we’ll discuss explicitly in this chapter, plus
perhaps ten less important ones such as surface tension, which we will not
bother with right now.
Professional physicists, however, are almost all obsessed with finding
simple patterns, so recognizing as many as fifteen or twenty types of forces
strikes them as distasteful and overly complex. Since about the year 1900,
physics has been on an aggressive program to discover ways in which these
many seemingly different types of forces arise from a smaller number of
fundamental ones. For instance, when you press your hands together, the
force that keeps them from passing through each other may seem to have
nothing to do with electricity, but at the atomic level, it actually does arise
from electrical repulsion between atoms. By about 1950, all the forces of
nature had been explained as arising from four fundamental types of forces
at the atomic and nuclear level, and the lumping-together process didn’t
stop there. By the 1960’s the length of the list had been reduced to three,
Chapter 5Analysis of Forces
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